PrintWindow Returns null data, Works in C++

Question

I have successfully gotten PrintWindow to work in C++, but when I try to do it in C#, I get back a bitmap with null data. Using VS 2012 Pro, on .NET 4.0. Here is my code:

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool GetWindowRect(IntPtr hWnd, out RECT lpRect);

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool PrintWindow(IntPtr hWnd, IntPtr hdcBlt, int nFlags);

    [DllImport("user32.dll")]
    public static extern ushort getLastError();

    static int count = 0;
    public Bitmap getWindow(IntPtr windowHandle)
    {
        RECT rect;
        Window.GetWindowRect(windowHandle, out rect);

        Console.WriteLine(rect.X);
        Console.WriteLine(rect.Y);
        Console.WriteLine(rect.Width);
        Console.WriteLine(rect.Height);
        Console.WriteLine();

        Bitmap bmp = new Bitmap(rect.Width, rect.Height);
        Graphics gfxBmp = Graphics.FromImage(bmp);
        IntPtr hdcBitmap = gfxBmp.GetHdc();

        bool success = PrintWindow(windowHandle, hdcBitmap, 0);
        if (!success)
        {
            Console.WriteLine("Error copying image");
            Console.WriteLine(getLastError());
        }

        bmp.Save("image_" + count++ + ".bmp");

        gfxBmp.ReleaseHdc(hdcBitmap);
        gfxBmp.Dispose();

        return bmp;

    }

Here is the output:

182
182
664
533

No error message printed and it looks like the handle is valid since the RECT bounds are correct. Following this example I got it working in C++ with the calculator program, but the same code with the calculator doesn't work in C#.

I need it to work in C#.

Answer 1

As requested :-) Try to place the ReleaseHdc directly after the PrintWinow() call.

See here.

I have to guess for the reason, but it might be something like "as long as the HDC is locked by unmanaged code, managed code cannot access it. Therefore the HDC must be released to see the bitmap in C#"