title: IMultiValueConverter ConvertBack problem
body: <p>I'm using wpf + mvvm and am trying to implement a conditional converter.  Here is what I'm doing in the xaml:</p>
  
  <pre><code>                &lt;CheckBox.IsChecked&gt;
                      &lt;MultiBinding Converter="{StaticResource pageSourceConverter}"&gt;
                          &lt;Binding Path="CurrentPage.Source"/&gt;
                          &lt;Binding Path="Project.Type1.MachineTypes.Rotating"/&gt;
                          &lt;Binding Path="Project.Type2.MachineTypes.Rotating" /&gt;
                          &lt;Binding Path="Project.Type3.MachineTypes.Rotating" /&gt;
                          &lt;Binding Path="Project.Type4.MachineTypes.Rotating" /&gt;
                      &lt;/MultiBinding&gt;
                  &lt;/CheckBox.IsChecked&gt;
  </code></pre>
  
  <p>And the MultiConverter:</p>
  
  <pre><code>public class PageSourceConverter : IMultiValueConverter
  
    {
        public object Convert(object[] values, Type targetType, object parameter,          CultureInfo culture)
        {
  
            String pageSource = values[0] as String;
  
            if (pageSource == "Type1")
                return values[1];
            else if (pageSource == "Type2")
                return values[2];
            else if (pageSource == "Type3")
                return values[3];
            else if (pageSource == "Type4")
                return values[4];
            else
                return null;
       }
  
       public object[] ConvertBack(object value, Type[] targetType, object parameter, CultureInfo culture)
       {
           return null;
       }
   }
  </code></pre>
  
  <p>So what I want to do is conditionally bind an element on a page to different Models I have in the viewController based on the page type the user is on.  </p>
  
  <p>What do I need to do for the convertback?  I don't know how to tell which element in the return array the value belongs to.  Any ideas?</p>