title: Custom DependencyProperty in ObservableCollection<t>
body: <p>How add custom DependencyProperty? This method does not work. </p>
  
  <pre><code>public class SongCollection : ObservableCollection&lt;Song&gt;
  {
  public static readonly DependencyProperty UrlProperty = DependencyProperty.Register(
    "Url", typeof(ObservableCollection&lt;Point&gt;), typeof(Arrow));
  public Uri Url
          {
              get { return (Uri)GetValue(UrlProperty);}
              ....
          }
  }
  </code></pre>