title: MVVM binding is not updating UI element
body: <p>In my .xaml I have a textbox "textBox_Input" that is bound to a string</p>
  
  <pre><code>&lt;Grid x:Name="LayoutRoot"&gt;
      &lt;Button Content="{Binding Source={StaticResource LocStrings}, Path=ConnectDongle}" Command="{Binding ConnectDongleCommand}" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Width="155" Height="21"/&gt;
      &lt;Button Content="{Binding Source={StaticResource LocStrings}, Path=ReadDongleFile}" Command="{Binding ReadDongleFileCommand}" HorizontalAlignment="Left" Margin="11,36,0,0" VerticalAlignment="Top" Width="154"/&gt;
      &lt;Button Content="{Binding Source={StaticResource LocStrings}, Path=WriteDongleFile}" Command="{Binding WriteDongleFileCommand}" HorizontalAlignment="Left" Margin="10,63,0,0" VerticalAlignment="Top" Width="155"/&gt;
      &lt;TextBox x:Name="textBox_Input" Text="{Binding Path=TextBoxInputText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Height="170" Margin="10,90,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="218"/&gt;
      &lt;TextBox Text="{Binding Path=TextBoxInputText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Height="170" Margin="233,90,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="218"/&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>The related ViewModel:</p>
  
  <pre><code>public RelayCommand ReadDongleFileCommand { private set; get; }
      public void ReadDongleFileCommandExecute()
      {
          TextBoxInputText += "a"; //this if just for testing
      }
  
      private String _textBoxInputText;
      public String TextBoxInputText
      {
          get
          {
              return _textBoxInputText;
          }
          set
          {
              _textBoxInputText = value;
          }
      }
  </code></pre>
  
  <p>When pressing the second Button bound to "ReadDongleFileCommand" both get and set of my TextBoxInputText are fired, so the command is working properly, but my TextBox is never updated. I never see any text in this TextBox.<br>
  I'm quite new to MVVM so probably I just forgot something simple, but I have no idea what it is.</p>