Jake Ginnivan’s blog

Taking Screenshots of WPF Apps

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:

Blank Modal Dialogue

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:

Working modal screenshot

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