title: Set value to null in WPF binding
body: <p>please take a look at the following line</p>
  
  <pre><code>&lt;TextBox Text="{Binding Price}"/&gt;
  </code></pre>
  
  <p>This Price property from above is a <code>Decimal?</code> (Nullable decimal).</p>
  
  <p>I want that if user deletes the content of the textbox (i.e. enters empty string, it should automatcally update source with null (Nothing in VB).</p>
  
  <p>Any ideas on how I can do it 'Xamly'?</p>