title: Getting the top left coordinates of a WPF UIElement
body: <p>I am working on extending the Microsoft resize <code>Adorner</code> example and need to be able to reposition the element after say the bottom left drag handle has been dragged.</p>
  
  <p>So if I have a textbox of say 150 wide, 35 high postitioned on my form, and the bottom left drag handle changes the width to 200 wide, the right hand of the text box remains unchanged but the left hand edge moves to the left.</p>
  
  <p>So I need to know the top left coordinates of the UIElement. I have tried Canvas.GetLeft and Canvas.GetTop but they return NaN which is confusing.</p>
  
  <p>I just tried VisualTreeHelper.GetOffset which does return an offset but when you try and use it in the arrange method of the element it disappears, presumably as the values in the offset are too high.</p>
  
  <p>In the days before Wpf the coordinate system was quite simple, wpf has overcomplicated things I think. </p>