对被遮挡窗口的拷屏

　　使用BitBlt需要把目标程序切到前台才可以正确拷屏，而PrintWindow就没这个限制了。

        //首先是获取目标组件的Handle，这个不用看，每个程序都不一样的
        HWND hMain, hSec, hThir;
        hMain = FindWindow("TFormMain", "窗口标题");
        hSec  = FindWindowEx(hMain, NULL, "TGroupBox", NULL);
        for(int i=1; i<6; i++)
        {
            hThir = FindWindowEx(hSec, NULL, NULL, NULL);
            if (hThir==NULL)
                break;
            hSec  = FindWindowEx(hMain, hSec, "TGroupBox", NULL);
        }

        //这里开始加载DLL了，整个拷屏操作的核心就是使用PrintWindow
        HINSTANCE Hdl;
        bool __stdcall (*PrintWindow)(HWND, HDC, UINT);     //定义函数原型
        Hdl = ::LoadLibrary("user32.dll");                  //载入DLL
        if(Hdl != NULL)
        {
            PrintWindow = (bool __stdcall (*)(HWND,HDC, UINT))::GetProcAddress(Hdl,"PrintWindow");     //获取函数入口地址
            if(PrintWindow != NULL)
            {
                RECT rc;
                ::GetClientRect(hSec, rc);

                HDC hdcScreen = GetDC(this->Handle);
                HDC memdc = CreateCompatibleDC(hdcScreen);

                HBITMAP hbmp = CreateCompatibleBitmap(hdcScreen, rc.right - rc.left, rc.bottom - rc.top);

                SelectObject(memdc, hbmp);
                //复制
                PrintWindow(hSec, memdc, 0);
                //此处是直接把拷屏的内容直接在img1中输出了
                BitBlt(img1->Canvas->Handle, 2, 2,  11, 18, memdc, 280, 16, SRCCOPY);
                img1->Refresh();
                //释放
                DeleteDC(memdc);
                DeleteObject(hbmp);
                ReleaseDC(this->Handle, hdcScreen);
            } else
            {
                ShowMessage("不能找到函数入口!");
            }
            ::FreeLibrary(Hdl);     //一定不要忘记调用完毕后释放DLL
        }
        else
        {
            ShowMessage("不能载入DLL!");
        }