title: XAML Binding to the opposite of another element
body: <p>I am developing a simple exercise and want to know if there is a way to bind to the opposite of another element using only XAML. For example. I have two buttons on a form, Start and Stop perhaps for a timer. I dont want both to be enables at the same time. When the program starts, the stop button should be disabled. When the start button is clicked, it should be disabled and the stop button enabled.  Then vice versa until the user quits.</p>
  
  <p>I know there are better controls for exactly this, and I know it is easy to do in code. I just wanted to know if there is some sort of NOT operator in XAML that could look like this:</p>
  
  <pre><code>&lt;Button Content="_Start" Name="btnStart"/&gt;
  &lt;Button Content="_Stop" Name="btnStop"
          IsEnabled="{Binding ElementName=btnStart,
                      Path=Not(IsEnabled)}"/&gt;
  </code></pre>
  
  <p>Or something like:</p>
  
  <pre><code>                    Path != IsEnabled}"/&gt;
  </code></pre>
  
  <p>Or even something like:</p>
  
  <pre><code>                    Path=IsEnabled.Not()}"/&gt;
  </code></pre>
  
  <p>I know I can bind directly to the start button, but when one is enabled so is the other, and likewise when it is disabled. See where I am comming from.</p>
  
  <p>I would be even willing to entertain some sort of XAML Validation thehnique which will first check the state of the start button and force the state of the stop one to be the opposite. Or any other similar hacks or workarounds that dont require codebehind.</p>
  
  <p>Thanks for any help on this.</p>
  
  <p>EDITS:
  Remember the key point for this is NO code behind. If I have to write a single line of code behind I might as well just use the onclick event handler and a simple ?: operator. So I want only something that will work directly in XAML.</p>