title: Binding the content of a Span
body: <p>I have a window that displays text.  There are two parts to the text: the first is fixed while the second needs to be the content of a DependencyProperty declared on the window.</p>
  
  <p>I considered using a TextBlock containing two Spans, the first of which contains the fixed content and the second of which contains the variable content, but I can't see anything obvious on the Span class that would allow me to bind to the aforementioned DependencyProperty.</p>
  
  <p>I'm currently using two Labels stacked side by side but this is ugly and doesn't help me if I wish to retrieve the content of the entire text block (as I do when displaying a ToolTip in the event that the window is too narrow to display the entire text block).</p>
  
  <p>Can anyone help me to crack this seemingly simple problem?  Thanks.</p>