Question

I m new to using WPF and GDI, and I m having trouble displaying an image. My eventual goal is to build something expose-like. So far, I gray-out the screens, gather all the active HWNDs, and capture the screens of all the windows. For now, I have a single Image element that I try to set the source for, but nothing ever shows up.

foreach (IntPtr hwnd in hwndlist)
{
    IntPtr windowhdc = GetDC((IntPtr) hwnd);
    IntPtr bmap = CreateBitmap(400, 400, 1, 32, null);
    IntPtr bitmaphdc = GetDC(bmap);
    BitBlt(bitmaphdc, 0, 0, System.Convert.ToInt32(this.Width), System.Convert.ToInt32(this.Height), windowhdc, 0, 0, TernaryRasterOperations.SRCCOPY);
    ReleaseDC(hwnd, windowhdc);
    BitmapSource bmapSource = System.Windows.Interop.Imaging.CreateBitmapSourceFromHBitmap(bmap, IntPtr.Zero, Int32Rect.Empty, System.Windows.Media.Imaging.BitmapSizeOptions.FromEmptyOptions());
    image1.Source = bmapSource;
    image1.BeginInit();
}

The complete code is here:
http://pastebin.com/m70af590 - code
http://pastebin.com/m38966750 - xaml

I know the way I have it now doesn t make much sense for what I m trying to do (running the loop and just writing to the same Image over and over), but I m hoping to have something on that Image by the end.

I ve tried hard coding the HWND of a visible window, and it still won t work.

Thanks for any help!

Answer 1

I think working with a Memory DC will solve your problem. To do this, first import:

[DllImport("gdi32.dll", EntryPoint = "CreateCompatibleDC")]
public static extern IntPtr CreateCompatibleDC(IntPtr hdc);

[DllImport("gdi32.dll", EntryPoint = "SelectObject")]
public static extern IntPtr SelectObject(IntPtr hdc, IntPtr bmp);

[DllImport("gdi32.dll",EntryPoint="DeleteDC")]
public static extern IntPtr DeleteDC(IntPtr hDc);

And instead of this:

IntPtr bitmaphdc = GetDC(bmap);
BitBlt(bitmaphdc, 0, 0, System.Convert.ToInt32(this.Width), System.Convert.ToInt32(this.Height), windowhdc, 0, 0, TernaryRasterOperations.SRCCOPY);

Do this:

IntPtr memdc = CreateCompatibleDC(windowhdc);
SelectObject(memdc, bmap);

BitBlt(memdc, 0, 0, System.Convert.ToInt32(this.Width), System.Convert.ToInt32(this.Height), windowhdc, 0, 0, TernaryRasterOperations.SRCCOPY);

Don t forget to delete the Memort DC later:

DeleteDC(memdc);

And BTW, you don t need image1.BeginInit();.

Also to check, you do not need to enumerate all windows. Use GetDesktopWindow method from user32.dll instead.

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