title: Is it possible to update a databinding source value with the set TargetNullValue in case it is 'null'?
body: <p>Just out of interest....</p>
  
  <p>In case I have a ViewModel with an uninitialized string, which is bound to a Textbox, I can use <code>TargetNullValue</code> to display a default value. 
  However, I was wondering if I can use the same value to update the string in case it is <code>null</code>?</p>
  
  <p>Basically instead of </p>
  
  <pre><code>    set
      {
          if(value != null) text = value;
          else value = "defaultstring";
          OnPropertyChanged();
      }  
  </code></pre>
  
  <p>just do the same thing from the databinding using <code>TargetNullValue</code>. </p>