C# WPF grab screenshot with SnippingTool effect

Question

I'm trying to integrate a screenshot grabbing feature in my WPF app and I'd like it to look like snipping tool.

So far I've managed accomplish something similar by creating a fullscreen window (with a canvas) with opacity set to 0.5 and dark background. When I click somewhere and start dragging, a white rectangle is drawn, generating an effect similar to this.

What I'd like to have is the inner part of that rectangle opening a opacity hole in the background canvas, so that I could see through the selected area - just like snipping tool.

Problem is, being fairly new to .NET, I have no idea how or where to start. Did some research and tests on the OpacityMask field of the screenshot window but got nowhere.

Here's a little vid to show the current effect.

Edit: Also, as bonus question, is there an easy way to grab a screenshot that spans across multiple monitors (virtual screen)? Graphics.CopyFromScreen() only seems to work for 1 screen.
Already fixed this and seems to work for all possible weird virtual desktop layouts:

// Capture screenie (rectangle is the area previously selected
double left = Canvas.GetLeft(this.rectangle);
double top = Canvas.GetTop(this.rectangle);

// Calculate left/top offset regarding to primary screen (where the app runs)
var virtualDisplay = System.Windows.Forms.SystemInformation.VirtualScreen;
var primaryScreen = System.Windows.Forms.Screen.PrimaryScreen.Bounds;
if (virtualDisplay.Left < primaryScreen.Left)
{
    left -= Math.Abs(virtualDisplay.Left - primaryScreen.Left);
}
if (virtualDisplay.Top < primaryScreen.Top)
{
    top -= Math.Abs(virtualDisplay.Top - primaryScreen.Top);
}

Answer 1

You can have a CombinedGeometry with GeometryCombineMode="Exclude" creating a "punched" effect. Sample:

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" AllowsTransparency="True" 
    WindowStyle="None" Background="Transparent">
    <Canvas >
        <Path Stroke="Black" Fill="White" Opacity=".5">
            <Path.Data>
                <CombinedGeometry GeometryCombineMode="Exclude">
                    <CombinedGeometry.Geometry1>
                        <RectangleGeometry Rect="0,0,800,600" >
                        </RectangleGeometry>
                    </CombinedGeometry.Geometry1>
                    <CombinedGeometry.Geometry2>
                        <RectangleGeometry  Rect="50,50,100,100" >
                        </RectangleGeometry>
                    </CombinedGeometry.Geometry2>
                </CombinedGeometry>
            </Path.Data>
        </Path>
    </Canvas>
</Window>