C++/CLI - Managed class to C# events

Question

I have a c++ class that triggers some method like an event.

class Blah {

   virtual void Event(EventArgs e);
}

How can I wrap it so whenever t

Answer 1

You need to do some work to reflect the Event() method call so it can be hooked by a managed class. Lets implement a concrete blah class that does so:

#pragma managed(push, off)

struct EvenAtrgs {};

class blah {
public:
   virtual void Event (EvenAtrgs e) = 0;
};

typedef void (* CallBack)(EvenAtrgs);

class blahImpl : blah {
    CallBack callback;
public:
    blahImpl(CallBack fp) {
        this->callback = fp;
    }
    virtual void Event(EvenAtrgs e) { 
        callback(e); 
    }
};
#pragma managed(pop)

You can now construct a blahImpl and pass it a function pointer that is called when the Event() method is called. You can use Marshal::GetFunctionPointerForDelegate() to get such a function pointer, it creates a stub for a delegate that makes the transition from unmanaged code to managed code and can store a instance as well. Combined with the boilerplate code to wrap an unmanaged class:

public ref class blahWrapper {
    blahImpl* instance;
    delegate void managedCallback(EvenAtrgs e);
    managedCallback^ callback;
    void fireEvent(EvenAtrgs e) {
        // Todo: convert e to a managed EventArgs derived class
        //...
        Event(this, EventArgs::Empty);
    }
public:
    event EventHandler^ Event;
    blahWrapper() {
        callback = gcnew managedCallback(this, &blahWrapper::fireEvent);
        instance = new blahImpl((CallBack)(void*)System::Runtime::InteropServices::Marshal::GetFunctionPointerForDelegate(callback));
    }
    ~blahWrapper() { delete instance; }
    !blahWrapper() { delete instance; }
};

The C# code can now write an event handler for the Event event. I left the spelling error in tact, you need to do some work to convert EvenAtrgs to a managed class that derives from EventArgs. Modify the managed Event accordingly.