title: WPF binding with StringFormat doesn't work on ToolTips
body: <p>The following code has a simple binding which binds the Text of the TextBlock named MyTextBlock to TextBox's Text and ToolTip property using the exact same Binding notation:</p>
  
  <pre><code>&lt;StackPanel&gt;
      &lt;TextBlock x:Name="MyTextBlock"&gt;Foo Bar&lt;/TextBlock&gt;
      &lt;TextBox    Text="{Binding ElementName=MyTextBlock, Path=Text, StringFormat='It is: \{0\}'}"
               ToolTip="{Binding ElementName=MyTextBlock, Path=Text, StringFormat='It is: \{0\}'}" /&gt;
  &lt;/StackPanel&gt;
  </code></pre>
  
  <p>The binding also uses the <a href="http://blogs.msdn.com/llobo/archive/2008/05/19/wpf-3-5-sp1-feature-stringformat.aspx" rel="noreferrer">StringFormat property introduced with .NET 3.5 SP1</a> which is working fine for the above Text property but seems to be broken for the ToolTip. The expected result is "It is: Foo Bar" but when you hover over the TextBox, the ToolTip shows only the binding value, not the string formatted value. Any ideas?</p>