title: INotifyPropertyChanged does not work when the property set to another value with mode = twoway
body: <p>My Xaml</p>
  
  <pre><code>&lt;TextBox Text="{Binding MyVal, Mode=TwoWay}" &gt;&lt;/TextBox&gt;
  </code></pre>
  
  <p>My Viewmodel </p>
  
  <pre><code>private string myVar;
  
      public string MyVal
      {
          get
          {
              return myVar;
          }
          set
          {
              if (value.Length &gt; 6)
                  myVar = value;
              else
                  myVar = "Not a valid INPUT";
              OnPropertyChanged("MyVal");
          }
      }
      public event PropertyChangedEventHandler PropertyChanged;
  
      public void OnPropertyChanged(string name)
      {
          PropertyChangedEventHandler handler = PropertyChanged;
          if (handler != null)
          {
              handler(this, new PropertyChangedEventArgs(name));
          }
      }
  </code></pre>
  
  <p>when ever user enters a less than 6 char string the textbox should disply error message. instead of that the textbox text is remains same as the user input. But the variable value is changing as expected. </p>
  
  <p>I'm using WinRT app please help Thanks In advance.</p>