title: ComboBox SelectedItem binding not updating
body: <p>I'm a bit puzzled:
  this works:</p>
  
  <pre><code>&lt;StackPanel Orientation="Horizontal" HorizontalAlignment="Right"&gt;
              &lt;Label Content="Rol" /&gt;
              &lt;ComboBox ItemTemplate="{StaticResource listRollen}"
                        Height="23" Width="150"
                        SelectedItem="{Binding Path=SelectedRol, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"
                        ItemsSource="{Binding Path=allRollen, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" /&gt;
          &lt;/StackPanel&gt;
  </code></pre>
  
  <p>and the property for SelectedRol is:</p>
  
  <pre><code>public TblRollen SelectedRol
      {
          get { return _selectedRol; }
          set
          {
              if (_selectedRol != value)
              {
                  _selectedRol = value;
                  OnPropertyChanged("SelectedRol");
              }
          }
      }
  </code></pre>
  
  <p>But this doesn't work:</p>
  
  <pre><code>&lt;StackPanel Orientation="Horizontal" HorizontalAlignment="Right"&gt;
              &lt;Label Content="Soort" /&gt;
              &lt;ComboBox ItemTemplate="{StaticResource listSoorten}"
                        Height="23" Width="150"
                        ItemsSource="{Binding Path=allSoorten}"
                        SelectedItem="{Binding Path=SelectedProduct, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/&gt;
          &lt;/StackPanel&gt;
  </code></pre>
  
  <p>with following property SelectedProduct:</p>
  
  <pre><code>public TblProduktSoorten SelectedProduct
      {
          get { return _selectedPSoort; }
          set
          {
              if (_selectedPSoort != value)
              {
                  _selectedPSoort = value;
                  OnPropertyChanged("SelectedProduct");
              }
          }
      }
  </code></pre>
  
  <p>somewhere in my code I set <code>SelectedProduct = p.TblProduktSoorten</code> and while debugging, I see the property gets set correctly...</p>