How to get the position of a user control?

I have an application. I want to show a ballon(UserControl) after I click a button on window. Th开发者_如何学编程e ballon must appear under the button. How to set the position to ballon using the position on button

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button is your button, root is your window and coordinates is the coordinates of button relative to root.

Point coordinates = button.TransformToAncestor(root).Transform(new Point(0, 0));

Take a loot at TransformToAncestor, it works for any visual.