Multi-viewport issue with function pointers.

[JunaMeinhold]

• Struct layouts are verified. (double checked)

Cpp signature:

ImVec2(*Platform_GetWindowPos)(ImGuiViewport *vp); 

Expected C# signature:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public unsafe delegate Vector2 PlatformGetWindowPos(ImGuiViewport* viewport);

Doesn't work on windows.

Working signature on windows: (Created using the disassembly)

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public unsafe delegate Vector2* PlatformGetWindowPos(Vector2* pos, ImGuiViewport* viewport);

Any thoughts why that could happen?
PS: I checked already the calling convention doesn't change anything. (set the cpp fn to __cdecl)

My thoughts:

• A issue in the .NET Runtime.

Screenshot of the problem with the "correct"/expected signature

Screenshot with the workaround:

[AhmedZero]

cimgui/cimgui#113 (comment)

[JunaMeinhold]

        private static unsafe Vector2* GetWindowSize(Vector2* size, ImGuiViewport* viewport)
        {
            ViewportData* vd = (ViewportData*)viewport->PlatformUserData;
            int w = 0, h = 0;
            SDL.SDLGetWindowSize(vd->Window, &w, &h);
            *size = new Vector2(w, h);
            return size;
        }

        private static unsafe Vector2* GetWindowPos(Vector2* size, ImGuiViewport* viewport)
        {
            ViewportData* vd = (ViewportData*)viewport->PlatformUserData;
            int x = 0, y = 0;
            SDL.SDLGetWindowPosition(vd->Window, &x, &y);
            *size = new Vector2(x, y);
            return size;
        }

        private static unsafe Vector2 GetWindowSize(ImGuiViewport* viewport)
        {
            ViewportData* vd = (ViewportData*)viewport->PlatformUserData;
            int w = 0, h = 0;
            SDL.SDLGetWindowSize(vd->Window, &w, &h);
            var size = new Vector2(w, h);
            return size;
        }

        private static unsafe Vector2 GetWindowPos(ImGuiViewport* viewport)
        {
            ViewportData* vd = (ViewportData*)viewport->PlatformUserData;
            int x = 0, y = 0;
            SDL.SDLGetWindowPosition(vd->Window, &x, &y);
            var size = new Vector2(x, y);
            return size;
        }

        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        public unsafe delegate Vector2 PlatformGetWindowPosDefault(ImGuiViewport* viewport);

        [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
        public unsafe delegate Vector2 PlatformGetWindowSizeDefault(ImGuiViewport* viewport);
        
        ...
        
            if (OperatingSystem.IsWindows())
            {
                platform_io->PlatformGetWindowPos = (void*)Marshal.GetFunctionPointerForDelegate<PlatformGetWindowPos>(GetWindowPos);
                platform_io->PlatformGetWindowSize = (void*)Marshal.GetFunctionPointerForDelegate<PlatformGetWindowSize>(GetWindowSize);
            }
            else
            {
                platform_io->PlatformGetWindowPos = (void*)Marshal.GetFunctionPointerForDelegate<PlatformGetWindowPosDefault>(GetWindowPos);
                platform_io->PlatformGetWindowSize = (void*)Marshal.GetFunctionPointerForDelegate<PlatformGetWindowSizeDefault>(GetWindowSize);
            }

I managed to get it working with linux too, it's kinda hacky, but it's a working workaround

[JunaMeinhold]

cimgui/cimgui#113 (comment)

Thanks for pointing that out

[AhmedZero]

You're welcome.