VS C++改变窗体背景色

static void changeBackground(HWND hwnd)
{
   /*
    RECT rect;
    GetClientRect(hwnd, &rect);
    int cxClient = rect.right;
    int cyClient = rect.bottom;
  */
    HDC hdc = GetDC(hwnd);//获取当前的显示设备上下文
    int w = 200;
    int h = 100;
    char* rgb = (char*)malloc(3*w*h);
    for (int i=0; i<w*h; i++)
    {
        rgb[i*3+0] = 0x12;
        rgb[i*3+1] = 0x78;
        rgb[i*3+2] = 0x34;
    }
    HDC  hdcsource = CreateCompatibleDC(NULL);//创建存放图象的显示缓冲
    HBITMAP bitmap = CreateCompatibleBitmap(hdc, w, h);
    SelectObject(hdcsource, bitmap); //将位图资源装入显示缓冲
    SetStretchBltMode(hdcsource, COLORONCOLOR);
    BITMAPINFO bmi;
    bmi.bmiHeader.biSize         = sizeof(BITMAPINFOHEADER);
    bmi.bmiHeader.biWidth        = w;
    bmi.bmiHeader.biHeight       = h;
    bmi.bmiHeader.biCompression  = BI_RGB;
    bmi.bmiHeader.biBitCount     = 24;
    bmi.bmiHeader.biPlanes       = 1;
    bmi.bmiHeader.biClrUsed      = 0;
    bmi.bmiHeader.biClrImportant = 0;
    bmi.bmiHeader.biSizeImage    = 0;
    StretchDIBits(hdcsource,
        0, 0, w, h,
        0, 0, w, h,
        rgb, &bmi,DIB_RGB_COLORS, SRCCOPY);
    BitBlt(hdc,0,0,w,h, hdcsource,0,0,SRCCOPY);//将图象显示缓冲的内容直接显示到屏幕
    DeleteObject(bitmap);
    DeleteDC(hdcsource);
    ReleaseDC(hwnd, hdc);
    free(rgb);
}