In my current project, we have quite an awesome UI automation setup running through every process (happy days path) in our WPF app.

Often we cannot resolve a UI element, and we will get an error like:

White.Core.AutomationException: Failed to get UIElement ControlType=button,AutomationId=OkButton from DialogWindow

While this is great, often it is hard to figure out why this happens, could be due to a validation error on the previous screen, or a error popup etc. So out infrastructure will automatically take a screenshot whenever it catches a AutomationException.

The problem is the modal window that was often captured (and was blocking a button press on the main window) was coming out as a tiny little square with no contents:

I began digging, then realised that using GDI+ to take a screenshot of a WPF app doesn’t seem to always work. It worked fine on my machine, but not our build machine.

After scouring the internet I put a all the solutions i could find into one big LinqPad script and ran it (this was my original code, so it went first):

var bounds = System.Windows.Forms.Screen.GetBounds(Point.Empty);
var bitmap = new Bitmap(bounds.Width, bounds.Height);
using (var g = Graphics.FromImage(bitmap))
{
    g.CopyFromScreen(Point.Empty, Point.Empty, bounds.Size);
}

bitmap.Dump();

.Dump() by the way works awesome with Bitmaps as well as everything else under the sun..

The class that ended up working for me is this:

public class ScreenCapture
{
    [DllImport("gdi32.dll")]
    static extern bool BitBlt(IntPtr hdcDest, int xDest, int yDest, int wDest, int hDest, IntPtr hdcSource, int xSrc, int ySrc, CopyPixelOperation rop);
    [DllImport("user32.dll")]
    static extern bool ReleaseDC(IntPtr hWnd, IntPtr hDc);
    [DllImport("gdi32.dll")]
    static extern IntPtr DeleteDC(IntPtr hDc);
    [DllImport("gdi32.dll")]
    static extern IntPtr DeleteObject(IntPtr hDc);
    [DllImport("gdi32.dll")]
    static extern IntPtr CreateCompatibleBitmap(IntPtr hdc, int nWidth, int nHeight);
    [DllImport("gdi32.dll")]
    static extern IntPtr CreateCompatibleDC(IntPtr hdc);
    [DllImport("gdi32.dll")]
    static extern IntPtr SelectObject(IntPtr hdc, IntPtr bmp);
    [DllImport("user32.dll")]
    public static extern IntPtr GetDesktopWindow();
    [DllImport("user32.dll")]
    public static extern IntPtr GetWindowDC(IntPtr ptr);

    public Bitmap CaptureScreenShot()
    {
        var sz = System.Windows.Forms.Screen.PrimaryScreen.Bounds.Size;
        var hDesk = GetDesktopWindow();
        var hSrce = GetWindowDC(hDesk);
        var hDest = CreateCompatibleDC(hSrce);
        var hBmp = CreateCompatibleBitmap(hSrce, sz.Width, sz.Height);
        var hOldBmp = SelectObject(hDest, hBmp);
        BitBlt(hDest, 0, 0, sz.Width, sz.Height, hSrce, 0, 0, CopyPixelOperation.SourceCopy | CopyPixelOperation.CaptureBlt);
        var bmp = Image.FromHbitmap(hBmp);
        SelectObject(hDest, hOldBmp);
        DeleteObject(hBmp);
        DeleteDC(hDest);
        ReleaseDC(hDesk, hSrce);

        return bmp;
    }
}

And now I have: