How to return C++ smart pointer to PowerPoint interface using C# Interface

Question

I am making a program partly written in C++ and C#. C# is mainly used for GUI. Inside my C++ code I create PowerPoint COM object and open a presentation in it.

At some point I need to reference PowerPoint::_PresentationPtr in my C# code. To call methods and so on.

PowerPoint::_PresentationPtr is a smart pointer made with

_COM_SMARTPTR_TYPEDEF(_Presentation, __uuidof(_Presentation));

It is defined by Visual studio in msppt.tlh Microsoft.Office.Interop.PowerPoint.dll binding:

struct __declspec(uuid("91493463-5a91-11cf-8700-00aa0060263b"))
/* interface */ PresEvents;
struct /* coclass */ Presentation;

struct __declspec(uuid("9149349d-5a91-11cf-8700-00aa0060263b"))
/* dual interface */ _Presentation;

/* PowerPoint::_PresentationPtr */
_COM_SMARTPTR_TYPEDEF(_Presentation, __uuidof(_Presentation)); 

So it makes PowerPoint::_PresentationPtr for ease of use the COM object in C++, calling methods etc.

And this is C# Microsoft.Office.Interop.PowerPoint.Presentation

namespace Microsoft.Office.Interop.PowerPoint
{
    [CoClass(typeof(PresentationClass))]
    [Guid("9149349D-5A91-11CF-8700-00AA0060263B")]
    public interface Presentation : _Presentation, PresEvents_Event
    {
    }
}

I have an Interface in C# with C++ implementation so I can access it in C# part of the application.

ApplicationHost.cs

namespace SampleWpfUserControlLibrary
{
    public interface IApplicationHostWindow
    {
        void OpenDocument();
        PowerPoint.Presentation GetPresentation();
        void Exit();
    }
}

ApplicastionHost.h

#pragma once
#using <SampleWpfUserControlLibrary.dll>

using namespace SampleWpfUserControlLibrary;

ref class ApplicationHostWrapper : IApplicationHostWindow
{
public:
    ApplicationHostWrapper(CMainFrame * pMainFrame)
    {
        _pMainFrame = pMainFrame;
    }

    virtual void __clrcall Exit() sealed
    {
        _pMainFrame->SendMessage(WM_CLOSE);
    }

    virtual void __clrcall OpenDocument() sealed
    {
        _pMainFrame->OpenDocument();
    }

    virtual PowerPoint::_PresentationPtr __clrcall GetPresentation() sealed
    {
        CMFCBindDoc * pDoc = _pMainFrame->GetDocView()->GetDocument();
        return pDoc->GetPresentation();
    }
};

However it does not compile. Arguing that GetPresentation() is not implemented. As the types do not match.

Update:

What should I return in C++ code PowerPoint::_Presentation or PowerPoint::_PresentationPtr and what should I map it to in C# part? Or How do convert it to Microsoft.Office.Interop.PowerPoint in the C#? I just want to reference it in C#.

These are the errors I get:**

• Error 3 error C2259: 'ApplicationHostWrapper' : cannot instantiate abstract class
  Source\Application\MainFrm.cpp 146 1 Application
• Error 1 error C3766: 'ApplicationHostWrapper' must provide an implementation for the interface method
  'Microsoft::Office::Interop::PowerPoint::Presentation
  ^SampleWpfUserControlLibrary::IApplicationHostWindow::getPresentation(void)' source\application\ApplicationHostWrapper.h 39 1 Application

In Object Browser getPresentation() method is this:

unknown-type^ getPresentation() Member of SampleWpfUserControlLibrary::IApplicationHostWindow

Answer 1

Okay, I found solution. in ref class ApplicationHostWrapper : IApplicationHostWindow by calling GetInterfacePtr() on a smart pointer we can get a pointer to interface.

virtual void* __clrcall GetPresentationPtr() sealed
{
    CMFCBindDoc * pDoc = _pMainFrame->GetDocView()->GetDocument();
    return (void*)pDoc->GetPresentation().GetInterfacePtr();
}

In C# Interface declaration ApplicationHost.cs:

namespace SampleWpfUserControlLibrary
{
    public interface IApplicationHostWindow
    {
        void OpenDocument();
        unsafe void* GetPresentationPtr();
        void Exit();
    }
}

And this allows us to make a method in managed code for example in a class:

private IApplicationHostWindow _hostWindow;
unsafe private PowerPoint.Presentation getPresentation(){
   IntPtr preIntPtr = new IntPtr(_hostWindow.GetPresentationPtr());
   return (PowerPoint.Presentation)Marshal.GetUniqueObjectForIUnknown(preIntPtr);
}

The only drawback is that it requires using /unsafe for compilation. And what could be the consequences I have no idea yet. If someone knows how to overcome /unsafe and do it differently you are welcome.