Differences in string compare methods in C#

Here are the rules for how these functions work:

stringValue.CompareTo(otherStringValue)

1. null comes before a string
2. it uses CultureInfo.CurrentCulture.CompareInfo.Compare, which means it will use a culture-dependent comparison. This might mean that ß will compare equal to SS in Germany, or similar

stringValue.Equals(otherStringValue)

1. null is not considered equal to anything
2. unless you specify a StringComparison option, it will use what looks like a direct ordinal equality check, i.e. ß is not the same as SS, in any language or culture

stringValue == otherStringValue

1. Is not the same as stringValue.Equals().
2. The == operator calls the static Equals(string a, string b) method (which in turn goes to an internal EqualsHelper to do the comparison.
3. Calling .Equals() on a null string gets null reference exception, while on == does not.

Object.ReferenceEquals(stringValue, otherStringValue)

Just checks that references are the same, i.e. it isn't just two strings with the same contents, you're comparing a string object with itself.

Note that with the options above that use method calls, there are overloads with more options to specify how to compare.

My advice if you just want to check for equality is to make up your mind whether you want to use a culture-dependent comparison or not, and then use .CompareTo or .Equals, depending on the choice.

From MSDN:

"The CompareTo method was designed primarily for use in sorting or alphabetizing operations. It should not be used when the primary purpose of the method call is to determine whether two strings are equivalent. To determine whether two strings are equivalent, call the Equals method."

They suggest using .Equals instead of .CompareTo when looking solely for equality. I am not sure if there is a difference between .Equals and == for the string class. I will sometimes use .Equals or Object.ReferenceEquals instead of == for my own classes in case someone comes along at a later time and redefines the == operator for that class.

If you are ever curious about differences in BCL methods, Reflector is your friend :-)

I follow these guidelines:

Exact match: _{EDIT: I previously always used == operator on the principle that inside Equals(string, string) the object == operator is used to compare the object references but it seems strA.Equals(strB) is still 1-11% faster overall than string.Equals(strA, strB), strA == strB, and string.CompareOrdinal(strA, strB). I loop tested with a StopWatch on both interned/non-interned string values, with same/different string lengths, and varying sizes (1B to 5MB).}

strA.Equals(strB)

Human-readable match (Western cultures, case-insensitive):

string.Compare(strA, strB, StringComparison.OrdinalIgnoreCase) == 0

Human-readable match (All other cultures, insensitive case/accent/kana/etc defined by CultureInfo):

string.Compare(strA, strB, myCultureInfo) == 0

Human-readable match with custom rules (All other cultures):

CompareOptions compareOptions = CompareOptions.IgnoreCase
                              | CompareOptions.IgnoreWidth
                              | CompareOptions.IgnoreNonSpace;
string.Compare(strA, strB, CultureInfo.CurrentCulture, compareOptions) == 0

As Ed said, CompareTo is used for sorting.

There is a difference, however, between .Equals and ==.

== resolves to essentially the following code:

if(object.ReferenceEquals(left, null) && 
   object.ReferenceEquals(right, null))
    return true;
if(object.ReferenceEquals(left, null))
    return right.Equals(left);
return left.Equals(right);

The simple reason is the following will throw an exception:

string a = null;
string b = "foo";

bool equal = a.Equals(b);

And the following will not:

string a = null;
string b = "foo";

bool equal = a == b;

Not that performance usually matters with 99% of the times you need to do this, but if you had to do this in a loop several million times I would highly suggest that you use .Equals or == because as soon as it finds a character that doesn't match it throws the whole thing out as false, but if you use the CompareTo it will have to figure out which character is less than the other, leading to slightly worse performance time.

If your app will be running in different countries, I'd recommend that you take a look at the CultureInfo implications and possibly use .Equals. Since I only really write apps for the US (and don't care if it doesn't work properly by someone), I always just use ==.

In the forms you listed here, there's not much difference between the two. CompareTo ends up calling a CompareInfo method that does a comparison using the current culture; Equals is called by the == operator.

If you consider overloads, then things get different. Compare and == can only use the current culture to compare a string. Equals and String.Compare can take a StringComparison enumeration argument that let you specify culture-insensitive or case-insensitive comparisons. Only String.Compare allows you to specify a CultureInfo and perform comparisons using a culture other than the default culture.

Because of its versatility, I find I use String.Compare more than any other comparison method; it lets me specify exactly what I want.

Using .Equals is also a lot easier to read.