title: WPF Binding Property not refreshing on update
body: <p>I've got a problem refreshing the binding to a property in my object. at initiating it shows fine result, but after i edit the source the value isn't refreshed in my window.</p>
  
  <pre><code>public class Marker : INotifyPropertyChanged 
  {
      public event PropertyChangedEventHandler PropertyChanged;
      protected virtual void OnPropertyChanged(PropertyChangedEventArgs e)
      {
          if (PropertyChanged != null)
          {
              PropertyChanged(this, e);
          }
      }
      private ObservableCollection&lt;bool?&gt; _repeat;
      [...]
      public ObservableCollection&lt;bool?&gt;    Repeat
      {
          get { return _repeat; }
          set 
          {
              _repeat = value;
              OnPropertyChanged(new PropertyChangedEventArgs("Repeat"));
              OnPropertyChanged(new PropertyChangedEventArgs("RepeatString"));
          }
      }
  
      [...]
  
      [XmlIgnore]
      public string RepeatString
      {
          get
          {
              string wt = string.Empty;
              wt += (bool)Repeat[0] ? "Mo, " : string.Empty;
              wt += (bool)Repeat[1] ? "Di, " : string.Empty;
              wt += (bool)Repeat[2] ? "Mi, " : string.Empty;
              wt += (bool)Repeat[3] ? "Do, " : string.Empty;
              wt += (bool)Repeat[4] ? "Fr, " : string.Empty;
              wt += (bool)Repeat[5] ? "Sa, " : string.Empty;
              wt += (bool)Repeat[6] ? "So, " : string.Empty;
              if (wt != string.Empty)
              {
                  wt = wt.Remove(wt.Length - 2);
              }
              return wt;
          }
          [...]
      }
      [...]
  }
  </code></pre>
  
  <p>In XAML part I've set bindings as following:</p>
  
  <pre><code>&lt;Grid DataContext="{Binding ElementName=lvMarker, Path=SelectedItem}"&gt;
      [...]
          &lt;CheckBox Content="Montag" IsChecked="{Binding Repeat[0], Mode=TwoWay}"/&gt;
          &lt;CheckBox Content="Dienstag" IsChecked="{Binding Repeat[1], Mode=TwoWay}"/&gt;
          &lt;CheckBox Content="Mittwoch" IsChecked="{Binding Repeat[2], Mode=TwoWay}"/&gt;
          &lt;CheckBox Content="Donnerstag" IsChecked="{Binding Repeat[3], Mode=TwoWay}"/&gt;
          &lt;CheckBox Content="Freitag" IsChecked="{Binding Repeat[4], Mode=TwoWay}"/&gt;
          &lt;CheckBox Content="Samstag" IsChecked="{Binding Repeat[5], Mode=TwoWay}"/&gt;
          &lt;CheckBox Content="Sonntag" IsChecked="{Binding Repeat[6], Mode=TwoWay}"/&gt;
      &lt;/StackPanel&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>this is updating the array just fine as I'm Clicking the Checkboxes. but not the following text block binding.</p>
  
  <pre><code>&lt;ListView x:Name="lvMarker" ItemsSource="{Binding MarkerCollection}" Width="Auto" ItemContainerStyle="{StaticResource lvItemStyle}"&gt;
      &lt;ListView.View&gt;
          &lt;GridView&gt;
              [...]
              &lt;GridViewColumn Header="Zeit" Width="Auto" CellTemplate="{StaticResource DateCell}"/&gt;
          &lt;/GridView&gt;
      &lt;/ListView.View&gt;
  &lt;/ListView&gt;
  </code></pre>
  
  <p>...</p>
  
  <pre><code>&lt;DataTemplate x:Key="DateCell"&gt;
      &lt;StackPanel&gt;
          &lt;TextBlock FontSize="14"&gt;
              [...]
          &lt;/TextBlock&gt;
          &lt;TextBlock FontSize="10" Text="{Binding RepeatString, Mode=OneWay}"/&gt;
      &lt;/StackPanel&gt;
  &lt;/DataTemplate&gt;
  </code></pre>
  
  <p>What would I have to do to refresh the TextBlock representing RepeatString, when I change the bool? of Repeat[] ?</p>