Delegate conversion via reflection with DynamicMethod issue

Test.cs

using System;
using System.Reflection.Emit;

class Program
{
  delegate object FuncObjectObject(Object o);

  static void Main(string[] args)
  {
    var dm = new DynamicMethod("", typeof(object), new [] { typeof(object) });
    var ilgen = dm.GetILGenerator();
    ilgen.Emit(OpCodes.Ldarg_0);
    ilgen.Emit(OpCodes.Ret);

    var dmd = (Func<object, object>)dm.CreateDelegate(typeof(Func<object, object>));
    Console.WriteLine(dmd(1)); // works

    // now do the same, but with reflection
    var rd = (FuncObjectObject) Delegate.CreateDelegate(typeof(FuncObjectObject), dmd.Target, dmd.Method); // kaboom!

    // wrap the delegate
    var sdm = new FuncObjectObject(dmd);
    Console.WriteLine(sdm(1)); // works

    var srd = (FuncObjectObject)Delegate.CreateDelegate(typeof(FuncObjectObject), sdm.Target, sdm.Method); // works
    Console.WriteLine(srd(1));

    // lets try something else
    var ad = (FuncObjectObject) Activator.CreateInstance(typeof(FuncObjectObject), dmd); // kaboom!
  }
}

The constructor for delegates actually takes two parameters; the C# compiler lies and splits out the target and method for you; the actual usage would be:

var ad = (FuncObjectObject)Activator.CreateInstance(typeof(FuncObjectObject),
     sdm.Target, sdm.Method.MethodHandle.GetFunctionPointer());

However: it doesn't work with dynamic methods.

For the rest: ultimately, dynamic methods have restrictions and limitations (but also: have some nice extra features, like the ability to spoof accessibility to a type outside your direct control). Some things don't work on them. If you need it to behave as a full method, you might need to use TypeBuilder and MethodBuilder to create a (presumably) static method on a dummy Type via CreateType() etc. Much more work.

yay, found a workaround!

var ad = (FuncObjectObject)Activator.CreateInstance(typeof(FuncObjectObject), 
     dmd, dmd.GetType().GetMethod("Invoke").MethodHandle.GetFunctionPointer()); // works!
Console.WriteLine(ad(1)); // works!

Thanks @mgravell for hinting towards to right direction 😄

heh; nice

Simpler:

var ad = (FuncObjectObject)Delegate.CreateDelegate(typeof(FuncObjectObject), dmd, "Invoke");