CreateWindowEx returns NULL

问题

The following is a portion of my code. Every time I run the program, CreateWindowEx of the SPanel returns NULL, triggering an error. Can anyone see what is wrong with this piece of code?

SPanelProc and MainWndProc are declared already, and is of the prototype LRESULT CALLBACK SPanelProc (HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);.

The environment I'm programming in is Visual C++ 2010.

Why is CreateWindowEx always returning NULL?

#include <Windows.h>
#include <WindowsX.h>

LRESULT CALLBACK SPanelProc (HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
    switch (msg)
    {
        case WM_MOUSEMOVE:

        break;
    }

    return 0;
}

void Init ()
{
    HWND child;
    HINSTANCE hInstance = (HINSTANCE)(GetWindowLongPtr (parent, GWLP_HINSTANCE));
    WNDCLASSEX wc;
    const char PanelClassName[] = "SPanel"; //ClassName of the panel. Do not reuse on another window

    //Create the window, initialize GWLP_USERDATA, check for errors, register the window class, and hide the window


    //Create the windowclass for the button
    wc.cbSize        = sizeof (WNDCLASSEX);
    wc.style         = 0;
    wc.lpfnWndProc   = SPanelProc;
    wc.cbClsExtra    = 0;
    wc.cbWndExtra    = 0;
    wc.hInstance     = hInstance;
    wc.hIcon         = LoadIcon (hInstance, IDI_APPLICATION);
    wc.hCursor       = LoadCursor (hInstance, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wc.lpszMenuName  = NULL;
    wc.lpszClassName = PanelClassName;
    wc.hIconSm       = LoadIcon (hInstance, IDI_APPLICATION);

    //If could not register window class
    if(!RegisterClassEx (&wc))
    {
        MessageBox (NULL, "Panel registration failed!", "Error!", MB_OK);
        PostQuitMessage (0);
        return;
    }

    //Create window
    child = CreateWindowEx (WS_EX_NOPARENTNOTIFY, PanelClassName, "TITLE_TEXT", WS_CHILD, 0, 0, 400, 200, parent, NULL, hInstance, NULL);

    //Check for error in window creation
    if (child == NULL)
    {
        MessageBox (NULL, "Panel creation failed!", "Error!", MB_OK);
        PostQuitMessage (0);
        return;
    }

    return;
}


int WINAPI WinMain (HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    WNDCLASSEX wc;
    HWND hWnd;
    MSG Msg;
    int length, width;

    //Register the window, create the window, and check for errors
    {
        //Window class
        wc.cbSize        = sizeof (WNDCLASSEX);
        wc.style         = 0;
        wc.lpfnWndProc   = MainWndProc;
        wc.cbClsExtra    = 0;
        wc.cbWndExtra    = 0;
        wc.hInstance     = hInstance; //Instance used to create the window
        wc.hIcon         = LoadIcon (hInstance, MAKEINTRESOURCE (IDI_CODERICON)); //Change here to change large application icon
        wc.hCursor       = LoadCursor (hInstance, IDC_ARROW); //Change here to change application cursor
        wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1); //Change here to change starting background color (white)
        wc.lpszMenuName  = NULL;
        wc.lpszClassName = Coder::Constants::MAINWNDCLASSNAME;
        wc.hIconSm       = (HICON)LoadImage (hInstance, MAKEINTRESOURCE (IDI_CODERICON), IMAGE_ICON, GetSystemMetrcs (SM_CXSMICON), GetSystemMetrics (SM_CYSMICON), 0); //Change here to change application icon (small, as in taskbar)

        //Register the window class so that the ClassName can be used to create the window and then check for error
        if (!RegisterClassEx (&wc))
        {
            MessageBox (NULL, "Main window registration failed!", "Error!", MB_OK);
            return 0;
        }

        //Get screen dimensions
        length = GetSystemMetrics (SM_CXSCREEN);
        width = GetSystemMetrics (SM_CYSCREEN);

        //Create window
        hWnd = CreateWindowEx (WS_EX_CLIENTEDGE, /*Border*/
            Coder::Constants::MAINWNDCLASSNAME,
            Coder::Constants::MAINWNDTITLETEXT, /*Title of window*/
            WS_OVERLAPPED | WS_SYSMENU | WS_MINIMIZEBOX | WS_THICKFRAME | WS_CLIPCHILDREN, /*Window style*/
            (length - Coder::Constants::XDEFAULT - Coder::Constants::XSHIFT) / 2, (width - Coder::Constants::YDEFAULT - Coder::Constants::YSHIFT) / 2, /*Top-left x and y coordinates*/
            Coder::Constants::XDEFAULT + Coder::Constants::XSHIFT, Coder::Constants::YDEFAULT + Coder::Constants::YSHIFT, /*Lengths and widths of window*/
            NULL, NULL, 
            hInstance, NULL);

        //Check for error in window creation
        if (hWnd == NULL)
        {
            MessageBox (NULL, "Main window creation failed!", "Error!", MB_OK);
            return 0;
        }
    }

    //Call function
            Init (hWnd);

    //Show and redraw the window
    ShowWindow (hWnd, nCmdShow);
    UpdateWindow (hWnd);

    //Put window in message loop
    while (GetMessage (&Msg, NULL, 0, 0) > 0) //While not terminated
    {
        TranslateMessage (&Msg);
        DispatchMessage (&Msg);
    }

    //Return
    return static_cast<int>(Msg.wParam);
}

回答1:

Every window procedure MUST return DefWindowProc(hwnd,msg,wparam,lparam) on every message, with exceptions on messages like WM_ERASEBKGND and WM_PAINT when you are using your custom painting procedure (you must return 0 so the OS doesn't the repaint the window with it's default procedure)

来源：https://stackoverflow.com/questions/11105507/createwindowex-returns-null