title: Error when Databinding with mode=twoway
body: <p>All,
    I'm pounding my head against the wall here. What I need is simple, and I'm sure there is a simple answer, but I can't seem to find it.
  Situation: I have a Silver light 4.0 app and I'm currently binding a list of strings to an Items control. In the data template for that I have a simple text box that I was doing very basic Binding "{Binding}". I need to update the binding to be twoway so the edits are automatically pushed back to my datacontext. </p>
  
  <p>Here is the Items control before I update the binding:</p>
  
  <pre><code>&lt;ItemsControl x:Name="spLiftHeader" ItemsSource="{Binding Path=WeekLabels}"&gt;
     &lt;ItemsControl.ItemsPanel&gt;
          &lt;ItemsPanelTemplate&gt;
               &lt;StackPanel x:Name="spLiftHeader" Orientation="Horizontal" /&gt;
          &lt;/ItemsPanelTemplate&gt;
     &lt;/ItemsControl.ItemsPanel&gt;
     &lt;ItemsControl.ItemTemplate&gt;
           &lt;DataTemplate&gt;
                **&lt;TextBox x:Name="txtWeekLbl" Text="{Binding}" Foreground="Black" Width="125" TextAlignment="Center"/&gt;**
           &lt;/DataTemplate&gt;
      &lt;/ItemsControl.ItemTemplate&gt;
  &lt;/ItemsControl&gt;
  </code></pre>
  
  <p>Here is the items control after the binding change:</p>
  
  <pre><code>&lt;ItemsControl x:Name="spLiftHeader" ItemsSource="{Binding Path=WeekLabels}"&gt;
     &lt;ItemsControl.ItemsPanel&gt;
          &lt;ItemsPanelTemplate&gt;
                &lt;StackPanel x:Name="spLiftHeader" Orientation="Horizontal" /&gt;
          &lt;/ItemsPanelTemplate&gt;
     &lt;/ItemsControl.ItemsPanel&gt;
     &lt;ItemsControl.ItemTemplate&gt;
          &lt;DataTemplate&gt;
              **&lt;TextBox x:Name="txtWeekLbl" Text="{Binding Mode=TwoWay}" Foreground="Black" Width="125" TextAlignment="Center"/&gt;**
          &lt;/DataTemplate&gt;
      &lt;/ItemsControl.ItemTemplate&gt;
  &lt;/ItemsControl&gt;
  </code></pre>
  
  <p>I've just simply added the "Mode=TwoWay" to the binding.
  After updating that, I get the amazingly useless error 4004 
  "System.Windows.Markup.XamlParseException: Provide value on 'System.Windows.Data.Binding' threw an exception" and the Line/Position reference points right do my updated Binding. 
  How does one add the Two Way mode to the simple binding?
  Thanks in advance.
  Nick</p>