使用兼容的dc将输出HBITMAP与BitBlt一起使用

Question

此代码不起作用。它必须将屏幕复制到程序窗口。

#include <windows.h>
LRESULT CALLBACK WndProc(HWND hwnd,UINT iMsg,WPARAM wParam,LPARAM lParam);
int WINAPI WinMain(HINSTANCE hInstance, 
    HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)
{
    static wchar_t szAppName[]=TEXT("App");
    HWND hwnd;
    MSG msg;
    WNDCLASSEX wndclass;

    wndclass.cbSize=sizeof(wndclass);
    wndclass.style=CS_HREDRAW|CS_VREDRAW;
    wndclass.lpfnWndProc=WndProc;
    wndclass.cbClsExtra=0;
    wndclass.cbWndExtra=0;
    wndclass.hInstance=hInstance;
    wndclass.hIcon=LoadIcon(NULL,IDI_APPLICATION);
    wndclass.hCursor=LoadCursor(NULL,IDC_ARROW);
    wndclass.hbrBackground=(HBRUSH)GetStockObject(WHITE_BRUSH);
    wndclass.lpszMenuName=NULL;
    wndclass.lpszClassName=szAppName;
    wndclass.hIconSm=LoadIcon(NULL,IDI_APPLICATION);

    RegisterClassEx(&wndclass);

    hwnd=CreateWindow(szAppName,szAppName,WS_OVERLAPPEDWINDOW,CW_USEDEFAULT,
        CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,NULL,NULL,hInstance,NULL);
    ShowWindow(hwnd,iCmdShow);
    UpdateWindow(hwnd);

    SetTimer(hwnd,0,100,NULL);

    while(BOOL lol=GetMessage(&msg,NULL,0,0))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}


HDC cdc;
HBITMAP hBmp;
HDC scr=GetDC(0);
LRESULT CALLBACK WndProc(HWND hwnd,UINT iMsg,WPARAM wParam,LPARAM lParam)
{
    HDC hdc;
    PAINTSTRUCT ps;
    switch(iMsg)
    {
    case WM_CREATE:
        cdc=CreateCompatibleDC(scr);
        hBmp=CreateCompatibleBitmap(scr,500,500);
        SelectObject(cdc,hBmp);
        return 0;
    case WM_PAINT:
        {
            hdc=BeginPaint(hwnd,&ps);
            EndPaint(hwnd,&ps);
            return 0;
        }
    case WM_TIMER:
        {
            hdc=GetDC(hwnd);
            BitBlt(cdc,0,0,500,500,scr,0,0,SRCCOPY);
            HDC compatibleDC=CreateCompatibleDC(scr);
            SelectObject(compatibleDC,hBmp);
            BitBlt(hdc,0,0,500,500,compatibleDC,0,0,SRCCOPY);
            return 0;
        }
    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }
    return DefWindowProc(hwnd,iMsg,wParam,lParam);
}

内部处理WM_TIMER有行BitBlt(hdc,0,0,500,500,compatibleDC,0,0,SRCCOPY);，我使用compatibleDC将位图从它复制到窗口的DC。但是，复制不会发生。为什么会这样。我同时在两个DC中选择HBITMAP。可能吗？或是导致这种行为的错误？