Constant flickering in C++ Windows desktop app when a child window is made the child of a C# desktop app?

Question

I have used the information from the following SO posts to make the parent of a Picture control in my C++ Desktop app, the child of a Panel control in my C# app. The C++ and C# apps are separate apps running in their own process:

Various issues using SetParent to embed window into external process

Embedding HWND into external process using SetParent

I am using the code shown below to do the re-parenting. The C# app launches the C++ app, passing it on the command line the Windows handle of the Panel control that should host the C++ app's Picture control. When I run the C# app I do see the outline of the C++ Picture control on the designated Panel control that is native to the C# app.

However, I am having the following problems:

1) Both the C++ and C# apps flicker terribly like they are both being repainted many times a second.

2) The picture control in the C++ app normally shows a video feed from my WebCam. I BitBlt() the frames from the webcam into the C++ Picture control. It works fine without the re-parenting but with it, I don't see any frames at all.

NOTE : The flicker is definitely the result of drawing into the now re-parented child window. If I disable the that operation, the flicker doesn't happen.

Does anyone know what is wrong and how I can fix it? Here is the code in the C++ app that does the re-parenting and the attachment of the C++ main input thread (the thread that owns the Picture control) to the input thread belonging to the main thread of the C# app (the thread that owns the Panel control):

// The following code executes in the wWinMain() function of the C++
//  app after the main dialog window and it's child controls have been
//  setup and initialized.  The value assigned to g_VideoHostContainerHWnd
//  was passed to the C++ app by the C# app that launched it, as a 
//  command line argument, using the Panel control's Handle property
//  converted to an unsigned integer.
g_OurMainThreadID = GetCurrentThreadId();

if (g_VideoHostContainerHWnd > 0)
{
    // Make our video stream window a parent of the designated window
    //  in the HiveMind client.

    // Get the window handle for the video stream window: IDC_PANEL_PICTURE.
    HWND videoStreamWindow =
        GetDlgItem(
        g_HwndDlg,
        IDC_PANEL_PICTURE);

    if (videoStreamWindow > 0)
    {
        // Get the thread ID for the thread that owns the video host container window.

         g_VideoHostThreadID = GetWindowThreadProcessId(
            g_VideoHostContainerHWnd,
            &g_VideoHostProcessID);

         if (g_VideoHostThreadID > 0)
         {
             // Make the video stream window a child of the HiveMindClient window.
             if (NULL == SetParent(videoStreamWindow, g_VideoHostContainerHWnd))
                 OutputDebugString(L"The reparenting of the video stream window FAILED.");
             else
                 OutputDebugString(L"The reparenting of the video stream window SUCCEEDED.");

             // Attach the our input thread to the thread ID for the process that launched us
             //  (i.e. - owns the video host window).
             if (AttachThreadInput(g_OurMainThreadID, g_VideoHostThreadID, true))
                 OutputDebugString(L"Attaching our input thread to the video host thread SUCCEEDED.");
             else
                 OutputDebugString(L"Attaching our input thread to the video host thread FAILED.");
         }
         else
         {
             OutputDebugString(L"The thread ID of the video host container's owner thread is ZERO.");
         } // else - if (g_VideoHostThreadID > 0)
    }
    else
        OutputDebugString(L"The video stream window handle is INVALID.");
} // if (g_VideoHostContainerHWnd > 0)

Answer 1

我也遇到了这个问题，解决方法是父hwnd需要标志WS_CLIPCHILDREN。