title: Read-only item in ComboBox
body: <p>I'm using MVVM to bind  a ComboBox to an ObservableCollection in my WPF-application.
  However, I need to make one or more items in this combobox "un-selectable".
  I should also mention, that the combobox is used in a DataGrid.</p>
  
  <p>I suppose, that I could use some kind of ValueConverter. But I can't figure out how.</p>
  
  <p>Current XAML:</p>
  
  <pre><code>&lt;DataGridTemplateColumn Width="Auto"&gt;
      &lt;DataGridTemplateColumn.CellTemplate&gt;
          &lt;DataTemplate&gt;
              &lt;TextBlock Text="{Binding Type.Name}" /&gt;
          &lt;/DataTemplate&gt;
      &lt;/DataGridTemplateColumn.CellTemplate&gt;
      &lt;DataGridTemplateColumn.CellEditingTemplate&gt;
          &lt;DataTemplate&gt;
              &lt;StackPanel DataContext="{Binding DataContext.CurrentListUser, 
                                                RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"&gt;
                  &lt;ComboBox ItemsSource="{Binding Types}"
                            DisplayMemberPath="Name"
                            SelectedValue="{Binding TypeId, 
                                            Mode=TwoWay, 
                                            UpdateSourceTrigger=PropertyChanged}"
                            SelectedValuePath="Id" /&gt;
              &lt;/StackPanel&gt;
          &lt;/DataTemplate&gt;
      &lt;/DataGridTemplateColumn.CellEditingTemplate&gt;
  &lt;/DataGridTemplateColumn&gt;
  </code></pre>