Coolest C# LINQ/Lambdas trick you've ever pulled?

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Saw a post about hidden features in C# but not a lot of people have written linq/lambdas example so... I wonder...

What's the coolest (as in the most elegant) use of the C# LINQ and/or Lambdas/anonymous delegates you have ever saw/written?

Bonus if it has went into production too!

lambda functional-programming linq c#
14 Answers

The LINQ Raytracer certainly tops my list =)

I'm not quite sure if qualifies as elegant but it is most certainly the coolest linq-expression I've ever seen!

Oh, and just to be extremely clear; I did not write it (Luke Hoban did)

Markus_21140

Some basic functionals:

public static class Functionals
{
    // One-argument Y-Combinator.
    public static Func<T, TResult> Y<T, TResult>(Func<Func<T, TResult>, Func<T, TResult>> F)
    {
        return t => F(Y(F))(t);
    }

    // Two-argument Y-Combinator.
    public static Func<T1, T2, TResult> Y<T1, T2, TResult>(Func<Func<T1, T2, TResult>, Func<T1, T2, TResult>> F)
    {
        return (t1, t2) => F(Y(F))(t1, t2);
    }

    // Three-arugument Y-Combinator.
    public static Func<T1, T2, T3, TResult> Y<T1, T2, T3, TResult>(Func<Func<T1, T2, T3, TResult>, Func<T1, T2, T3, TResult>> F)
    {
        return (t1, t2, t3) => F(Y(F))(t1, t2, t3);
    }

    // Four-arugument Y-Combinator.
    public static Func<T1, T2, T3, T4, TResult> Y<T1, T2, T3, T4, TResult>(Func<Func<T1, T2, T3, T4, TResult>, Func<T1, T2, T3, T4, TResult>> F)
    {
        return (t1, t2, t3, t4) => F(Y(F))(t1, t2, t3, t4);
    }

    // Curry first argument
    public static Func<T1, Func<T2, TResult>> Curry<T1, T2, TResult>(Func<T1, T2, TResult> F)
    {
        return t1 => t2 => F(t1, t2);
    }

    // Curry second argument.
    public static Func<T2, Func<T1, TResult>> Curry2nd<T1, T2, TResult>(Func<T1, T2, TResult> F)
    {
        return t2 => t1 => F(t1, t2);
    }

    // Uncurry first argument.
    public static Func<T1, T2, TResult> Uncurry<T1, T2, TResult>(Func<T1, Func<T2, TResult>> F)
    {
        return (t1, t2) => F(t1)(t2);
    }

    // Uncurry second argument.
    public static Func<T1, T2, TResult> Uncurry2nd<T1, T2, TResult>(Func<T2, Func<T1, TResult>> F)
    {
        return (t1, t2) => F(t2)(t1);
    }
}

Don't do much good if you don't know how to use them. In order to know that, you need to know what they're for:

Turbulent_Intellect_27290

Progress Reporting for long running LINQ queries. In the blog post you can find an extension method WithProgressReporting() that lets you discover and report the progress of a linq query as it executes.

user_17270
Drakiula_24370
Slace_113880
Slace_113880
Tomas_Petricek_335180

I was trying to come up with a cool way to build a navigation control for a website I was building. I wanted to use regular HTML unordered list elements (employing the standard CSS "Sucker Fish" look) with a top-navigation mouse-over effect that reveals the drop down items. I had a sql dependent cached DataSet with two tables (NavigationTopLevels & NavigationBottomLevels). Then all I had to was create two class objects (TopNav & SubNav) with the few required properties (the TopNav class had to have a generic list of bottomnav items -> List<SubNav> SubItems).



var TopNavs = from n in ds.NavigationTopLevels
                          select new TopNav
                          {
                              NavigateUrl = String.Format("{0}/{1}", tmpURL, n.id),
                              Text = n.Text,
                              id = n.id,
                              SubItems = new List<SubNav>(
                                  from si in ds.NavigationBottomLevels
                                  where si.parentID == n.id
                                  select new SubNav
                                  {
                                      id = si.id,
                                      level = si.NavLevel,
                                      NavigateUrl = String.Format("{0}/{1}/{2}", tmpURL, n.id, si.id),
                                      parentID = si.parentID,
                                      Text = si.Text
                                  }
                                )

                          };

 List<TopNav> TopNavigation = TopNavs.ToList();

It might not be the "coolest" but for a lot of people who want to have dynamic navigation, its sweet not to have to muddle around in the usual looping logic that comes with that. LINQ is, if anything a time saver in this case.

Ian_Patrick_Hughes_22130

I think that LINQ is a major change to .NET and it is a very powerful tool.

I use LINQ to XML in production to parse and filter records from a 6MB XML file (with 20+ node levels) into a dataset in two lines of code.

Before LINQ this would have taken hundreds of lines of code and days to debug.

That's what I call elegant!

ThatBloke_70500

Working with attributes:

private void WriteMemberDescriptions(Type type)
{
    var descriptions =
        from member in type.GetMembers()
        let attributes = member.GetAttributes<DescriptionAttribute>(true)
        let attribute = attributes.FirstOrDefault()
        where attribute != null
        select new
        {
            Member = member.Name,
            Text = attribute.Description
        };

        foreach(var description in descriptions)
        {
            Console.WriteLine("{0}: {1}", description.Member, description.Text);
        }
}

The GetAttributes extension method:

public static class AttributeSelection
{
    public static IEnumerable<T> GetAttributes<T>(this ICustomAttributeProvider provider, bool inherit) where T : Attribute
    {
        if(provider == null)
        {
            throw new ArgumentNullException("provider");
        }

        return provider.GetCustomAttributes(typeof(T), inherit).Cast<T>();
    }
}

AttributeSelection is production code and also defines GetAttribute and HasAttribute. I chose to use the let and where clauses in this example.

Bryan_Watts_378150
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