Saving a screenshot of a window using C#, WPF, and DWM

Question

This is a follow up question to this question

The solution to the above uses DWM to display a thumbnail of an active window. If I understand correctly, it works by letting you specify the window handle of the application you want to view and then having you provide a window handle and a location on that window where windows should draw the contents of the target Window.

Is there a way to render the window screen shot directly to BitmapImage or Image instead of directly drawing it somewhere in your window? (Basically to just grab a screen shot of the window - even if it's covered by another window - with out using an updating thumbnail.)

Thanks for you help!

Answer 1

The Control.DrawToBitmap doesn't always work so I resorted to the following native API calls that provide more consistent results:

The Utilities class. Call Utilities.CaptureWindow(Control.Handle) to capture a specific control:

public static class Utilities
{
    public static Image CaptureScreen()
    {
        return CaptureWindow(User32.GetDesktopWindow());
    }

    public static Image CaptureWindow(IntPtr handle)
    {

        IntPtr hdcSrc = User32.GetWindowDC(handle);

        RECT windowRect = new RECT();
        User32.GetWindowRect(handle, ref windowRect);

        int width = windowRect.right - windowRect.left;
        int height = windowRect.bottom - windowRect.top;

        IntPtr hdcDest = Gdi32.CreateCompatibleDC(hdcSrc);
        IntPtr hBitmap = Gdi32.CreateCompatibleBitmap(hdcSrc, width, height);

        IntPtr hOld = Gdi32.SelectObject(hdcDest, hBitmap);
        Gdi32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, ApiConstants.SRCCOPY);
        Gdi32.SelectObject(hdcDest, hOld);
        Gdi32.DeleteDC(hdcDest);
        User32.ReleaseDC(handle, hdcSrc);

        Image image = Image.FromHbitmap(hBitmap);
        Gdi32.DeleteObject(hBitmap);

        return image;
    }
}

The Gdi32 class:

public class Gdi32
{
    [DllImport("gdi32.dll")]
    public static extern bool BitBlt(IntPtr hObject, int nXDest, int nYDest, int nWidth, int nHeight, IntPtr hObjectSource, int nXSrc, int nYSrc, int dwRop);
    [DllImport("gdi32.dll")]
    public static extern IntPtr CreateCompatibleBitmap(IntPtr hDC, int nWidth, int nHeight);
    [DllImport("gdi32.dll")]
    public static extern IntPtr CreateCompatibleDC(IntPtr hDC);
    [DllImport("gdi32.dll")]
    public static extern bool DeleteDC(IntPtr hDC);
    [DllImport("gdi32.dll")]
    public static extern bool DeleteObject(IntPtr hObject);
    [DllImport("gdi32.dll")]
    public static extern IntPtr SelectObject(IntPtr hDC, IntPtr hObject);
}

The User32 class:

public static class User32
{
    [DllImport("user32.dll")]
    public static extern IntPtr GetDesktopWindow();
    [DllImport("user32.dll")]
    public static extern IntPtr GetWindowDC(IntPtr hWnd);
    [DllImport("user32.dll")]
    public static extern IntPtr GetWindowRect(IntPtr hWnd, ref RECT rect);
    [DllImport("user32.dll")]
    public static extern IntPtr ReleaseDC(IntPtr hWnd, IntPtr hDC);
}

The constants used:

    public const int SRCCOPY = 13369376;

The structs used:

[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    public int left;
    public int top;
    public int right;
    public int bottom;
}

A friendly Control extension method:

public static class ControlExtensions
{
    public static Image DrawToImage(this Control control)
    {
        return Utilities.CaptureWindow(control.Handle);
    }
}