What are your favorite extension methods for C#? (codeplex.com/extensionoverflow)

public static bool In<T>(this T source, params T[] list)
{
  if(null==source) throw new ArgumentNullException("source");
  return list.Contains(source);
}

Allows me to replace:

if(reallyLongIntegerVariableName == 1 || 
    reallyLongIntegerVariableName == 6 || 
    reallyLongIntegerVariableName == 9 || 
    reallyLongIntegerVariableName == 11)
{
  // do something....
}

and

if(reallyLongStringVariableName == "string1" || 
    reallyLongStringVariableName == "string2" || 
    reallyLongStringVariableName == "string3")
{
  // do something....
}

and

if(reallyLongMethodParameterName == SomeEnum.Value1 || 
    reallyLongMethodParameterName == SomeEnum.Value2 || 
    reallyLongMethodParameterName == SomeEnum.Value3 || 
    reallyLongMethodParameterName == SomeEnum.Value4)
{
  // do something....
}

With:

if(reallyLongIntegerVariableName.In(1,6,9,11))
{
      // do something....
}

and

if(reallyLongStringVariableName.In("string1","string2","string3"))
{
      // do something....
}

and

if(reallyLongMethodParameterName.In(SomeEnum.Value1, SomeEnum.Value2, SomeEnum.Value3, SomeEnum.Value4)
{
  // do something....
}

I have various extension methods in my MiscUtil project (full source is available there - I'm not going to repeat it here). My favourites, some of which involve other classes (such as ranges):

Date and time stuff - mostly for unit tests. Not sure I'd use them in production :)

var birthday = 19.June(1976);
var workingDay = 7.Hours() + 30.Minutes();

Ranges and stepping - massive thanks to Marc Gravell for his operator stuff to make this possible:

var evenNaturals = 2.To(int.MaxValue).Step(2);
var daysSinceBirth = birthday.To(DateTime.Today).Step(1.Days());

Comparisons:

var myComparer = ProjectionComparer.Create(Person p => p.Name);
var next = myComparer.ThenBy(p => p.Age);
var reversed = myComparer.Reverse();

Argument checking:

x.ThrowIfNull("x");

LINQ to XML applied to anonymous types (or other types with appropriate properties):

// <Name>Jon</Name><Age>32</Age>
new { Name="Jon", Age=32}.ToXElements();
// Name="Jon" Age="32" (as XAttributes, obviously)
new { Name="Jon", Age=32}.ToXAttributes()

Push LINQ - would take too long to explain here, but search for it.

string.Format shortcut:

public static class StringExtensions
{
    // Enable quick and more natural string.Format calls
    public static string F(this string s, params object[] args)
    {
        return string.Format(s, args);
    }
}

Example:

var s = "The co-ordinate is ({0}, {1})".F(point.X, point.Y);

For quick copy-and-paste go here.

Don't you find it more natural to type "some string".F("param") instead of string.Format("some string", "param") ?

For a more readable name, try one of these suggestion:

s = "Hello {0} world {1}!".Fmt("Stack", "Overflow");
s = "Hello {0} world {1}!".FormatBy("Stack", "Overflow");
s = "Hello {0} world {1}!".FormatWith("Stack", "Overflow");
s = "Hello {0} world {1}!".Display("Stack", "Overflow");
s = "Hello {0} world {1}!".With("Stack", "Overflow");

..

Are these any use?

public static bool CoinToss(this Random rng)
{
    return rng.Next(2) == 0;
}

public static T OneOf<T>(this Random rng, params T[] things)
{
    return things[rng.Next(things.Length)];
}

Random rand;
bool luckyDay = rand.CoinToss();
string babyName = rand.OneOf("John", "George", "Radio XBR74 ROCKS!");

public static class ComparableExtensions
{
  public static bool Between<T>(this T actual, T lower, T upper) where T : IComparable<T>
  {
    return actual.CompareTo(lower) >= 0 && actual.CompareTo(upper) < 0;
  }
}

Example:

if (myNumber.Between(3,7))
{
  // ....
}

The extension method:

public static void AddRange<T, S>(this ICollection<T> list, params S[] values)
    where S : T
{
    foreach (S value in values)
        list.Add(value);
}

The method applies for all types and lets you add a range of items to a list as parameters.

Example:

var list = new List<Int32>();
list.AddRange(5, 4, 8, 4, 2);

By all means put this in the codeplex project.

Serializing / Deserializing objects to XML:

/// <summary>Serializes an object of type T in to an xml string</summary>
/// <typeparam name="T">Any class type</typeparam>
/// <param name="obj">Object to serialize</param>
/// <returns>A string that represents Xml, empty otherwise</returns>
public static string XmlSerialize<T>(this T obj) where T : class, new()
{
    if (obj == null) throw new ArgumentNullException("obj");

    var serializer = new XmlSerializer(typeof(T));
    using (var writer = new StringWriter())
    {
        serializer.Serialize(writer, obj);
        return writer.ToString();
    }
}

/// <summary>Deserializes an xml string in to an object of Type T</summary>
/// <typeparam name="T">Any class type</typeparam>
/// <param name="xml">Xml as string to deserialize from</param>
/// <returns>A new object of type T is successful, null if failed</returns>
public static T XmlDeserialize<T>(this string xml) where T : class, new()
{
    if (xml == null) throw new ArgumentNullException("xml");

    var serializer = new XmlSerializer(typeof(T));
    using (var reader = new StringReader(xml))
    {
        try { return (T)serializer.Deserialize(reader); }
        catch { return null; } // Could not be deserialized to this type.
    }
}

ForEach for IEnumerables

public static class FrameworkExtensions
{
    // a map function
    public static void ForEach<T>(this IEnumerable<T> @enum, Action<T> mapFunction)
    {
        foreach (var item in @enum) mapFunction(item);
    }
}

Naive example:

var buttons = GetListOfButtons() as IEnumerable<Button>;

// click all buttons
buttons.ForEach(b => b.Click());

Cool example:

// no need to type the same assignment 3 times, just
// new[] up an array and use foreach + lambda
// everything is properly inferred by csc :-)
new { itemA, itemB, itemC }
    .ForEach(item => {
        item.Number = 1;
        item.Str = "Hello World!";
    });

Note:

This is not like Select because Select expects your function to return something as for transforming into another list.

ForEach simply allows you to execute something for each of the items without any transformations/data manipulation.

I made this so I can program in a more functional style and I was surprised that List has a ForEach while IEnumerable does not.

Put this in the codeplex project

My conversion extensions which allow you to do:

int i = myString.To<int>();

Here it is, as posted on TheSoftwareJedi.com

public static T To<T>(this IConvertible obj)
{
  return (T)Convert.ChangeType(obj, typeof(T));
}

public static T ToOrDefault<T>
             (this IConvertible obj)
{
    try
    {
        return To<T>(obj);
    }
    catch
    {
        return default(T);
    }
}

public static bool ToOrDefault<T>
                    (this IConvertible obj,
                     out T newObj)
{
    try
    {
        newObj = To<T>(obj); 
        return true;
    }
    catch
    {
        newObj = default(T); 
        return false;
    }
}

public static T ToOrOther<T>
                       (this IConvertible obj,
                       T other)
{
  try
  {
      return To<T>obj);
  }
  catch
  {
      return other;
  }
}

public static bool ToOrOther<T>
                         (this IConvertible obj,
                         out T newObj,
                         T other)
{
    try
    {
        newObj = To<T>(obj);
        return true;
    }
    catch
    {
        newObj = other;
        return false;
    }
}

public static T ToOrNull<T>
                      (this IConvertible obj)
                      where T : class
{
    try
    {
        return To<T>(obj);
    }
    catch
    {
        return null;
    }
}

public static bool ToOrNull<T>
                  (this IConvertible obj,
                  out T newObj)
                  where T : class
{
    try
    {
        newObj = To<T>(obj);
        return true;
    }
    catch
    {
        newObj = null;
        return false;
    }
}

You can ask for default (calls blank constructor or "0" for numerics) on failure, specify a "default" value (I call it "other"), or ask for null (where T : class). I've also provided both silent exception models, and a typical TryParse model that returns a bool indicating the action taken, and an out param holds the new value. So our code can do things like this

int i = myString.To<int>();
string a = myInt.ToOrDefault<string>();
//note type inference
DateTime d = myString.ToOrOther(DateTime.MAX_VALUE);
double d;
//note type inference
bool didItGiveDefault = myString.ToOrDefault(out d);
string s = myDateTime.ToOrNull<string>();

I couldn't get Nullable types to roll into the whole thing very cleanly. I tried for about 20 minutes before I threw in the towel.

I have an extension method for logging exceptions:

public static void Log(this Exception obj)
{
  //your logging logic here
}

And it is used like this:

try
{
    //Your stuff here
}
catch(Exception ex)
{
    ex.Log();
}

[sorry for posting twice; the 2nd one is better designed :-)]

public static class StringExtensions {

    /// <summary>
    /// Parses a string into an Enum
    /// </summary>
    /// <typeparam name="T">The type of the Enum</typeparam>
    /// <param name="value">String value to parse</param>
    /// <returns>The Enum corresponding to the stringExtensions</returns>
    public static T EnumParse<T>(this string value) {
        return StringExtensions.EnumParse<T>(value, false);
    }

    public static T EnumParse<T>(this string value, bool ignorecase) {

        if (value == null) {
            throw new ArgumentNullException("value");
        }

        value = value.Trim();

        if (value.Length == 0) {
            throw new ArgumentException("Must specify valid information for parsing in the string.", "value");
        }

        Type t = typeof(T);

        if (!t.IsEnum) {
            throw new ArgumentException("Type provided must be an Enum.", "T");
        }

        return (T)Enum.Parse(t, value, ignorecase);
    }
}

Useful to parse a string into an Enum.

public enum TestEnum
{
    Bar,
    Test
}

public class Test
{
    public void Test()
    {
        TestEnum foo = "Test".EnumParse<TestEnum>();
    }
 }

Credit goes to Scott Dorman

--- Edit for Codeplex project ---

I have asked Scott Dorman if he would mind us publishing his code in the Codeplex project. This is the reply I got from him:

Thanks for the heads-up on both the SO post and the CodePlex project. I have upvoted your answer on the question. Yes, the code is effectively in the public domain currently under the CodeProject Open License (http://www.codeproject.com/info/cpol10.aspx).

I have no problems with this being included in the CodePlex project, and if you want to add me to the project (username is sdorman) I will add that method plus some additional enum helper methods.

DateTimeExtensions

Examples:

DateTime firstDayOfMonth = DateTime.Now.First();
DateTime lastdayOfMonth = DateTime.Now.Last();
DateTime lastFridayInMonth = DateTime.Now.Last(DayOfWeek.Friday);
DateTime nextFriday = DateTime.Now.Next(DayOfWeek.Friday);
DateTime lunchTime = DateTime.Now.SetTime(11, 30);
DateTime noonOnFriday = DateTime.Now.Next(DayOfWeek.Friday).Noon();
DateTime secondMondayOfMonth = DateTime.Now.First(DayOfWeek.Monday).Next(DayOfWeek.Monday).Midnight();

gitorious.org/cadenza is a full library of some of the most useful extension methods I've seen.

Here is one I use frequently for presentation formatting.

public static string ToTitleCase(this string mText)
{
    if (mText == null) return mText;

    System.Globalization.CultureInfo cultureInfo = System.Threading.Thread.CurrentThread.CurrentCulture;
    System.Globalization.TextInfo textInfo = cultureInfo.TextInfo;

    // TextInfo.ToTitleCase only operates on the string if is all lower case, otherwise it returns the string unchanged.
    return textInfo.ToTitleCase(mText.ToLower());
}

Here's a to-and-from for Roman numerals. Not often used, but could be handy. Usage:

if ("IV".IsValidRomanNumeral())
{
   // Do useful stuff with the number 4.
}

Console.WriteLine("MMMDCCCLXXXVIII".ParseRomanNumeral());
Console.WriteLine(3888.ToRomanNumeralString());

The source:

    public static class RomanNumeralExtensions
    {
        private const int NumberOfRomanNumeralMaps = 13;

        private static readonly Dictionary<string, int> romanNumerals =
            new Dictionary<string, int>(NumberOfRomanNumeralMaps)
            {
                { "M", 1000 }, 
                { "CM", 900 }, 
                { "D", 500 }, 
                { "CD", 400 }, 
                { "C", 100 }, 
                { "XC", 90 }, 
                { "L", 50 }, 
                { "XL", 40 }, 
                { "X", 10 }, 
                { "IX", 9 }, 
                { "V", 5 }, 
                { "IV", 4 }, 
                { "I", 1 }
            };

        private static readonly Regex validRomanNumeral = new Regex(
            "^(?i:(?=[MDCLXVI])((M{0,3})((C[DM])|(D?C{0,3}))"
            + "?((X[LC])|(L?XX{0,2})|L)?((I[VX])|(V?(II{0,2}))|V)?))$", 
            RegexOptions.Compiled);

        public static bool IsValidRomanNumeral(this string value)
        {
            return validRomanNumeral.IsMatch(value);
        }

        public static int ParseRomanNumeral(this string value)
        {
            if (value == null)
            {
                throw new ArgumentNullException("value");
            }

            value = value.ToUpperInvariant().Trim();

            var length = value.Length;

            if ((length == 0) || !value.IsValidRomanNumeral())
            {
                throw new ArgumentException("Empty or invalid Roman numeral string.", "value");
            }

            var total = 0;
            var i = length;

            while (i > 0)
            {
                var digit = romanNumerals[value[--i].ToString()];

                if (i > 0)
                {
                    var previousDigit = romanNumerals[value[i - 1].ToString()];

                    if (previousDigit < digit)
                    {
                        digit -= previousDigit;
                        i--;
                    }
                }

                total += digit;
            }

            return total;
        }

        public static string ToRomanNumeralString(this int value)
        {
            const int MinValue = 1;
            const int MaxValue = 3999;

            if ((value < MinValue) || (value > MaxValue))
            {
                throw new ArgumentOutOfRangeException("value", value, "Argument out of Roman numeral range.");
            }

            const int MaxRomanNumeralLength = 15;
            var sb = new StringBuilder(MaxRomanNumeralLength);

            foreach (var pair in romanNumerals)
            {
                while (value / pair.Value > 0)
                {
                    sb.Append(pair.Key);
                    value -= pair.Value;
                }
            }

            return sb.ToString();
        }
    }

The ThrowIfArgumentIsNull is a nice way to do that null check we all should do.

public static class Extensions
{
    public static void ThrowIfArgumentIsNull<T>(this T obj, string parameterName) where T : class
    {
        if (obj == null) throw new ArgumentNullException(parameterName + " not allowed to be null");
    }
}

Below is the way to use it and it works on all classes in your namespace or wherever you use the namespace its within.

internal class Test
{
    public Test(string input1)
    {
        input1.ThrowIfArgumentIsNull("input1");
    }
}

It's ok to use this code on the CodePlex project.

I miss the Visual Basic's With statement when moving to C#, so here it goes:

public static void With<T>(this T obj, Action<T> act) { act(obj); }

And here's how to use it in C#:

someVeryVeryLonggggVariableName.With(x => {
    x.Int = 123;
    x.Str = "Hello";
    x.Str2 = " World!";
});

Saves a lot of typing!

Compare this to:

someVeryVeryLonggggVariableName.Int = 123;
someVeryVeryLonggggVariableName.Str = "Hello";
someVeryVeryLonggggVariableName.Str2 = " World!";

put in codeplex project

Takes a camelCaseWord or PascalCaseWord and "wordifies" it, ie camelCaseWord => camel Case Word

public static string Wordify( this string camelCaseWord )
{
    // if the word is all upper, just return it
    if( !Regex.IsMatch( camelCaseWord, "[a-z]" ) )
        return camelCaseWord;

    return string.Join( " ", Regex.Split( camelCaseWord, @"(?<!^)(?=[A-Z])" ) );
}

I often use it in conjuction with Capitalize

public static string Capitalize( this string word )
{
    return word[0].ToString( ).ToUpper( ) + word.Substring( 1 );
}

Example usage

SomeEntityObject entity = DataAccessObject.GetSomeEntityObject( id );
List<PropertyInfo> properties = entity.GetType().GetPublicNonCollectionProperties( );

// wordify the property names to act as column headers for an html table or something
List<string> columns = properties.Select( p => p.Name.Capitalize( ).Wordify( ) ).ToList( );

Free to use in codeplex project

I found this one helpful

public static IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> pSeq)
{
    return pSeq ?? Enumerable.Empty<T>();
}

It removes the null check in the calling code. You could now do

MyList.EmptyIfNull().Where(....)

Convert a double to string formatted using the specified culture:

public static class ExtensionMethods 
{
  public static string ToCurrency(this double value, string cultureName)
  {
    CultureInfo currentCulture = new CultureInfo(cultureName);
    return (string.Format(currentCulture, "{0:C}", value));
  }
}

Example:

double test = 154.20;
string testString = test.ToCurrency("en-US"); // $154.20

Below is an extension method that adapts Rick Strahl's code (and the comments too) to stop you having to guess or read the byte order mark of a byte array or text file each time you convert it to a string.

The snippet allows you to simply do:

byte[] buffer = File.ReadAllBytes(@"C:\file.txt");
string content = buffer.GetString();

If you find any bugs please add to the comments. Feel free to include it in the Codeplex project.

public static class Extensions
{
    /// <summary>
    /// Converts a byte array to a string, using its byte order mark to convert it to the right encoding.
    /// Original article: http://www.west-wind.com/WebLog/posts/197245.aspx
    /// </summary>
    /// <param name="buffer">An array of bytes to convert</param>
    /// <returns>The byte as a string.</returns>
    public static string GetString(this byte[] buffer)
    {
        if (buffer == null || buffer.Length == 0)
            return "";

        // Ansi as default
        Encoding encoding = Encoding.Default;       

        /*
            EF BB BF    UTF-8 
            FF FE UTF-16    little endian 
            FE FF UTF-16    big endian 
            FF FE 00 00 UTF-32, little endian 
            00 00 FE FF UTF-32, big-endian 
         */

        if (buffer[0] == 0xef && buffer[1] == 0xbb && buffer[2] == 0xbf)
            encoding = Encoding.UTF8;
        else if (buffer[0] == 0xfe && buffer[1] == 0xff)
            encoding = Encoding.Unicode;
        else if (buffer[0] == 0xfe && buffer[1] == 0xff)
            encoding = Encoding.BigEndianUnicode; // utf-16be
        else if (buffer[0] == 0 && buffer[1] == 0 && buffer[2] == 0xfe && buffer[3] == 0xff)
            encoding = Encoding.UTF32;
        else if (buffer[0] == 0x2b && buffer[1] == 0x2f && buffer[2] == 0x76)
            encoding = Encoding.UTF7;

        using (MemoryStream stream = new MemoryStream())
        {
            stream.Write(buffer, 0, buffer.Length);
            stream.Seek(0, SeekOrigin.Begin);
            using (StreamReader reader = new StreamReader(stream, encoding))
            {
                return reader.ReadToEnd();
            }
        }
    }
}

I got tired of tedious null-checking while pulling values from MySqlDataReader, so:

public static DateTime? GetNullableDateTime(this MySqlDataReader dr, string fieldName)
{
    DateTime? nullDate = null;
    return dr.IsDBNull(dr.GetOrdinal(fieldName)) ? nullDate : dr.GetDateTime(fieldName);
}

public static string GetNullableString(this MySqlDataReader dr, string fieldName)
{
    return dr.IsDBNull(dr.GetOrdinal(fieldName)) ? String.Empty : dr.GetString(fieldName);
}

public static char? GetNullableChar(this MySqlDataReader dr, string fieldName)
{
    char? nullChar = null;
    return dr.IsDBNull(dr.GetOrdinal(fieldName)) ? nullChar : dr.GetChar(fieldName);
}

Of course this could be used with any SqlDataReader.

Both hangy and Joe had some good comments on how to do this, and I have since had an opportunity to implement something similar in a different context, so here is another version:

public static int? GetNullableInt32(this IDataRecord dr, int ordinal)
{
    int? nullInt = null;
    return dr.IsDBNull(ordinal) ? nullInt : dr.GetInt32(ordinal);
}

public static int? GetNullableInt32(this IDataRecord dr, string fieldname)
{
    int ordinal = dr.GetOrdinal(fieldname);
    return dr.GetNullableInt32(ordinal);
}

public static bool? GetNullableBoolean(this IDataRecord dr, int ordinal)
{
    bool? nullBool = null;
    return dr.IsDBNull(ordinal) ? nullBool : dr.GetBoolean(ordinal);
}

public static bool? GetNullableBoolean(this IDataRecord dr, string fieldname)
{
    int ordinal = dr.GetOrdinal(fieldname);
    return dr.GetNullableBoolean(ordinal);
}

"Please mark your answers with an acceptance to put the code in the Codeplex project."

Why? All the Stuff on this site under CC-by-sa-2.5, so just put your Extension overflow Project under the same license and you can freely use it.

Anyway, here is a String.Reverse function, based on this question.

/// <summary>
/// Reverse a String
/// </summary>
/// <param name="input">The string to Reverse</param>
/// <returns>The reversed String</returns>
public static string Reverse(this string input)
{
    char[] array = input.ToCharArray();
    Array.Reverse(array);
    return new string(array);
}

It irritated me that LINQ gives me an OrderBy that takes a class implementing IComparer as an argument, but does not support passing in a simple anonymous comparer function. I rectified that.

This class creates an IComparer from your comparer function...

/// <summary>
///     Creates an <see cref="IComparer{T}"/> instance for the given
///     delegate function.
/// </summary>
internal class ComparerFactory<T> : IComparer<T>
{
    public static IComparer<T> Create(Func<T, T, int> comparison)
    {
        return new ComparerFactory<T>(comparison);
    }

    private readonly Func<T, T, int> _comparison;

    private ComparerFactory(Func<T, T, int> comparison)
    {
        _comparison = comparison;
    }

    #region IComparer<T> Members

    public int Compare(T x, T y)
    {
        return _comparison(x, y);
    }

    #endregion
}

...and these extension methods expose my new OrderBy overloads on enumerables. I doubt this works for LINQ to SQL, but it's great for LINQ to Objects.

public static class EnumerableExtensions
{
    /// <summary>
    /// Sorts the elements of a sequence in ascending order by using a specified comparison delegate.
    /// </summary>
    public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector,
                                                                     Func<TKey, TKey, int> comparison)
    {
        var comparer = ComparerFactory<TKey>.Create(comparison);
        return source.OrderBy(keySelector, comparer);
    }

    /// <summary>
    /// Sorts the elements of a sequence in descending order by using a specified comparison delegate.
    /// </summary>
    public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector,
                                                                               Func<TKey, TKey, int> comparison)
    {
        var comparer = ComparerFactory<TKey>.Create(comparison);
        return source.OrderByDescending(keySelector, comparer);
    }
}

You're welcome to put this on codeplex if you like.

This one is for MVC it adds the ability to generate a <label /> tag to the Html variable that is available in every ViewPage. Hopefully it will be of use to others trying to develop similar extensions.

Use:

<%= Html.Label("LabelId", "ForId", "Text")%>

Output:

<label id="LabelId" for="ForId">Text</label>

Code:

public static class HtmlHelperExtensions
{
    public static string Label(this HtmlHelper Html, string @for, string text)
    {
        return Html.Label(null, @for, text);
    }

    public static string Label(this HtmlHelper Html, string @for, string text, object htmlAttributes)
    {
        return Html.Label(null, @for, text, htmlAttributes);
    }

    public static string Label(this HtmlHelper Html, string @for, string text, IDictionary<string, object> htmlAttributes)
    {
        return Html.Label(null, @for, text, htmlAttributes);
    }

    public static string Label(this HtmlHelper Html, string id, string @for, string text)
    {
        return Html.Label(id, @for, text, null);
    }

    public static string Label(this HtmlHelper Html, string id, string @for, string text, object htmlAttributes)
    {
        return Html.Label(id, @for, text, new RouteValueDictionary(htmlAttributes));
    }

    public static string Label(this HtmlHelper Html, string id, string @for, string text, IDictionary<string, object> htmlAttributes)
    {
        TagBuilder tag = new TagBuilder("label");

        tag.MergeAttributes(htmlAttributes);

        if (!string.IsNullOrEmpty(id))
            tag.MergeAttribute("id", Html.AttributeEncode(id));

        tag.MergeAttribute("for", Html.AttributeEncode(@for));

        tag.SetInnerText(Html.Encode(text));

        return tag.ToString(TagRenderMode.Normal);
    }
}

Find more examples here: www.extensionmethod.net

Another useful one for me:

/// <summary>
/// Converts any type in to an Int32
/// </summary>
/// <typeparam name="T">Any Object</typeparam>
/// <param name="value">Value to convert</param>
/// <returns>The integer, 0 if unsuccessful</returns>
public static int ToInt32<T>(this T value)
{
  int result;
  if (int.TryParse(value.ToString(), out result))
  {
    return result;
  }
  return 0;
}

/// <summary>
/// Converts any type in to an Int32 but if null then returns the default
/// </summary>
/// <param name="value">Value to convert</param>
/// <typeparam name="T">Any Object</typeparam>
/// <param name="defaultValue">Default to use</param>
/// <returns>The defaultValue if unsuccessful</returns>
public static int ToInt32<T>(this T value, int defaultValue)
{
  int result;
  if (int.TryParse(value.ToString(), out result))
  {
    return result;
  }
  return defaultValue;
}

Example:

int number = "123".ToInt32();

or:

int badNumber = "a".ToInt32(100); // Returns 100 since a is nan

IEnumerable<> Shuffle

I used the Fisher-Yates the algorithm to implement a shuffle function.

By using yield return and breaking the code in two functions, it achieves proper argument validation and deferred execution. (thanks, Dan, for pointing this flaw in my first version)

static public IEnumerable<T> Shuffle<T>(this IEnumerable<T> source)
{
    if (source == null) throw new ArgumentNullException("source");

    return ShuffleIterator(source);
}

static private IEnumerable<T> ShuffleIterator<T>(this IEnumerable<T> source)
{
    T[] array = source.ToArray();
    Random rnd = new Random();          
    for (int n = array.Length; n > 1;)
    {
        int k = rnd.Next(n--); // 0 <= k < n

        //Swap items
        if (n != k)
        {
            T tmp = array[k];
            array[k] = array[n];
            array[n] = tmp;
        }
    }

    foreach (var item in array) yield return item;
}

There's a lot of functionality you can get from the Random class.

Below are some extension methods I use from time to time. With these, in addition to Next and NextDouble, the Random class gives you NextBool, NextChar, NextDateTime, NextTimeSpan, NextDouble (accepting minValue and maxValue parameters), and my personal favorite: NextString. There are more (NextByte, NextShort, NextLong, etc.); but those are mostly for completeness and don't get used as much. So I didn't include them here (this code is long enough as it is!).

// todo: implement additional CharType values (e.g., AsciiAny)
public enum CharType {
    AlphabeticLower,
    AlphabeticUpper,
    AlphabeticAny,
    AlphanumericLower,
    AlphanumericUpper,
    AlphanumericAny,
    Numeric
}

public static class RandomExtensions {
    // 10 digits vs. 52 alphabetic characters (upper & lower);
    // probability of being numeric: 10 / 62 = 0.1612903225806452
    private const double AlphanumericProbabilityNumericAny = 10.0 / 62.0;

    // 10 digits vs. 26 alphabetic characters (upper OR lower);
    // probability of being numeric: 10 / 36 = 0.2777777777777778
    private const double AlphanumericProbabilityNumericCased = 10.0 / 36.0;

    public static bool NextBool(this Random random, double probability) {
        return random.NextDouble() <= probability;
    }

    public static bool NextBool(this Random random) {
        return random.NextDouble() <= 0.5;
    }

    public static char NextChar(this Random random, CharType mode) {
        switch (mode) {
            case CharType.AlphabeticAny:
                return random.NextAlphabeticChar();
            case CharType.AlphabeticLower:
                return random.NextAlphabeticChar(false);
            case CharType.AlphabeticUpper:
                return random.NextAlphabeticChar(true);
            case CharType.AlphanumericAny:
                return random.NextAlphanumericChar();
            case CharType.AlphanumericLower:
                return random.NextAlphanumericChar(false);
            case CharType.AlphanumericUpper:
                return random.NextAlphanumericChar(true);
            case CharType.Numeric:
                return random.NextNumericChar();
            default:
                return random.NextAlphanumericChar();
        }
    }

    public static char NextChar(this Random random) {
        return random.NextChar(CharType.AlphanumericAny);
    }

    private static char NextAlphanumericChar(this Random random, bool uppercase) {
        bool numeric = random.NextBool(AlphanumericProbabilityNumericCased);

        if (numeric)
            return random.NextNumericChar();
        else
            return random.NextAlphabeticChar(uppercase);
    }

    private static char NextAlphanumericChar(this Random random) {
        bool numeric = random.NextBool(AlphanumericProbabilityNumericAny);

        if (numeric)
            return random.NextNumericChar();
        else
            return random.NextAlphabeticChar(random.NextBool());
    }

    private static char NextAlphabeticChar(this Random random, bool uppercase) {
        if (uppercase)
            return (char)random.Next(65, 91);
        else
            return (char)random.Next(97, 123);
    }

    private static char NextAlphabeticChar(this Random random) {
        return random.NextAlphabeticChar(random.NextBool());
    }

    private static char NextNumericChar(this Random random) {
        return (char)random.Next(48, 58);
    }

    public static DateTime NextDateTime(this Random random, DateTime minValue, DateTime maxValue) {
        return DateTime.FromOADate(
            random.NextDouble(minValue.ToOADate(), maxValue.ToOADate())
        );
    }

    public static DateTime NextDateTime(this Random random) {
        return random.NextDateTime(DateTime.MinValue, DateTime.MaxValue);
    }

    public static double NextDouble(this Random random, double minValue, double maxValue) {
        if (maxValue < minValue)
            throw new ArgumentException("Minimum value must be less than maximum value.");

        double difference = maxValue - minValue;
        if (!double.IsInfinity(difference))
            return minValue + (random.NextDouble() * difference);

        else {
            // to avoid evaluating to Double.Infinity, we split the range into two halves:
            double halfDifference = (maxValue * 0.5) - (minValue * 0.5);

            // 50/50 chance of returning a value from the first or second half of the range
            if (random.NextBool())
                return minValue + (random.NextDouble() * halfDifference);
            else
                return (minValue + halfDifference) + (random.NextDouble() * halfDifference);
        }
    }

    public static string NextString(this Random random, int numChars, CharType mode) {
        char[] chars = new char[numChars];

        for (int i = 0; i < numChars; ++i)
            chars[i] = random.NextChar(mode);

        return new string(chars);
    }

    public static string NextString(this Random random, int numChars) {
        return random.NextString(numChars, CharType.AlphanumericAny);
    }

    public static TimeSpan NextTimeSpan(this Random random, TimeSpan minValue, TimeSpan maxValue) {
        return TimeSpan.FromMilliseconds(
            random.NextDouble(minValue.TotalMilliseconds, maxValue.TotalMilliseconds)
        );
    }

    public static TimeSpan NextTimeSpan(this Random random) {
        return random.NextTimeSpan(TimeSpan.MinValue, TimeSpan.MaxValue);
    }
}

I'm disappointed that the .NET Framework prefers that files and directories be represented as strings rather than objects, and that the FileInfo and DirectoryInfo types aren't as powerful as I'd wish. So, I started to write fluent extension methods as I needed them, e.g.:

public static FileInfo SetExtension(this FileInfo fileInfo, string extension)
{
    return new FileInfo(Path.ChangeExtension(fileInfo.FullName, extension));
}

public static FileInfo SetDirectory(this FileInfo fileInfo, string directory)
{
    return new FileInfo(Path.Combine(directory, fileInfo.Name));
}

Yes, you can put this in the codeplex

Some of my best method extensions (I have a lot!):

public static T ToEnum<T>(this string str) where T : struct
{
    return (T)Enum.Parse(typeof(T), str);
}

//DayOfWeek sunday =  "Sunday".ToEnum<DayOfWeek>();

public static string ToString<T>(this IEnumerable<T> collection, string separator)
{
    return ToString(collection, t => t.ToString(), separator);
}

public static string ToString<T>(this IEnumerable<T> collection, Func<T, string> stringElement, string separator)
{
    StringBuilder sb = new StringBuilder();
    foreach (var item in collection)
    {
        sb.Append(stringElement(item));
        sb.Append(separator);
    }
    return sb.ToString(0, Math.Max(0, sb.Length - separator.Length));  // quita el ultimo separador
}

//new []{1,2,3}.ToString(i=>i*2, ", ")  --> "2, 4, 6"

Also, the next ones are meant to be able to continue in the same line in almost any situation, not declaring new variables and then removing state:

public static R Map<T, R>(this T t, Func<T, R> func)
{
    return func(t);
}

ExpensiveFindWally().Map(wally=>wally.FirstName + " " + wally.LastName)

public static R TryCC<T, R>(this T t, Func<T, R> func)
    where T : class
    where R : class
{
    if (t == null) return null;
    return func(t);
}

public static R? TryCS<T, R>(this T t, Func<T, R> func)
    where T : class
    where R : struct
{
    if (t == null) return null;
    return func(t);
}

public static R? TryCS<T, R>(this T t, Func<T, R?> func)
    where T : class
    where R : struct
{
    if (t == null) return null;
    return func(t);
}

public static R TrySC<T, R>(this T? t, Func<T, R> func)
    where T : struct
    where R : class
{
    if (t == null) return null;
    return func(t.Value);
}

public static R? TrySS<T, R>(this T? t, Func<T, R> func)
    where T : struct
    where R : struct
{
    if (t == null) return null;
    return func(t.Value);
}

public static R? TrySS<T, R>(this T? t, Func<T, R?> func)
    where T : struct
    where R : struct
{
    if (t == null) return null;
    return func(t.Value);
}

//int? bossNameLength =  Departament.Boss.TryCC(b=>b.Name).TryCS(s=>s.Length);


public static T ThrowIfNullS<T>(this T? t, string mensaje)
    where T : struct
{
    if (t == null)
        throw new NullReferenceException(mensaje);
    return t.Value;
}

public static T ThrowIfNullC<T>(this T t, string mensaje)
    where T : class
{
    if (t == null)
        throw new NullReferenceException(mensaje);
    return t;
}

public static T Do<T>(this T t, Action<T> action)
{
    action(t);
    return t;
}

//Button b = new Button{Content = "Click"}.Do(b=>Canvas.SetColumn(b,2));

public static T TryDo<T>(this T t, Action<T> action) where T : class
{
    if (t != null)
        action(t);
    return t;
}

public static T? TryDoS<T>(this T? t, Action<T> action) where T : struct
{
    if (t != null)
        action(t.Value);
    return t;
}

Hope it doesn't look like coming from Mars :)

Timespan-related extensions like:

public static TimeSpan Seconds(this int seconds)
{
  return TimeSpan.FromSeconds(seconds);
}

public static TimeSpan Minutes(this int minutes)
{
  return TimeSpan.FromMinutes(minutes);
}

That allow to use:

1.Seconds()
20.Minutes()

Lock extensions like:

public static IDisposable GetReadLock(this ReaderWriterLockSlim slimLock)
{
  slimLock.EnterReadLock();
  return new DisposableAction(slimLock.ExitReadLock);
}

public static IDisposable GetWriteLock(this ReaderWriterLockSlim slimLock)
{
  slimLock.EnterWriteLock();
  return new DisposableAction(slimLock.ExitWriteLock);
}

public static IDisposable GetUpgradeableReadLock(this ReaderWriterLockSlim slimLock)
{
  slimLock.EnterUpgradeableReadLock();
  return new DisposableAction(slimLock.ExitUpgradeableReadLock);
}

That allow to use locks like:

using (lock.GetUpgradeableReadLock())
{
  // try read
  using (lock.GetWriteLock())
  {
    //do write
  }
}

And many other from the Lokad Shared Libraries

HTH. These are some of my main ones.

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;

namespace Insert.Your.Namespace.Here.Helpers
{
    public static class Extensions
    {
        public static bool IsNullOrEmpty<T>(this IEnumerable<T> iEnumerable)
        {
            // Cheers to Joel Mueller for the bugfix. Was .Count(), now it's .Any()
            return iEnumerable == null ||
                   !iEnumerable.Any();
        }

        public static IList<T> ToListIfNotNullOrEmpty<T>(this IList<T> iList)
        {
            return iList.IsNullOrEmpty() ? null : iList;
        }

        public static PagedList<T> ToPagedListIfNotNullOrEmpty<T>(this PagedList<T> pagedList)
        {
            return pagedList.IsNullOrEmpty() ? null : pagedList;
        }

        public static string ToPluralString(this int value)
        {
            return value == 1 ? string.Empty : "s";
        }

        public static string ToReadableTime(this DateTime value)
        {
            TimeSpan span = DateTime.Now.Subtract(value);
            const string plural = "s";


            if (span.Days > 7)
            {
                return value.ToShortDateString();
            }

            switch (span.Days)
            {
                case 0:
                    switch (span.Hours)
                    {
                        case 0:
                            if (span.Minutes == 0)
                            {
                                return span.Seconds <= 0
                                           ? "now"
                                           : string.Format("{0} second{1} ago",
                                                           span.Seconds,
                                                           span.Seconds != 1 ? plural : string.Empty);
                            }
                            return string.Format("{0} minute{1} ago",
                                                 span.Minutes,
                                                 span.Minutes != 1 ? plural : string.Empty);
                        default:
                            return string.Format("{0} hour{1} ago",
                                                 span.Hours,
                                                 span.Hours != 1 ? plural : string.Empty);
                    }
                default:
                    return string.Format("{0} day{1} ago",
                                         span.Days,
                                         span.Days != 1 ? plural : string.Empty);
            }
        }

        public static string ToShortGuidString(this Guid value)
        {
            return Convert.ToBase64String(value.ToByteArray())
                .Replace("/", "_")
                .Replace("+", "-")
                .Substring(0, 22);
        }

        public static Guid FromShortGuidString(this string value)
        {
            return new Guid(Convert.FromBase64String(value.Replace("_", "/")
                                                         .Replace("-", "+") + "=="));
        }

        public static string ToStringMaximumLength(this string value, int maximumLength)
        {
            return ToStringMaximumLength(value, maximumLength, "...");
        }

        public static string ToStringMaximumLength(this string value, int maximumLength, string postFixText)
        {
            if (string.IsNullOrEmpty(postFixText))
            {
                throw new ArgumentNullException("postFixText");
            }

            return value.Length > maximumLength
                       ? string.Format(CultureInfo.InvariantCulture,
                                       "{0}{1}",
                                       value.Substring(0, maximumLength - postFixText.Length),
                                       postFixText)
                       :
                           value;
        }

        public static string SlugDecode(this string value)
        {
            return value.Replace("_", " ");
        }

        public static string SlugEncode(this string value)
        {
            return value.Replace(" ", "_");
        }
    }
}

public static class EnumerableExtensions
{
    [Pure]
    public static U MapReduce<T, U>(this IEnumerable<T> enumerable, Func<T, U> map, Func<U, U, U> reduce)
    {
        CodeContract.RequiresAlways(enumerable != null);
        CodeContract.RequiresAlways(enumerable.Skip(1).Any());
        CodeContract.RequiresAlways(map != null);
        CodeContract.RequiresAlways(reduce != null);
        return enumerable.AsParallel().Select(map).Aggregate(reduce);
    }
    [Pure]
    public static U MapReduce<T, U>(this IList<T> list, Func<T, U> map, Func<U, U, U> reduce)
    {
        CodeContract.RequiresAlways(list != null);
        CodeContract.RequiresAlways(list.Count >= 2);
        CodeContract.RequiresAlways(map != null);
        CodeContract.RequiresAlways(reduce != null);
        U result = map(list[0]);
        for (int i = 1; i < list.Count; i++)
        {
            result = reduce(result,map(list[i]));
        }
        return result;
    }

    //Parallel version; creates garbage
    [Pure]
    public static U MapReduce<T, U>(this IList<T> list, Func<T, U> map, Func<U, U, U> reduce)
    {
        CodeContract.RequiresAlways(list != null);
        CodeContract.RequiresAlways(list.Skip(1).Any());
        CodeContract.RequiresAlways(map != null);
        CodeContract.RequiresAlways(reduce != null);

        U[] mapped = new U[list.Count];
        Parallel.For(0, mapped.Length, i =>
            {
                mapped[i] = map(list[i]);
            });
        U result = mapped[0];
        for (int i = 1; i < list.Count; i++)
        {
            result = reduce(result, mapped[i]);
        }
        return result;
    }

}

Sometimes its handy to write out a string on a selected element in a list with a custom seperator.

For instance if you have a List<Person> and want to loop out lastname seperated with a comma you could do this.

string result = string.Empty;
foreach (var person in personList) {
   result += person.LastName + ", ";
}
result = result.Substring(0, result.Length - 2);
return result;

Or you could use this handy extension method

public static string Join<T>(this IEnumerable<T> collection, Func<T, string> func, string separator)
{
  return String.Join(separator, collection.Select(func).ToArray());
}

And use it like this

personList.Join(x => x.LastName, ", ");

Which produces the same result, in this case a list of lastnames seperated by a comma.

Function to compare Files/Directories through the OS File System Info. This is useful to compare shares with local files.

Usage:

DirectoryInfo dir = new DirectoryInfo(@"C:\test\myShareDir");
Console.WriteLine(dir.IsSameFileAs(@"\\myMachineName\myShareDir"));

FileInfo file = new FileInfo(@"C:\test\myShareDir\file.txt");
Console.WriteLine(file.IsSameFileAs(@"\\myMachineName\myShareDir\file.txt"));

Code:

public static class FileExtensions
{
    struct BY_HANDLE_FILE_INFORMATION
    {
        public uint FileAttributes;
        public System.Runtime.InteropServices.ComTypes.FILETIME CreationTime;
        public System.Runtime.InteropServices.ComTypes.FILETIME LastAccessTime;
        public System.Runtime.InteropServices.ComTypes.FILETIME LastWriteTime;
        public uint VolumeSerialNumber;
        public uint FileSizeHigh;
        public uint FileSizeLow;
        public uint NumberOfLinks;
        public uint FileIndexHigh;
        public uint FileIndexLow;
    }

    //
    // CreateFile constants
    //
    const uint FILE_SHARE_READ = 0x00000001;
    const uint OPEN_EXISTING = 3;
    const uint GENERIC_READ = (0x80000000);
    const uint FILE_FLAG_BACKUP_SEMANTICS = 0x02000000;


    [DllImport("kernel32.dll", SetLastError = true)]
    static extern IntPtr CreateFile(
        string lpFileName,
        uint dwDesiredAccess,
        uint dwShareMode,
        IntPtr lpSecurityAttributes,
        uint dwCreationDisposition,
        uint dwFlagsAndAttributes,
        IntPtr hTemplateFile);

    [DllImport("kernel32.dll", SetLastError = true)]
    static extern bool GetFileInformationByHandle(IntPtr hFile, out BY_HANDLE_FILE_INFORMATION lpFileInformation);

    public static bool IsSameFileAs(this FileSystemInfo file, string path)
    {
        BY_HANDLE_FILE_INFORMATION fileInfo1, fileInfo2;
        IntPtr ptr1 = CreateFile(file.FullName, GENERIC_READ, FILE_SHARE_READ, IntPtr.Zero, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, IntPtr.Zero);
        if ((int)ptr1 == -1)
        {
            System.ComponentModel.Win32Exception e = new System.ComponentModel.Win32Exception(Marshal.GetLastWin32Error());
            throw e;
        }
        IntPtr ptr2 = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, IntPtr.Zero, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, IntPtr.Zero);
        if ((int)ptr2 == -1)
        {
            System.ComponentModel.Win32Exception e = new System.ComponentModel.Win32Exception(Marshal.GetLastWin32Error());
            throw e;
        }
        GetFileInformationByHandle(ptr1, out fileInfo1);
        GetFileInformationByHandle(ptr2, out fileInfo2);

        return ((fileInfo1.FileIndexHigh == fileInfo2.FileIndexHigh) &&
            (fileInfo1.FileIndexLow == fileInfo2.FileIndexLow));
    }
}

Pythonic methods for Dictionaries:

/// <summary>
/// If a key exists in a dictionary, return its value, 
/// otherwise return the default value for that type.
/// </summary>
public static U GetWithDefault<T, U>(this Dictionary<T, U> dict, T key)
{
    return dict.GetWithDefault(key, default(U));
}

/// <summary>
/// If a key exists in a dictionary, return its value,
/// otherwise return the provided default value.
/// </summary>
public static U GetWithDefault<T, U>(this Dictionary<T, U> dict, T key, U defaultValue)
{
    return dict.ContainsKey(key)
        ? dict[key]
        : defaultValue;
}

Useful for when you want to append a timestamp to a filename to assure uniqueness.

/// <summary>
/// Format a DateTime as a string that contains no characters
//// that are banned from filenames, such as ':'.
/// </summary>
/// <returns>YYYY-MM-DD_HH.MM.SS</returns>
public static string ToFilenameString(this DateTime dt)
{
    return dt.ToString("s").Replace(":", ".").Replace('T', '_');
}

I use this extension method usually with anonymous types to get a dictionary ala ruby

public static Dictionary<string, object> ToDictionary(this object o)
{
    var dictionary = new Dictionary<string, object>();

    foreach (var propertyInfo in o.GetType().GetProperties())
    {
        if (propertyInfo.GetIndexParameters().Length == 0)
        {
            dictionary.Add(propertyInfo.Name, propertyInfo.GetValue(o, null));
        }
    }

    return dictionary;
}

You can use it

var dummy = new { color = "#000000", width = "100%", id = "myid" };
Dictionary<string, object> dict = dummy.ToDictionary();

And with an extended method as

public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
{
    foreach (T item in source)
    {
        action(item);
    }
}

You can do it

dummy.ToDictionary().ForEach((p) => Console.Write("{0}='{1}' ", p.Key, p.Value));

Output

color='#000000' width='100%' id='myid'

static string Format( this string str,
                    , params Expression<Func<string,object>>[] args)
{
    var parameters = args.ToDictionary
                        ( e=>string.Format("{{{0}}}",e.Parameters[0].Name)
                        , e=>e.Compile()(e.Parameters[0].Name));

    var sb = new StringBuilder(str);
    foreach(var kv in parameters)
    {
        sb.Replace( kv.Key
                  , kv.Value != null ? kv.Value.ToString() : "");
    }

    return sb.ToString();
}

With the above extension you can write this:

var str = "{foo} {bar} {baz}".Format(foo=>foo, bar=>2, baz=>new object());

and you'll get "foo 2 System.Object".

Simple but nicer than "Enumerable.Range", IMHO:

/// <summary>
/// Replace "Enumerable.Range(n)" with "n.Range()":
/// </summary>
/// <param name="n">iterations</param>
/// <returns>0..n-1</returns>
public static IEnumerable<int> Range(this int n)
{
    for (int i = 0; i < n; i++)
        yield return i;
}

You all probably already know that an interesting usage for extension methods is as a kind of mixin. Some extension methods, like the XmlSerializable, pollute almost every class; and it doesn't make sense to most of them, like Thread and SqlConnection.

Some functionality should be explicitly mixed in to the classes that want to have it. I propose a new notation to this kind of type, with the M prefix.

The XmlSerializable then, is this:

public interface MXmlSerializable { }
public static class XmlSerializable {
  public static string ToXml(this MXmlSerializable self) {
    if (self == null) throw new ArgumentNullException();
    var serializer = new XmlSerializer(self.GetType());
    using (var writer = new StringWriter()) {
      serializer.Serialize(writer, self);
      return writer.GetStringBuilder().ToString();
    }
  }
  public static T FromXml<T>(string xml) where T : MXmlSerializable {
    var serializer = new XmlSerializer(typeof(T));
    return (T)serializer.Deserialize(new StringReader(xml));
  }
}

A class then mixes it in:

public class Customer : MXmlSerializable {
  public string Name { get; set; }
  public bool Preferred { get; set; }
}

And the usage is simply:

var customer = new Customer { 
  Name = "Guybrush Threepwood", 
  Preferred = true };
var xml = customer.ToXml();

If you like the idea, you can create a new namespace for useful mixins in the project. What do you think?

Oh, and by the way, I think most extension methods should explicitly test for null.

Here's a bitmap extension which can convert bitmaps to grayscale;

public static Bitmap GrayScale(this Bitmap bitmap)
{
    Bitmap newBitmap = new Bitmap(bitmap.Width, bitmap.Height);
    Graphics g = Graphics.FromImage(newBitmap);

    //the grayscale ColorMatrix
    ColorMatrix colorMatrix = new ColorMatrix(new float[][] {
            new float[] {.3f, .3f, .3f, 0, 0},
            new float[] {.59f, .59f, .59f, 0, 0},
            new float[] {.11f, .11f, .11f, 0, 0},
            new float[] {0, 0, 0, 1, 0},
            new float[] {0, 0, 0, 0, 1}
    });

    ImageAttributes attributes = new ImageAttributes();
    attributes.SetColorMatrix(colorMatrix);
    g.DrawImage(bitmap, new Rectangle(0, 0, bitmap.Width, bitmap.Height), 0, 0, bitmap.Width, bitmap.Height, GraphicsUnit.Pixel, attributes);
    g.Dispose();
    return newBitmap;
}

Sample usage:

Bitmap grayscaled = bitmap.GrayScale()

I didn't want to add anything that was already said, so here are some that I use that haven't been mentioned. (Sorry if this is too lengthy):

public static class MyExtensions
{
    public static bool IsInteger(this string input)
    {
        int temp;

        return int.TryParse(input, out temp);
    }

    public static bool IsDecimal(this string input)
    {
        decimal temp;

        return decimal.TryParse(input, out temp);
    }

    public static int ToInteger(this string input, int defaultValue)
    {
        int temp;

        return (int.TryParse(input, out temp)) ? temp : defaultValue;
    }

    public static decimal ToDecimal(this string input, decimal defaultValue)
    {
        decimal temp;

        return (decimal.TryParse(input, out temp)) ? temp : defaultValue;
    }

    public static DateTime ToFirstOfTheMonth(this DateTime input)
    {
        return input.Date.AddDays(-1 * input.Day + 1);
    }

    // Intentionally returns 0 if the target date is before the input date.
    public static int MonthsUntil(this DateTime input, DateTime targetDate)
    {
        input = input.ToFirstOfTheMonth();

        targetDate = targetDate.ToFirstOfTheMonth();

        int result = 0;

        while (input < targetDate)
        {
        input = input.AddMonths(1);
            result++;
        }

        return result;
    }

    // Used for backwards compatibility in a system built before my time.
    public static DataTable ToDataTable(this IEnumerable input)
    {
        // too much code to show here right now...
    }
}

String.As<T>, which can be used to convert a string value as some type (intended to be used primarily with primitives and types that support IConvertable. Works great with Nullable types and even Enums!

public static partial class StringExtensions
{
    /// <summary>
    /// Converts the string to the specified type, using the default value configured for the type.
    /// </summary>
    /// <typeparam name="T">Type the string will be converted to. The type must implement IConvertable.</typeparam>
    /// <param name="original">The original string.</param>
    /// <returns>The converted value.</returns>
    public static T As<T>(this String original)
    {
        return As(original, CultureInfo.CurrentCulture,
                  default(T));
    }

    /// <summary>
    /// Converts the string to the specified type, using the default value configured for the type.
    /// </summary>
    /// <typeparam name="T">Type the string will be converted to.</typeparam>
    /// <param name="original">The original string.</param>
    /// <param name="defaultValue">The default value to use in case the original string is null or empty, or can't be converted.</param>
    /// <returns>The converted value.</returns>
    public static T As<T>(this String original, T defaultValue)
    {
        return As(original, CultureInfo.CurrentCulture, defaultValue);
    }

    /// <summary>
    /// Converts the string to the specified type, using the default value configured for the type.
    /// </summary>
    /// <typeparam name="T">Type the string will be converted to.</typeparam>
    /// <param name="original">The original string.</param>
    /// <param name="provider">Format provider used during the type conversion.</param>
    /// <returns>The converted value.</returns>
    public static T As<T>(this String original, IFormatProvider provider)
    {
        return As(original, provider, default(T));
    }

    /// <summary>
    /// Converts the string to the specified type.
    /// </summary>
    /// <typeparam name="T">Type the string will be converted to.</typeparam>
    /// <param name="original">The original string.</param>
    /// <param name="provider">Format provider used during the type conversion.</param>
    /// <param name="defaultValue">The default value to use in case the original string is null or empty, or can't be converted.</param>
    /// <returns>The converted value.</returns>
    /// <remarks>
    /// If an error occurs while converting the specified value to the requested type, the exception is caught and the default is returned. It is strongly recommended you
    /// do NOT use this method if it is important that conversion failures are not swallowed up.
    ///
    /// This method is intended to be used to convert string values to primatives, not for parsing, converting, or deserializing complex types.
    /// </remarks>
    public static T As<T>(this String original, IFormatProvider provider,
                          T defaultValue)
    {
        T result;
        Type type = typeof (T);

        if (String.IsNullOrEmpty(original)) result = defaultValue;
        else
        {
            // need to get the underlying type if T is Nullable<>.

            if (type.IsNullableType())
            {
                type = Nullable.GetUnderlyingType(type);
            }

            try
            {
                // ChangeType doesn't work properly on Enums
                result = type.IsEnum
                             ? (T) Enum.Parse(type, original, true)
                             : (T) Convert.ChangeType(original, type, provider);
            }
            catch // HACK: what can we do to minimize or avoid raising exceptions as part of normal operation? custom string parsing (regex?) for well-known types? it would be best to know if you can convert to the desired type before you attempt to do so.
            {
                result = defaultValue;
            }
        }

        return result;
    }
}

This relies on another simple extension for Type:

/// <summary>
/// Extension methods for <see cref="Type"/>.
/// </summary>
public static class TypeExtensions
{
    /// <summary>
    /// Returns whether or not the specified type is <see cref="Nullable{T}"/>.
    /// </summary>
    /// <param name="type">A <see cref="Type"/>.</param>
    /// <returns>True if the specified type is <see cref="Nullable{T}"/>; otherwise, false.</returns>
    /// <remarks>Use <see cref="Nullable.GetUnderlyingType"/> to access the underlying type.</remarks>
    public static bool IsNullableType(this Type type)
    {
        if (type == null) throw new ArgumentNullException("type");

        return type.IsGenericType && type.GetGenericTypeDefinition().Equals(typeof (Nullable<>));
    }
}

Usage:

var someInt = "1".As<int>();
var someIntDefault = "bad value".As(1); // "bad value" won't convert, so the default value 1 is returned.
var someEnum = "Sunday".As<DayOfWeek>();
someEnum = "0".As<DayOfWeek>(); // returns Sunday
var someNullableEnum = "".As<DayOfWeek?>(null); // returns a null value since "" can't be converted

I created a nice Each extension that has the same behavior as jQuery's each function.

It allows something like below, where you can get the index of the current value and break out of the loop by returning false:

new[] { "first", "second", "third" }.Each((value, index) =>
{
    if (value.Contains("d"))
        return false;
    Console.Write(value);
    return true;
});

Here's the code

/// <summary>
/// Generic iterator function that is useful to replace a foreach loop with at your discretion.  A provided action is performed on each element.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="source"></param>
/// <param name="action">Function that takes in the current value in the sequence. 
/// <returns></returns>
public static IEnumerable<T> Each<T>(this IEnumerable<T> source, Action<T> action)
{
    return source.Each((value, index) =>
    {
        action(value);
        return true;
    });
}


/// <summary>
/// Generic iterator function that is useful to replace a foreach loop with at your discretion.  A provided action is performed on each element.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="source"></param>
/// <param name="action">Function that takes in the current value and its index in the sequence.  
/// <returns></returns>
public static IEnumerable<T> Each<T>(this IEnumerable<T> source, Action<T, int> action)
{
    return source.Each((value, index) =>
    {
        action(value, index);
        return true;
    });
}

/// <summary>
/// Generic iterator function that is useful to replace a foreach loop with at your discretion.  A provided action is performed on each element.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="source"></param>
/// <param name="action">Function that takes in the current value in the sequence.  Returns a value indicating whether the iteration should continue.  So return false if you don't want to iterate anymore.</param>
/// <returns></returns>
public static IEnumerable<T> Each<T>(this IEnumerable<T> source, Func<T, bool> action)
{
    return source.Each((value, index) =>
    {
        return action(value);
    });
}

/// <summary>
/// Generic iterator function that is useful to replace a foreach loop with at your discretion.  A provided action is performed on each element.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="source"></param>
/// <param name="action">Function that takes in the current value and its index in the sequence.  Returns a value indicating whether the iteration should continue.  So return false if you don't want to iterate anymore.</param>
/// <returns></returns>
public static IEnumerable<T> Each<T>(this IEnumerable<T> source, Func<T, int, bool> action)
{
    if (source == null)
        return source;

    int index = 0;
    foreach (var sourceItem in source)
    {
        if (!action(sourceItem, index))
            break;
        index++;
    }
    return source;
}

http://www.codeplex.com/linqext/

I use these in my web projects, mainly with MVC. I have a handful of these written for the ViewData and TempData

/// <summary>
/// Checks the Request.QueryString for the specified value and returns it, if none 
/// is found then the default value is returned instead
/// </summary>
public static T QueryValue<T>(this HtmlHelper helper, string param, T defaultValue) {
    object value = HttpContext.Current.Request.QueryString[param] as object;
    if (value == null) { return defaultValue; }
    try {
        return (T)Convert.ChangeType(value, typeof(T));
    } catch (Exception) {
        return defaultValue;
    }
}

That way I can write something like...

<% if (Html.QueryValue("login", false)) { %>
    <div>Welcome Back!</div>

<% } else { %>
    <%-- Render the control or something --%>

<% } %>

The Substring method on the string class has always felt inadequate to me. Usually when you do a substring, you know the character(s) from where you want to start, and the charachter(s) where you want to end. Thus, I've always felt that have to specify length as the second parameter is stupid. Therefore, I've written my own extension methods. One that takes a startIndex and an endIndex. And one, that takes a startText (string) and endText (string) so you can just specify the text from where to start the substring, and the text for where to end it.

NOTE: I couldn't name the method Substring as in .NET because my first overload takes the same parameter types as one of the .NET overloads. Therefore I named them Subsetstring. Feel free to add to the CodePlex...

public static class StringExtensions
{
    /// <summary>
    /// Returns a Subset string starting at the specified start index and ending and the specified end
    /// index.
    /// </summary>
    /// <param name="s">The string to retrieve the subset from.</param>
    /// <param name="startIndex">The specified start index for the subset.</param>
    /// <param name="endIndex">The specified end index for the subset.</param>
    /// <returns>A Subset string starting at the specified start index and ending and the specified end
    /// index.</returns>
    public static string Subsetstring(this string s, int startIndex, int endIndex)
    {
        if (startIndex > endIndex)
        {
            throw new InvalidOperationException("End Index must be after Start Index.");
        }

        if (startIndex < 0)
        {
            throw new InvalidOperationException("Start Index must be a positive number.");
        }

        if(endIndex <0)
        {
            throw new InvalidOperationException("End Index must be a positive number.");
        }

        return s.Substring(startIndex, (endIndex - startIndex));
    }

    /// <summary>
    /// Finds the specified Start Text and the End Text in this string instance, and returns a string
    /// containing all the text starting from startText, to the begining of endText. (endText is not
    /// included.)
    /// </summary>
    /// <param name="s">The string to retrieve the subset from.</param>
    /// <param name="startText">The Start Text to begin the Subset from.</param>
    /// <param name="endText">The End Text to where the Subset goes to.</param>
    /// <param name="ignoreCase">Whether or not to ignore case when comparing startText/endText to the string.</param>
    /// <returns>A string containing all the text starting from startText, to the begining of endText.</returns>
    public static string Subsetstring(this string s, string startText, string endText, bool ignoreCase)
    {
        if (string.IsNullOrEmpty(startText) || string.IsNullOrEmpty(endText))
        {
            throw new ArgumentException("Start Text and End Text cannot be empty.");
        }
        string temp = s;
        if (ignoreCase)
        {
            temp = s.ToUpperInvariant();
            startText = startText.ToUpperInvariant();
            endText = endText.ToUpperInvariant();
        }
        int start = temp.IndexOf(startText);
        int end = temp.IndexOf(endText, start);
        return Subsetstring(s, start, end);
    }
}

Usage:

string s = "This is a tester for my cool extension method!!";
       s = s.Subsetstring("tester", "cool",true);

Output: "tester for my "

While working with MVC and having lots of if statements where i only care about either true or false, and printing null, or string.Empty in the other case, I came up with:

public static TResult WhenTrue<TResult>(this Boolean value, Func<TResult> expression)
{
    return value ? expression() : default(TResult);
}

public static TResult WhenTrue<TResult>(this Boolean value, TResult content)
{
    return value ? content : default(TResult);
}

public static TResult WhenFalse<TResult>(this Boolean value, Func<TResult> expression)
{
    return !value ? expression() : default(TResult);
}

public static TResult WhenFalse<TResult>(this Boolean value, TResult content)
{
    return !value ? content : default(TResult);
}

It allows me to change <%= (someBool) ? "print y" : string.Empty %> into <%= someBool.WhenTrue("print y") %> .

I only use it in my Views where I mix code and HTML, in code files writing the "longer" version is more clear IMHO.

An easier way to load default settings from a collection (in real life I use it to populate the settings from any source, including the command line, ClickOnce URL parameters etc.):

public static void LoadFrom(this ApplicationSettingsBase settings, NameValueCollection configuration)
{
    if (configuration != null)
        foreach (string key in configuration.AllKeys)
            if (!String.IsNullOrEmpty(key))
                try
                {
                    settings[key] = configuration.Get(key);
                }
                catch (SettingsPropertyNotFoundException)
                {
                  // handle bad arguments as you wish
                }
}

Example:

Settings.Default.LoadFrom(new NameValueCollection() { { "Setting1", "Value1" }, { "Setting2", "Value2" } });

Comes in useful for unit testing:

public static IList<T> Clone<T>(this IList<T> list) where T : ICloneable
{
    var ret = new List<T>(list.Count);
    foreach (var item in list)
        ret.Add((T)item.Clone());

    // done
    return ret;
}

A series of these like TWith2Sugars, alternate shortened syntax:

public static long? ToNullableInt64(this string val)
{
    long ret;
    return Int64.TryParse(val, out ret) ? ret : new long?();
}

And finally this - is there something already in the BCL that does the following?

public static void Split<T>(this T[] array, 
    Func<T,bool> determinator, 
    IList<T> onTrue, 
    IList<T> onFalse)
{
    if (onTrue == null)
        onTrue = new List<T>();
    else
        onTrue.Clear();

    if (onFalse == null)
        onFalse = new List<T>();
    else
        onFalse.Clear();

    if (determinator == null)
        return;

    foreach (var item in array)
    {
        if (determinator(item))
            onTrue.Add(item);
        else
            onFalse.Add(item);
    }
}

I'm using this one quite a lot...

Original code:

if (guid != Guid.Empty) return guid;
else return Guid.NewGuid();

New code:

return guid.NewGuidIfEmpty();

Extension method:

public static Guid NewGuidIfEmpty(this Guid uuid)
{
    return (uuid != Guid.Empty ? uuid : Guid.NewGuid());
}

These extension methods are pretty useful for me when parsing form input before putting into the database

public static int? ToInt(this string input) 
{
    int val;
    if (int.TryParse(input, out val))
        return val;
    return null;
}

public static DateTime? ToDate(this string input)
{
    DateTime val;
    if (DateTime.TryParse(input, out val))
        return val;
    return null;
}

public static decimal? ToDecimal(this string input)
{
    decimal val;
    if (decimal.TryParse(input, out val))
        return val;
    return null;
}

I like these methods for dealing with enums that have the Flags attribute set:

public static bool AnyOf(this object mask, object flags)
{
    return ((int)mask & (int)flags) != 0;
}
public static bool AllOf(this object mask, object flags)
{
    return ((int)mask & (int)flags) == (int)flags;
}
public static object SetOn(this object mask, object flags)
{
    return (int)mask | (int)flags;
}
etc.

Example usage:

var options = SomeOptions.OptionA;
options = options.SetOn(OptionB);
options = options.SetOn(OptionC);

if (options.AnyOf(SomeOptions.OptionA | SomeOptions.OptionB))
{
etc.

The original methods were from this article: http://www.codeproject.com/KB/cs/masksandflags.aspx?display=Print I just converted them to extension methods.

The one problem with them though is that the parameters of object type, which means that all objects end up being extended with these methods, whereas ideally they should only apply to enums.

Update As per the comments, you can get around the "signature pollution", at the expense of performance, like this:

public static bool AnyOf(this Enum mask, object flags)
{
    return (Convert.ToInt642(mask) & (int)flags) != 0;
}

GetMemberName allows to get the string with the name of a member with compile time safety.

public static string GetMemberName<T, TResult>(
    this T anyObject, 
    Expression<Func<T, TResult>> expression)
{
    return ((MemberExpression)expression.Body).Member.Name;
}

Usage:

"blah".GetMemberName(x => x.Length); // returns "Length"

It comes together with a non-extension static method if you don't have a instance:

public static string GetMemberName<T, TReturn>(
    Expression<Func<T, TReturn>> expression)
    where T : class
{
    return ((MemberExpression)expression.Body).Member.Name;
}

But the call doesn't look as pretty of course:

ReflectionUtility.GetMemberName((string) s => s.Length); // returns "Length"

You can put it on Codeplex if you want.

I have implemented a package of extension methods (available at http://foop.codeplex.com/) and some of my daily used ones are:

// the most beloved extension method for me is Pipe:
<%= variable.Pipe(x => this.SomeFunction(x)).Pipe(y =>
{
    ...;
    return this.SomeOtherFunction(y);
}) %>

var d = 28.December(2009); // some extension methods for creating DateTime
DateTime justDatePart = d.JustDate();
TimeSpan justTimePart = d.JustTime();
var nextTime = d.Add(5.Hours());

using(StreamReader reader = new StreamReader("lines-of-data-file-for-example")) {
    ...
    // for reading streams line by line and usable in LINQ
    var query = from line in reader.Lines(); 
                where line.Contains(_today)
                select new { Parts = PartsOf(line), Time = _now };
}

500.Sleep();

XmlSerialize and XmlDeserialize

IsNull and IsNotNull

IfTrue, IfFalse and Iff:
true.IfTrue(() => Console.WriteLine("it is true then!");

IfNull and IfNotNull

I thought I'd seen this somewhere before, but couldn't find it suggested anywhere here. MS has a TryGetValue function on the IDictionary interface, but it returns a bool and gives the value in an out parameter, so here's a simpler, cleaner implementation:

public static TVal GetValueOrDefault<TKey, TVal>(this IDictionary<TKey, TVal> d, TKey key) {
  if (d.ContainsKey(key))
    return d[key];
  return default(TVal);
}

Here's another control-extension i've been using though i don't know if it's posted here before.

public static class ControlExtensions
{
    public static void DoubleBuffer(this Control control) 
    {
        // http://stackoverflow.com/questions/76993/how-to-double-buffer-net-controls-on-a-form/77233#77233
        // Taxes: Remote Desktop Connection and painting: http://blogs.msdn.com/oldnewthing/archive/2006/01/03/508694.aspx

        if (System.Windows.Forms.SystemInformation.TerminalServerSession) return;
        System.Reflection.PropertyInfo dbProp = typeof(System.Windows.Forms.Control).GetProperty("DoubleBuffered", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
        dbProp.SetValue(control, true, null);
    }
}

usage:

this.someControl.DoubleBuffer();

I like these NUnit Assert extensions: http://svn.caffeine-it.com/openrasta/trunk/src/Rasta.Testing/AssertExtensions.cs

Here is another ThrowIfNull implementation:

[ThreadStatic]
private static string lastMethodName = null;

[ThreadStatic]
private static int lastParamIndex = 0;

[MethodImpl(MethodImplOptions.NoInlining)]
public static void ThrowIfNull<T>(this T parameter)
{
    var currentStackFrame = new StackFrame(1);
    var props = currentStackFrame.GetMethod().GetParameters();

    if (!String.IsNullOrEmpty(lastMethodName)) {
        if (currentStackFrame.GetMethod().Name != lastMethodName) {
            lastParamIndex = 0;
        } else if (lastParamIndex >= props.Length - 1) {
            lastParamIndex = 0;
        } else {
            lastParamIndex++;
        }
    } else {
        lastParamIndex = 0;
    }

    if (!typeof(T).IsValueType) {
        for (int i = lastParamIndex; i &lt; props.Length; i++) {
            if (props[i].ParameterType.IsValueType) {
                lastParamIndex++;
            } else {
                break;
            }
        }
    }

    if (parameter == null) {
        string paramName = props[lastParamIndex].Name;
        throw new ArgumentNullException(paramName);
    }

    lastMethodName = currentStackFrame.GetMethod().Name;
}

It's not as efficient as the other impementations, but has cleaner usage:

public void Foo()
{
    Bar(1, 2, "Hello", "World"); //no exception
    Bar(1, 2, "Hello", null); //exception
    Bar(1, 2, null, "World"); //exception
}

public void Bar(int x, int y, string someString1, string someString2)
{
    //will also work with comments removed
    //x.ThrowIfNull();
    //y.ThrowIfNull();
    someString1.ThrowIfNull();
    someString2.ThrowIfNull();

    //Do something incredibly useful here!
}

Changing the parameters to int? will also work.

-bill

Some extensions for working with lists:

/// <summary>
/// Wrap an object in a list
/// </summary>
public static IList<T> WrapInList<T>(this T item)
{
    List<T> result = new List<T>();
    result.Add(item);
    return result;
}

use eg:

myList = someObject.InList();

To make an IEnumerable that contains items from one or more sources, in order to make IEnumerable work more like lists. This may not be a good idea for high-performance code but useful for making tests:

public static IEnumerable<T> Append<T>(this IEnumerable<T> enumerable, T newItem)
{
    foreach (T item in enumerable)
    {
        yield return item;
    }

    yield return newItem;
}

public static IEnumerable<T> Append<T>(this IEnumerable<T> enumerable, params T[] newItems)
{
    foreach (T item in enumerable)
    {
        yield return item;
    }

    foreach (T newItem in newItems)
    {
        yield return newItem;
    }
}

use e.g.

someEnumeration = someEnumeration.Append(newItem);

Other variations of this are possible - e.g.

someEnumeration = someEnumeration.Append(otherEnumeration);

If you are cloning items, you may also want to clone lists of them:

public static IList<T> Clone<T>(this IEnumerable<T> source) where T: ICloneable
{
    List<T> result = new List<T>();

    foreach (T item in source)
    {
        result.Add((T)item.Clone());
    }

    return result;
}

When I am working with ObservableCollection<T>, I generally extend it with an AddRange method. Other answers here give implementations of this.

You may put this code in the Codeplex project if you want.

This is an extension method for the ASP.Net MVC action link helper method that allows it to use the controller's authorize attributes to decide if the link should be enabled, disabled or hidden from the current user's view. I saves you from having to enclose your restricted actions in "if" clauses that check for user membership in all the views. Thanks to Maarten Balliauw for the idea and the code bits that showed me the way :)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Security.Principal;
using System.Web.Routing;
using System.Web.Mvc;
using System.Collections;
using System.Reflection;
namespace System.Web.Mvc.Html
{
    public static class HtmlHelperExtensions
    {

        /// <summary>
        /// Shows or hides an action link based on the user's membership status
        /// and the controller's authorize attributes
        /// </summary>
        /// <param name="linkText">The link text.</param>
        /// <param name="action">The controller action name.</param>
        /// <param name="controller">The controller name.</param>
        /// <returns></returns>
        public static string SecurityTrimmedActionLink(
            this HtmlHelper htmlHelper,
            string linkText,
            string action,
            string controller)
        {
            return SecurityTrimmedActionLink(htmlHelper, linkText, action, controller, false, null);
        }

        /// <summary>
        /// Enables, disables or hides an action link based on the user's membership status
        /// and the controller's authorize attributes
        /// </summary>
        /// <param name="linkText">The link text.</param>
        /// <param name="action">The action name.</param>
        /// <param name="controller">The controller name.</param>
        /// <param name="showDisabled">if set to <c>true</c> [show link as disabled - 
        /// using a span tag instead of an anchor tag ].</param>
        /// <param name="disabledAttributeText">Use this to add attributes to the disabled
        /// span tag.</param>
        /// <returns></returns>
        public static string SecurityTrimmedActionLink(
            this HtmlHelper htmlHelper, 
            string linkText, 
            string action, 
            string controller, 
            bool showDisabled, 
            string disabledAttributeText)
        {
            if (IsAccessibleToUser(action, controller, HttpContext.Current ))
            {
                return htmlHelper.ActionLink(linkText, action, controller);
            }
            else
            {
                return showDisabled ? 
                    String.Format(
                        "<span{1}>{0}</span>", 
                        linkText, 
                        disabledAttributeText==null?"":" "+disabledAttributeText
                        ) : "";
            }
        }

        private static IController GetControllerInstance(string controllerName)
        {
            Assembly assembly = Assembly.GetExecutingAssembly();
            Type controllerType = GetControllerType(controllerName);
            return (IController)Activator.CreateInstance(controllerType);
        }

        private static ArrayList GetControllerAttributes(string controllerName, HttpContext context)
        {
            if (context.Cache[controllerName + "_ControllerAttributes"] == null)
            {
                var controller = GetControllerInstance(controllerName);

                context.Cache.Add(
                    controllerName + "_ControllerAttributes",
                    new ArrayList(controller.GetType().GetCustomAttributes(typeof(AuthorizeAttribute), true)),
                    null,
                    Caching.Cache.NoAbsoluteExpiration,
                    Caching.Cache.NoSlidingExpiration,
                    Caching.CacheItemPriority.Default,
                    null);

            }
            return (ArrayList)context.Cache[controllerName + "_ControllerAttributes"];

        }

        private static ArrayList GetMethodAttributes(string controllerName, string actionName, HttpContext context)
        {
            if (context.Cache[controllerName + "_" + actionName + "_ActionAttributes"] == null)
            {
                ArrayList actionAttrs = new ArrayList();
                var controller = GetControllerInstance(controllerName);
                MethodInfo[] methods = controller.GetType().GetMethods();

                foreach (MethodInfo method in methods)
                {
                    object[] attributes = method.GetCustomAttributes(typeof(ActionNameAttribute), true);

                    if ((attributes.Length == 0 && method.Name == actionName)
                        ||
                        (attributes.Length > 0 && ((ActionNameAttribute)attributes[0]).Name == actionName))
                    {
                        actionAttrs.AddRange(method.GetCustomAttributes(typeof(AuthorizeAttribute), true));
                    }
                }

                context.Cache.Add(
                    controllerName + "_" + actionName + "_ActionAttributes",
                    actionAttrs,
                    null,
                    Caching.Cache.NoAbsoluteExpiration,
                    Caching.Cache.NoSlidingExpiration,
                    Caching.CacheItemPriority.Default,
                    null);

            }

            return (ArrayList)context.Cache[controllerName + "_" + actionName+ "_ActionAttributes"]; 
        }

        public static bool IsAccessibleToUser(string actionToAuthorize, string controllerToAuthorize, HttpContext context)
        {
            IPrincipal principal = context.User;

            //cache the attribute list for both controller class and it's methods

            ArrayList controllerAttributes = GetControllerAttributes(controllerToAuthorize, context);

            ArrayList actionAttributes = GetMethodAttributes(controllerToAuthorize, actionToAuthorize, context);                        

            if (controllerAttributes.Count == 0 && actionAttributes.Count == 0)
                return true;

            string roles = "";
            string users = "";
            if (controllerAttributes.Count > 0)
            {
                AuthorizeAttribute attribute = controllerAttributes[0] as AuthorizeAttribute;
                roles += attribute.Roles;
                users += attribute.Users;
            }
            if (actionAttributes.Count > 0)
            {
                AuthorizeAttribute attribute = actionAttributes[0] as AuthorizeAttribute;
                roles += attribute.Roles;
                users += attribute.Users;
            }

            if (string.IsNullOrEmpty(roles) && string.IsNullOrEmpty(users) && principal.Identity.IsAuthenticated)
                return true;

            string[] roleArray = roles.Split(',');
            string[] usersArray = users.Split(',');
            foreach (string role in roleArray)
            {
                if (role == "*" || principal.IsInRole(role))
                    return true;
            }
            foreach (string user in usersArray)
            {
                if (user == "*" && (principal.Identity.Name == user))
                    return true;
            }
            return false;
        }

        private static Type GetControllerType(string controllerName)
        {
            Assembly assembly = Assembly.GetExecutingAssembly();
            foreach (Type type in assembly.GetTypes())
            {
                if (
                    type.BaseType!=null 
                    && type.BaseType.Name == "Controller" 
                    && (type.Name.ToUpper() == (controllerName.ToUpper() + "Controller".ToUpper())))
                {
                    return type;
                }
            }
            return null;
        }

    }
}

Equivalent to Python's Join method:

/// <summary>
/// same as python 'join'
/// </summary>
/// <typeparam name="T">list type</typeparam>
/// <param name="separator">string separator </param>
/// <param name="list">list of objects to be ToString'd</param>
/// <returns>a concatenated list interleaved with separators</returns>
static public string Join<T>(this string separator, IEnumerable<T> list)
{
    var sb = new StringBuilder();
    bool first = true;

    foreach (T v in list)
    {
        if (!first)
            sb.Append(separator);
        first = false;

        if (v != null)
            sb.Append(v.ToString());
    }

    return sb.ToString();
}

Convert any string to type Int32

// Calls the underlying int.TryParse method to convert a string
// representation of a number to its 32-bit signed integer equivalent.
// Returns Zero if conversion fails. 
public static int ToInt32(this string s)
{
    int retInt;
    int.TryParse(s, out retInt);
    return retInt;
}

SAMPLE USE:
string s = "999";
int i = s.ToInt32();

A generic Try:

class Program
{
    static void Main(string[] args)
    {
        var z = 0;
        var a = 0.AsDefaultFor(() => 1 / z);
        Console.WriteLine(a);
        Console.ReadLine();
    }
}

public static class TryExtensions
{
    public static T AsDefaultFor<T>(this T @this, Func<T> operation)
    {
        try
        {
            return operation();
        }
        catch
        {
            return @this;
        }
    }
}

Put it up on the CodePlex project if you want.

The WhereIf() Method

var query = dc.Reviewer 
    .Where(r => r.FacilityID == facilityID) 
    .WhereIf(CheckBoxActive.Checked, r => r.IsActive); 

public static IEnumerable<TSource> WhereIf<TSource>(
    this IEnumerable<TSource> source,
    bool condition, Func<TSource, bool> predicate) 
{ 
    if (condition) 
        return source.Where(predicate); 
    else 
        return source; 
}

public static IQueryable<TSource> WhereIf<TSource>(
    this IQueryable<TSource> source,
    bool condition, Expression<Func<TSource, bool>> predicate) 
{ 
    if (condition) 
        return source.Where(predicate); 
    else 
        return source; 
}

I also added overloads for the index predicate in the Where() extension method. For more fun, add a flavor that includes an additional 'else' predicate.

Inline Conversions: I like this little pattern. Completed it for Boolean, Double and DateTime. Designed to follow the C# is and as operators.

public static Int32? AsInt32(this string s)
{
    Int32 value;
    if (Int32.TryParse(s, out value))
        return value;

    return null;
}

public static bool IsInt32(this string s)
{
    return s.AsInt32().HasValue;
}

public static Int32 ToInt32(this string s)
{
    return Int32.Parse(s);
{

Several times I found myself wanting something like, I think, Groovy's "Safe navigation".

From http://groovy.codehaus.org/Statements:

If you are walking a complex object graph and don't want to have NullPointerExceptions thrown you can use the ?. operator rather than . to perform your navigation.

def foo = null def bar = foo?.something?.myMethod() assert bar == null

So, do you think is a good idea adding an extension method for it? Something like:

obj.SafelyNavigate(x => x.SomeProperty.MaybeAMethod().AnotherProperty);

I think it would be nice even if it can also bring some trouble.

If you think it's a good idea:

• What would you think it should happen for value types?, return default? throw?, disable it by generic constraint?.
• Swallowing NullReferenceException to implement it would be too risky?, What do you propose?, Walking the expression tree executing every call or member access seems difficult and kind of overkill (if at all possible) doesn't it?.

Maybe it's just a bad idea :D, but I see it like something that can be useful if done right. If there's nothing like it and you think it holds some value, I may give it a shot and edit the answer afterwards.

For ASP.NET, I use these extensions to HttpSessionState to load objects in session. It allows you to load session objects in a clean manner, and will create and initialize them if they do not exist. I use the two extension methods like so:

private bool CreateMode;
private MyClass SomeClass;

protected override void OnInit (EventArgs e)
{
    CreateMode = Session.GetSessionValue<bool> ("someKey1", () => true);
    SomeClass = Session.GetSessionClass<MyClass> ("someKey2", () => new MyClass () 
    { 
       MyProperty = 123 
    });
}

Here are the extension classes:

public static class SessionExtensions    
{
    public delegate object UponCreate ();
    public static T GetSessionClass<T> (this HttpSessionState session, 
       string key, UponCreate uponCreate) where T : class
    {
        if (null == session[key])
        {
            var item = uponCreate () as T;
            session[key] = item;
            return item;
        }
        return session[key] as T;
    }
    public static T GetSessionValue<T> (this HttpSessionState session, 
       string key, UponCreate uponCreate) where T : struct
    {
        if (null == session[key])
        {
            var item = uponCreate();
            session[key] = item;
            return (T)item;
        }
        return (T)session[key];
    }
}

Hate this kind of code?

CloneableClass cc1 = new CloneableClass ();
CloneableClass cc2 = null;
CloneableClass cc3 = null;

cc3 = (CloneableClass) cc1.Clone (); // this is ok
cc3 = cc2.Clone (); // this throws null ref exception
// code to handle both cases
cc3 = cc1 != null ? (CloneableClass) cc1.Clone () : null;

It's a bit clunky, so I replace it with this extension, which I call CloneOrNull -

public static T CloneOrNull<T> (this T self) where T : class, ICloneable
{
    if (self == null) return null;
    return (T) self.Clone ();
}

Usage is like:

CloneableClass cc1 = new CloneableClass ();
CloneableClass cc2 = null;
CloneableClass cc3 = null;

cc3 = cc1.CloneOrNull (); // clone of cc1
cc3 = cc2.CloneOrNull (); // null
// look mom, no casts!

Please feel free to use this anywhere!

Some tools to IEnumerable: ToString(Format), ToString(Function) and Join(Separator).

For example:

var names = new[] { "Wagner", "Francine", "Arthur", "Bernardo" };

names.ToString("Name: {0}\n");
// Name: Wagner
// Name: Francine
// Name: Arthur
// Name: Bernardo

names.ToString(name => name.Length > 6 ? String.Format("{0} ", name) : String.Empty);
// Francine Bernardo

names.Join(" - ");
// Wagner - Francine - Arthur - Bernardo

Extensions:

public static string ToString<T>(this IEnumerable<T> self, string format)
{
    return self.ToString(i => String.Format(format, i));
}

public static string ToString<T>(this IEnumerable<T> self, Func<T, object> function)
{
    var result = new StringBuilder();

    foreach (var item in self) result.Append(function(item));

    return result.ToString();
}

public static string Join<T>(this IEnumerable<T> self, string separator)
{
    return String.Join(separator, values: self.ToArray());
}

• For adding multiple elements to a collection that doesn't have AddRange, e.g., collection.Add(item1, item2, itemN);

  static void Add<T>(this ICollection<T> coll, params T[] items)
   { foreach (var item in items) coll.Add(item);
   }
  

• The following is like string.Format() but with custom string representation of arguments, e.g., "{0} {1} {2}".Format<Custom>(c=>c.Name,"string",new object(),new Custom()) results in "string {System.Object} Custom1Name"

  static string Format<T>(  this string format
                          , Func<T,object> select
                          , params object[] args)
   { for(int i=0; i < args.Length; ++i)
      { var x = args[i] as T;
        if (x != null) args[i] = select(x);
      }
     return string.Format(format, args);
   }
  

// Values ordered true/false
// True/false values separated by a capital letter
// Only two values allowed
// ---------------------------
// Limited, but could be useful
public enum BooleanFormat
{
    OneZero,
    YN,
    YesNo,
    TF,
    TrueFalse,
    PassFail,
    YepNope
}

public static class BooleanExtension
{
    /// <summary>
    /// Converts the boolean value of this instance to the specified string value. 
    /// </summary>
    private static string ToString(this bool value, string passValue, string failValue)
    {
        return value ? passValue : failValue;
    }

    /// <summary>
    /// Converts the boolean value of this instance to a string. 
    /// </summary>
    /// <param name="booleanFormat">A BooleanFormat value. 
    /// Example: BooleanFormat.PassFail would return "Pass" if true and "Fail" if false.</param>
    /// <returns>Boolean formatted string</returns>
    public static string ToString(this bool value, BooleanFormat booleanFormat)
    {
        string booleanFormatString = Enum.GetName(booleanFormat.GetType(), booleanFormat);
        return ParseBooleanString(value, booleanFormatString);      
    }

    // Parses boolean format strings, not optimized
    private static string ParseBooleanString(bool value, string booleanFormatString)
    {
        StringBuilder trueString = new StringBuilder();
        StringBuilder falseString = new StringBuilder();

        int charCount = booleanFormatString.Length;

        bool isTrueString = true;

        for (int i = 0; i != charCount; i++)
        {
            if (char.IsUpper(booleanFormatString[i]) && i != 0)
                isTrueString = false;

            if (isTrueString)
                trueString.Append(booleanFormatString[i]);
            else
                falseString.Append(booleanFormatString[i]);
        }

        return (value == true ? trueString.ToString() : falseString.ToString());
    }

Perhaps the most useful extension methods I've written and used are here:

http://www.codeproject.com/KB/cs/fun-with-cs-extensions.aspx?msg=2838918#xx2838918xx

// Checks for an empty collection, and sends the value set in the default constructor for the desired field
public static TResult MinGuarded<T, TResult>(this IEnumerable<T> items, Func<T, TResult> expression) where T : new() {
    if(items.IsEmpty()) {
        return (new List<T> { new T() }).Min(expression);
    }
    return items.Min(expression);
}

// Checks for an empty collection, and sends the value set in the default constructor for the desired field
public static TResult MaxGuarded<T, TResult>(this IEnumerable<T> items, Func<T, TResult> expression) where T : new() {
    if(items.IsEmpty()) {
        return (new List<T> { new T() }).Max(expression);
    }
    return items.Max(expression);
}

I am not sure if there is a better way to do this, but this extension is very helpful whenever I want to have control over the default values of fields in my object.
For instance, if I want to control the value of a DateTime and want to be set as per my business logic, then I can do so in the default constructor. Otherwise, it comes out to be DateTime.MinDate.

public static class StringHelper
{
    public static String F(this String str, params object[] args)
    {
        return String.Format(str, args);
    }
}

Using like:

"Say {0}".F("Hello");

public static class DictionaryExtensions
{
    public static Nullable<TValue> GetValueOrNull<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key)
        where TValue : struct
    {
        TValue result;
        if (dictionary.TryGetValue(key, out result))
            return result;
        else
            return null;
    }
}

Free to use, just mention my name (Janko Röbisch) in the code.

Ive created a extension method to select an item in a dropdown in ASP.NET.

Below is the code

 public static class Utilities
{
    public enum DropDownListSelectionType
    {
        ByValue,
        ByText
    }

    public static void SelectItem(this  System.Web.UI.WebControls.DropDownList drp, string selectedValue, DropDownListSelectionType type)
    {
        drp.ClearSelection();
        System.Web.UI.WebControls.ListItem li;
        if (type == DropDownListSelectionType.ByValue)
            li = drp.Items.FindByValue(selectedValue.Trim());
        else
            li = drp.Items.FindByText(selectedValue.Trim());
        if (li != null)
            li.Selected = true;
    }}

This method can be called by the following lines of code to either select by text

DropDownList1.SelectItem("ABCD", Utilities.DropDownListSelectionType.ByText);

or select by value

DropDownList1.SelectItem("11", Utilities.DropDownListSelectionType.ByValue);

The above code doesnt select anything if it cant find the text/value passed in.

And here's the control-invoke extension i'm using regularly;

public static class InvokeExtensions
{
    public static void InvokeHandler(this Control control, MethodInvoker del) // Sync. control-invoke extension.
    {
        if (control.InvokeRequired)
        {
            control.Invoke(del);
            return; 
        }
        del(); // run the actual code.
    }

    public static void AsyncInvokeHandler(this Control control, MethodInvoker del) // Async. control-invoke extension.
    {
        if (control.InvokeRequired)
        {
            control.BeginInvoke(del);
            return; 
        }
        del(); // run the actual code.
    }
}

sample;

this.TreeView.AsyncInvokeHandler(() =>
        {
            this.Text = 'xyz'
        });

which allows cross-thread gui-updates.

I use the following extensions to extend all collections (maybe someone find these useful):

/// <summary>
/// Collection Helper
/// </summary>
/// <remarks>
/// Use IEnumerable by default, but when altering or getting item at index use IList.
/// </remarks>
public static class CollectionHelper
{

    #region Alter;

    /// <summary>
    /// Swap item to another place
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="IndexA">Index a</param>
    /// <param name="IndexB">Index b</param>
    /// <returns>New collection</returns>
    public static IList<T> Swap<T>(this IList<T> @this, Int32 IndexA, Int32 IndexB)
    {
        T Temp = @this[IndexA];
        @this[IndexA] = @this[IndexB];
        @this[IndexB] = Temp;
        return @this;
    }

    /// <summary>
    /// Swap item to the left
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="Index">Index</param>
    /// <returns>New collection</returns>
    public static IList<T> SwapLeft<T>(this IList<T> @this, Int32 Index)
    {
        return @this.Swap(Index, Index - 1);
    }

    /// <summary>
    /// Swap item to the right
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="Index">Index</param>
    /// <returns>New collection</returns>
    public static IList<T> SwapRight<T>(this IList<T> @this, Int32 Index)
    {
        return @this.Swap(Index, Index + 1);
    }

    #endregion Alter;

    #region Action;

    /// <summary>
    /// Execute action at specified index
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="Index">Index</param>
    /// <param name="ActionAt">Action to execute</param>
    /// <returns>New collection</returns>
    public static IList<T> ActionAt<T>(this IList<T> @this, Int32 Index, Action<T> ActionAt)
    {
        ActionAt(@this[Index]);
        return @this;
    }

    #endregion Action;

    #region Randomize;

    /// <summary>
    /// Take random items
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="Count">Number of items to take</param>
    /// <returns>New collection</returns>
    public static IEnumerable<T> TakeRandom<T>(this IEnumerable<T> @this, Int32 Count)
    {
        return @this.Shuffle().Take(Count);
    }

    /// <summary>
    /// Take random item
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <returns>Item</returns>
    public static T TakeRandom<T>(this IEnumerable<T> @this)
    {
        return @this.TakeRandom(1).Single();
    }

    /// <summary>
    /// Shuffle list
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <returns>New collection</returns>
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> @this)
    {
        return @this.OrderBy(Item => Guid.NewGuid());
    }

    #endregion Randomize;

    #region Navigate;

    /// <summary>
    /// Get next item in collection and give first item, when last item is selected;
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="Index">Index in collection</param>
    /// <returns>Next item</returns>
    public static T Next<T>(this IList<T> @this, ref Int32 Index)
    {
        Index = ++Index >= 0 && Index < @this.Count ? Index : 0;
        return @this[Index];
    }

    /// <summary>
    /// Get previous item in collection and give last item, when first item is selected;
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <param name="Index">Index in collection</param>
    /// <returns>Previous item</returns>
    public static T Previous<T>(this IList<T> @this, ref Int32 Index)
    {
        Index = --Index >= 0 && Index < @this.Count ? Index : @this.Count - 1;
        return @this[Index];
    }

    #endregion Navigate;

    #region Clone;

    /// <summary>
    /// 
    /// </summary>
    /// <typeparam name="T">Collection type</typeparam>
    /// <param name="this">Collection</param>
    /// <returns>Cloned collection</returns>
    public static IEnumerable<T> Clone<T>(this IEnumerable<T> @this) where T : ICloneable
    {
        return @this.Select(Item => (T)Item.Clone());
    }

    #endregion Clone;

    #region String;

    /// <summary>
    /// Joins multiple string with Separator
    /// </summary>
    /// <param name="this">Collection</param>
    /// <param name="Separator">Separator</param>
    /// <returns>Joined string</returns>
    public static String Join(this IEnumerable<String> @this, String Separator = "")
    {
        return String.Join(Separator, @this);
    }

    #endregion String;

}

How about ...

public static bool IsWinXPOrHigher(this OperatingSystem OS)
{
  return (OS.Platform == PlatformID.Win32NT)
    && ((OS.Version.Major > 5) || ((OS.Version.Major == 5) && (OS.Version.Minor >= 1)));
}

public static bool IsWinVistaOrHigher(this OperatingSystem OS)
{
  return (OS.Platform == PlatformID.Win32NT)
    && (OS.Version.Major >= 6);
}

public static bool IsWin7OrHigher(this OperatingSystem OS)
{
  return (OS.Platform == PlatformID.Win32NT)
    && ((OS.Version.Major > 6) || ((OS.Version.Major == 6) && (OS.Version.Minor >= 1)));
}

public static bool IsWin8OrHigher(this OperatingSystem OS)
{
  return (OS.Platform == PlatformID.Win32NT)
    && ((OS.Version.Major > 6) || ((OS.Version.Major == 6) && (OS.Version.Minor >= 2)));
}

Usage:

if (Environment.OSVersion.IsWinXPOrHigher())
{
  // do stuff
}

if (Environment.OSVersion.IsWinVistaOrHigher())
{
  // do stuff
}

if (Environment.OSVersion.IsWin7OrHigher())
{
  // do stuff
}

if (Environment.OSVersion.IsWin8OrHigher())
{
  // do stuff
}

Another one, this time to make UriBuilder more friendly when dealing with query params.

    /// <summary>
    /// Adds the specified query parameter to the URI builder.
    /// </summary>
    /// <param name="builder">The builder.</param>
    /// <param name="parameterName">Name of the parameter.</param>
    /// <param name="value">The URI escaped value.</param>
    /// <returns>The final full query string.</returns>
    public static string AddQueryParam(this UriBuilder builder, string parameterName, string value)
    {
        if (parameterName == null)
            throw new ArgumentNullException("parameterName");

        if (parameterName.Length == 0)
            throw new ArgumentException("The parameter name is empty.");

        if (value == null)
            throw new ArgumentNullException("value");

        if (value.Length == 0)
            throw new ArgumentException("The value is empty.");

        if (builder.Query.Length == 0)
        {
            builder.Query = String.Concat(parameterName, "=", value);
        }
        else if
            (builder.Query.Contains(String.Concat("&", parameterName, "="))
            || builder.Query.Contains(String.Concat("?", parameterName, "=")))
        {
            throw new InvalidOperationException(String.Format("The parameter {0} already exists.", parameterName));
        }
        else
        {
            builder.Query = String.Concat(builder.Query.Substring(1), "&", parameterName, "=", value);
        }

        return builder.Query;
    }

Easily serialize objects into XML:

public static string ToXml<T>(this T obj) where T : class
{
    XmlSerializer s = new XmlSerializer(obj.GetType());
    using (StringWriter writer = new StringWriter())
    {
        s.Serialize(writer, obj);
        return writer.ToString();
    }
}

"<root><child>foo</child</root>".ToXml<MyCustomType>();