title: WPF Bind IsEnabled to Listbox SelectedItem
body: <p>Using MVVM style I have successfully bound an <code>ObservableCollection&lt;string&gt;</code> to a <code>ListBox</code>, showing up the values as <code>RadioButton</code>s. The control behaves exactly as expected.</p>
  
  <p>Now I have an issue regarding some <code>TextBox</code>es bound to this <code>ListBox</code>: I want whenever the <code>SelectedItem</code> in the <code>ListBox</code> is equal to a specific value (e.g. <code>ValueForEnabled</code>) the <code>TextBox</code>es to be enabled otherwise they should be disabled.</p>
  
  <p>I know I have to bind to <code>SeletedItem</code> of the <code>ListBox</code> (named <code>lbSource</code>) but how exactly is this done? </p>
  
  <p>I want something like this (Pseudo code):</p>
  
  <pre><code>&lt;TextBox  ...
  
      IsEnabled="{Binding ElementName=lbSource, Path=SelectedItem='ValueForEnabled',
                  Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" 
                  ...            
  /&gt;
  </code></pre>