passing an unmanaged function pointer callback to managed code using C++ interop

I'm trying to add some managed code into an existing c++ GUI application. I'd like to get a functional callback working...specifically, I'd like to pass a method pointer from UNMANAGED code to MANAGED code and have the man开发者_JAVA技巧aged code invoke the callback.

I'm looking at something like this:

typedef int (__stdcall *ANSWERCB)(int);  //Delegate declaration

class UnmanagedObject
{
public:

 UnmanagedObject() {}
 int MethodD(int n) { return n; }
};

with a corresponding managed class:

public delegate int CallbackDelegate(int i);

public class ManagedClass
{
    public ManagedClass() {}
    public void MethodD( CallbackDelegate cd ) { cd.Invoke( 5 ); }
}

The problem is that I can't for the life of me figure out how to actually invoke this from managed code:

UnmanagedObject* obj = new UnmanagedObject();
ManagedLibrary::ManagedClass^ mc = gcnew ManagedLibrary::ManagedClass();
mc->MethodD( /* what do I pass here? */ );

I've tried:

ManagedLibrary::CallbackDelegate^ cd = gcnew CallbackDelegate(obj, &UnmanagedObject::MethodD);

but it generates a compiler error "delegate target needs to be a pointer to a member function".

Any interop guru's able to help out?

Thanks!

The problem starts at your ANSWERCB function pointer declaration. It isn't actually suitable for calling the MethodD() function, that's an instance method of the class. Make this work in native C++ first, you need a member function pointer.

From there, you'll have a lot less problems making it work from managed code, Marshal::GetDelegateForFunctionPointer() gets you what you need.

Half your trouble is that the member function in question doesn't actually have the same signature as the callback delegate. It takes a function pointer itself. First, you need to alter UnmanagedClass to be

class UnmanagedObject
{
public:
    UnmanagedObject() {} 
    int MethodD(int n) { return n; } 
};

I suspect for the gcnew call that you simply have the parameters the wrong way around- i.e., mem func ptr, then obj.