Please see my comment to the question. Is that what you need?
If so, one of the way to do it is to assign some instance of
System.Windows.Media.Geometry of non-rectangular shape to the property
System.Windows.Window.VisualClip:
http://msdn.microsoft.com/en-us/library/system.windows.window.aspx[
^],
http://msdn.microsoft.com/en-us/library/system.windows.media.visual.visualclip.aspx[
^],
http://msdn.microsoft.com/en-us/library/system.windows.media.geometry.aspx[
^].
This method is the cleanest one: not only a window looks non-rectangular visually (like it would look with transparency), but is also transparent to mouse events, for example: you click mouse in the corner; and the window of the application beneath is activated — this is something you really need.
It's also good to use
WindowStyle="None"; to avoid showing non-client areas of the window.
Another way is related to
Background,
AllowsTransparency, and
WindowStyle properties:
http://msdn.microsoft.com/en-us/library/ms748948.aspx[
^] (read: "Non-Rectangular Window Style"),
http://msdn.microsoft.com/en-us/library/aa358516%28v=vs.85%29.aspx[
^].
Good luck,
—SA
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