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

Answer 1

Here's one I just created today.

// requires .NET 4

public static TReturn NullOr(this TIn obj, Func func,
        TReturn elseValue = default(TReturn)) where TIn : class
    { return obj != null ? func(obj) : elseValue; }

// versions for CLR 2, which doesn't support optional params

public static TReturn NullOr(this TIn obj, Func func,
        TReturn elseValue) where TIn : class
    { return obj != null ? func(obj) : elseValue; }
public static TReturn NullOr(this TIn obj, Func func)
        where TIn : class
    { return obj != null ? func(obj) : default(TReturn); }

It lets you do this:

var lname = thingy.NullOr(t => t.Name).NullOr(n => n.ToLower());

which is more fluent and (IMO) easier to read than this:

var lname = (thingy != null ? thingy.Name : null) != null
    ? thingy.Name.ToLower() : null;