Why do some C# lambda expressions compile to static methods?

Question

As you can see in the code below, I have declared an Action<> object as a variable.

Would anybody please let me know why this action method delega

Answer 1

Delegate caching behavior was changed in Roslyn. Previously, as stated, any lambda expression which didn't capture variables was compiled into a static method at the call site. Roslyn changed this behavior. Now, any lambda, which captures variables or not, is transformed into a display class:

Given this example:

public class C
{
    public void M()
    {
        var x = 5;
        Action action = y => Console.WriteLine(y);
    }
}

Native compiler output:

public class C
{
    [CompilerGenerated]
    private static Action CS$<>9__CachedAnonymousMethodDelegate1;
    public void M()
    {
        if (C.CS$<>9__CachedAnonymousMethodDelegate1 == null)
        {
            C.CS$<>9__CachedAnonymousMethodDelegate1 = new Action(C.b__0);
        }
        Action arg_1D_0 = C.CS$<>9__CachedAnonymousMethodDelegate1;
    }
    [CompilerGenerated]
    private static void b__0(int y)
    {
        Console.WriteLine(y);
    }
}

Roslyn:

public class C
{
    [CompilerGenerated]
    private sealed class <>c__DisplayClass0
    {
        public static readonly C.<>c__DisplayClass0 CS$<>9__inst;
        public static Action CS$<>9__CachedAnonymousMethodDelegate2;
        static <>c__DisplayClass0()
        {
            // Note: this type is marked as 'beforefieldinit'.
            C.<>c__DisplayClass0.CS$<>9__inst = new C.<>c__DisplayClass0();
        }
        internal void b__1(int y)
        {
            Console.WriteLine(y);
        }
    }
    public void M()
    {
        Action arg_22_0;
        if (arg_22_0 = C.
                       <>c__DisplayClass0.CS$<>9__CachedAnonymousMethodDelegate2 == null)
        {
            C.<>c__DisplayClass0.CS$<>9__CachedAnonymousMethodDelegate2 =
          new Action(C.<>c__DisplayClass0.CS$<>9__inst.b__1);
        }
    }
}

Delegate caching behavior changes in Roslyn talks about why this change was made.