C++/CLI: Trigger Events from native code and handle them in Managed Code, part II

In the previous post, we saw how to trigger an event from a C++ native class which has a C++/CLI wrapper as an interface to .NET. We dealt with the general case in which we wish to pass non trivial information (a class or a struct) along with the event. Today we'll see the more simple case in which we just need to pass a float or some other basic CLR type. As expected, things are quite simpler.

1. In the native class, just for convenience, define a pointer to function type. Both returnType and parameters should be basic CLR types.

   typedef <returnType> ( __stdcall * PFOnEventCallback )( <parameters> );

2. In the same class, declare a field of that type:

   PFOnEventCallback m_fireEvent;

3. Still in the native class, go to where you wish to trigger the event:

   if (m_fireEvent)
      m_fireEvent( <parameters> );

   (It's good practice to initialize m_fireEvent to 0 or NULL, your choice)
4. Define a registration method:

   void CNativeClass::registerOnEventCallback( PFOnEventCallback newVal ){
      m_fireEvent = newVal;
   }

5. In the C++/CLI wrapper, define a delegate analogous to the native pointer to function. They should have the same returnType and parameters; since they are basic types, no explicit marshalling is needed.

   delegate <returnType> DCallback( <parameters> );
   // ...Alongside the other fields
   DCallback ^ m_hDCallback;

6. Declare the managed event:

   event DCallback ^ Event{
      void add( DCallback ^p ){
         m_hDCallback += p;
         m_pNativeInstance->registerCallback(
            PFOnEventCallback(
               Marshal::GetFunctionPointerForDelegate( p ).ToPointer()
            )
         );
      }
   
      void remove( DCallback ^p ){
         m_hDCallback -= p;                 
         m_pNativeInstance->registerCallback( 0 );
      }
   }

7. Finally, subscribe to the managed event from pure managed code. For example, in VB.NET:

   AddHandler CLIWrapper.Event, AddressOf SomeSubWithTheSameSignatureAsDCallback
   'When you are done or the object is disposed'
   RemoveHandler CLIWrapper.OnEvent, AddressOf SomeSubWithTheSameSignatureAsDCallback

It's a bit less convoluted than the general case, but I'm sure Java people would still punch me in the gonads. (No offense intended, on the contrary, I'd rather work in Java)