You need the following Polygon
elements in the Button
element in your XAML code:
<Polygon x:Name="TopTriangle"
Points="0,0 15,0 15,15"
Fill="Green"
Stroke="Cyan"
HorizontalAlignment="Right"
VerticalAlignment="Top"
Margin="3"/>
<Polygon x:Name="BottomTriangle"
Points="0,0 0,15 15,15"
Fill="Cyan"
Stroke="Cyan"
HorizontalAlignment="Left"
VerticalAlignment="Bottom"
Margin="3"/>
Consider inverted cartesian product, top triangle points are:
And bottom triangle points are: