title: WPF TextBox content not updated when the round trip by the view model change the actual value
body: <p>I have below WPF TextBox:</p>
  <pre><code>&lt;TextBox Grid.Column=&quot;0&quot;
           Grid.Row=&quot;1&quot; 
           Text=&quot;{Binding myPath, UpdateSourceTrigger=PropertyChanged}&quot;                 
           Margin=&quot;5,8,8,5&quot; /&gt; 
  </code></pre>
  <p>And its property in the view model is:</p>
  <pre><code>public string myPath
  {
     get =&gt; myObject.path;
     set
     {
         // Do some comprobations before assigning the new value
         if (comprobationsOk(value))
         {
            Uri myUri = new Uri(value);
            myObject.path = myUri.LocalPath;
            this.OnPropertyChanged();
         }
     }
  }
  </code></pre>
  <p>And this is my OnPropertyChanged method in my ViewModelBase class which implements INotifyPropertyChanged:</p>
  <pre><code>    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
      {
          this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
      }
  </code></pre>
  <p>As you can see, the actual view model value is transformed in the setter.</p>
  <p>When the user starts typing in the TextBox <code>///my/absolute/path</code> the property in the view model gets <code>//my/absolute/path</code>.<br />
  So I expect the <code>TextBox</code> to be updated with <code>//my/absolute/path</code>, but it is not and remains as <code>///my/absolute/path</code></p>