title: Can't set focus to a child of UserControl
body: <p>I have a <code>UserControl</code> which contains a <code>TextBox</code>.  When my main window loads I want to set the focus to this textbox so I added <code>Focusable="True" GotFocus="UC_GotFocus"</code> to the <code>UserControl</code>s definition and <code>FocusManager.FocusedElement="{Binding ElementName=login}"</code> to my main windows definition.  In the <code>UC_GotFocus</code> method i simply call <code>.Focus()</code> on the control i want to focus on but this doesn't work.</p>
  
  <p>All i need to do is have a <code>TextBox</code> in a <code>UserControl</code> receive focus when the application starts.</p>
  
  <p>Any help would be appreciated, thanks.</p>