title: Setting ViewModel's Property from XAML
body: <p>I have some <code>UserControl</code>, It's <code>DataContext</code> is binded to the ViewModel,
  How to set a ViewModel's property from <code>XAML</code>? Is it possible?</p>
  
  <p>UPD : 
  Sorry for being not very clear,
  I'm trying to get something like this : 
  UserControl's DataContext is binded to ViewModel, I need to set ViewModel's property to something (let's say, UserControl's Width property).
  Is it possible?</p>
  
  <p>UPD2: It seems to be not possible.I know about TwoWay binding mode, etc, thing I wanted to do - to set ViewModel's property to UserControl's one</p>
  
  <p>This example should be very clear</p>
  
  <pre><code>&lt;Set Property={Binding SomePropertyOnViewModel} 
       Value={Binding RelativeSource={RelativeSource Self}, 
                      Path=SomePropertyOnUserControl}&gt;
  </code></pre>