How to pass a delegate or function pointer from C# to C++ and call it there using InternalCall

After some more hours of digging I finally found a (the?) solution.
Basically, what works for the PInvoke approach works here as well, you can pass a function pointer instead of a delegate from C# to C(++).
I'd prefer a solution where you can pass a delegate directly, but you can always add some wrapper code in C# to at least make it look like that.

Solution:

C#:

public delegate void CallbackDelegate(string message);
[MethodImplAttribute(MethodImplOptions.InternalCall)]
public static extern void setCallback(IntPtr aCallback);

private CallbackDelegate del; 
public void testCallbacks()
{
    System.Console.Write("Registering C# callback...\n");
    del = new CallbackDelegate(callback01);
    setCallback(Marshal.GetFunctionPointerForDelegate(del));

    System.Console.Write("Calling passed C++ callback...\n");
}

public void callback01(string message)
{
    System.Console.Write("callback 01 called. Message: " + message + "\n");
}

C++:

typedef void (*CallbackFunction)(MonoString*);
void setCallback(CallbackFunction delegate)
{
    std::cout << &delegate << std::endl;
    delegate(mono_string_new(mono_domain_get(), "Test string set in C++"));
}

Watch out, though: You need to keep the delegate around in C# somehow (which is why I assigned it to "del") or it will be caught by the GC and your callback will become invalid.
It makes sense, of course, but I feel this is easy to forget in this case.

you can pass function pointer as parameter in c++ to c# using intptr_t.

MSDN isn't accurate, below code works.

  // c++
  static void func(int param)
  {
    //...
  }  

  void other_func()
  {
    ptr->SetCallback( reinterpret_cast<intptr_t>(func));
  }

  // c#
  public static mydelegatetype somefunc = null;

  public void SetCallback(IntPtr function_pointer)
  {
    somefunc = (mydelegatetype)
      Marshal.GetDelegateForFunctionPointer(function_pointer, typeof(mydelegatetype));
  }