How to set the focus on a user control item in WPF?

I have a WPF app that uses different instances of a User Control; when the user clicks a button on the mai开发者_StackOverflow中文版n window I need to set the focus on the textbox inside the currently active instance of the user control.

Which is the best way to set the focus on that child item?

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child.Focus();

or

VisualTreeHelper.GetChild(parent, 0).Focus(); // you might want to test it for nulls