title: DataGridComboBoxColumn binding issue
body: <p>I have DataGridComboBoxColumn in DataGrid:</p>
  
  <pre><code>  &lt;DataGridComboBoxColumn Header="{x:Static Properties:Resources.Unit}" Width="Auto"
  
                                      SelectedValueBinding="{Binding UnitUnitId}"
                                      SelectedValuePath="UnitId"
                                      DisplayMemberPath="Name" &gt;
                  &lt;DataGridComboBoxColumn.ElementStyle&gt;
                      &lt;Style TargetType="ComboBox"&gt;
                          &lt;Setter Property="ItemsSource" Value="{Binding Path=DataContext.Units,RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/&gt;
                      &lt;/Style&gt;
                  &lt;/DataGridComboBoxColumn.ElementStyle&gt;
                  &lt;DataGridComboBoxColumn.EditingElementStyle&gt;
                      &lt;Style TargetType="ComboBox"&gt;
                          &lt;Setter Property="ItemsSource" Value="{Binding Path=DataContext.Units,RelativeSource={RelativeSource AncestorType={x:Type UserControl}}}"/&gt;
                      &lt;/Style&gt;
                  &lt;/DataGridComboBoxColumn.EditingElementStyle&gt;
  
              &lt;/DataGridComboBoxColumn&gt;
  </code></pre>
  
  <p>and ComboBox:</p>
  
  <pre><code>&lt;ComboBox x:Name="unitBox" ItemsSource="{Binding Path=Units}" 
                SelectedItem="{Binding TaxSubGroup.Unit}"
                 Grid.Row="2"
                Grid.Column="1" Margin="11,11,0,0" HorizontalAlignment="Stretch"&gt;
          &lt;ComboBox.ItemTemplate&gt;
              &lt;DataTemplate&gt;
                  &lt;TextBlock Text="{Binding Converter={x:Static c:UnitToStringConverter.Default}}"/&gt;
              &lt;/DataTemplate&gt;
          &lt;/ComboBox.ItemTemplate&gt;
      &lt;/ComboBox&gt;
  </code></pre>
  
  <p>Units property:</p>
  
  <pre><code>   public ObservableCollection&lt;Unit&gt; Units
      {
          get
          {
              return _units;
          }
          set
          {
              if(_units!=value)
              {
                  _units = value;
                  RaisePropertyChanged("Units");
              }
          }
      }
  </code></pre>
  
  <p>when i'm changing ComboBoxes value, DataGridComboBoxColumn's value updating automatically, but changing DataGrid's comboboxcolumn's value not updating ComboBoxes value. Why?</p>
  
  <p>UPDATED</p>
  
  <p>The problem was in DataGridComboBoxCulumn's SelectedValueBinding property. All i needed is to add UpdateSourceTrigger property:</p>
  
  <pre><code>SelectedValueBinding="{Binding UnitUnitId,UpdateSourceTrigger=PropertyChanged}"
  </code></pre>