Last time we took a look at how anonymous methods are implemented. Today we'll look at a puzzle that can be solved with what we've learned. Consider the following program fragment:

using System;

class MyClass {
 delegate void DelegateA();
 delegate void DelegateB();

 static DelegateB ConvertDelegate(DelegateA d)
 {
  return (DelegateB)
   Delegate.CreateDelegate(typeof(DelegateB), d.Method);
 }

 static public void Main()
 {
  int i = 0;
  ConvertDelegate(delegate { Console.WriteLine(0); });
 }
}

The ConvertDelegate method merely converts a DelegateA to a DelegateB by creating a DelegateB with the same underlying method. Since the two delegate types use the same signature, this conversion goes off without a hitch.

But now let's make a small change to that Main function:

 static public void Main()
 {
  int i = 0;
  // one character change - 0 becomes i
  ConvertDelegate(delegate { Console.WriteLine(i); });
 }

Now the program crashes with a System.ArgumentException at the point where we try to create the DelegateB. What's going on?

First, observe that the overload of Delegate.CreateDelegate that was used is one that can only be used to create delegates from static methods. Next, note that in Test1, the anonymous method references neither its own members nor any local variables from its lexically-enclosing method. Therefore, the resulting anonymous method is a "static anonymous method of the easy type". Since the anonymous method is a static member, the use of the "static members only" overload of Delegate.CreateDelegate succeeds.

However, in Test2, the anonymous method dereferences the i variable from its lexically-enclosing method. This forces the anonymous method to be a "anonymous method of the hard type", and those anonymous methods use an anonymous instance member function of an anonymous helper class. As a result, d.Method is an instance method, and the chosen overload of Delegate.CreateDelegate throws an invalid parameter exception since it works only with static methods.

The solution is to use a different overload of Delegate.CreateDelegate, one that work with either static or instance member functions.

 DelegateB ConvertDelegate(DelegateA d)
 {
  return (DelegateB)
   Delegate.CreateDelegate(typeof(DelegateB), d.Target, d.Method);
 }

The Delegate.CreateDelegate(Type, Object, MethodInfo) overload creates a delegate for a static method if the Object parameter is null or a delegate for an instance method if the Object parameter is non-null. Hardly by coincidence, that is exactly what d.Target produces. If the original delegate is for a static method, then d.Target is null; otherwise, it is the object for which the instance method is to be invoked on.

This fix, therefore, makes the ConvertDelegate function handle conversion of delegates for either static or instance methods. Which is a good thing, because it may now be called upon to convert delegates for instance methods as well as static ones.

Okay, this time we were lucky that the hidden gotcha of anonymous methods resulted in an exception. Next time, we'll see a gotcha that merely results in incorrect behavior that will probably take you forever to track down.

Update: This behavior changed in Visual Studio 2015 with the switch to the Roslyn compiler. For performance reasons, anonymous methods are now always instance methods, even if they capture nothing.