title: C#/WPF: Binding Combobox ItemSource in Datagrid to element outside of the DataContext
body: <p>I'd like to do following:</p>
  
  <pre><code>public List&lt;Users&gt; PreLoadedUserList { get; set; }
  public List&lt;RowEntries&gt; SomeDataRowList { get; set; }
  
  public class Users
  {
      public int Age { get; set; }
      public string Name { get; set; }
  }
  public class SomeDataRowList 
  {
      public int UserAge { get; set;
  }
  </code></pre>
  
  <p>Now my (WPF Toolkit) DataGrid looks like this:</p>
  
  <pre><code>&lt;my:DataGrid AutoGenerateColumns="False" MinHeight="200" 
               ItemsSource="{Binding Path=SomeDataRowList}"&gt;
      &lt;my:DataGridComboBoxColumn Header="Age" 
                                 ItemsSource="{Binding Path=PreLoadedUserList}" 
                                 DisplayMemberPath="Name" 
                                 SelectedValueBinding="{Binding Path=UserAge}"/&gt;
  
  &lt;/my:DataGrid&gt;
  </code></pre>
  
  <p>Now my problem is, that PreLoadedUserList is outside of the ItemSource (SomeDataRowList) and I don't know how to bind to something outside of it. What I actually want it:
  - Display in the ComboBox PreLoadedUserList 
  - Set the Value of (RowEntries) SelectedItem.UserAge to the Value of the selected ComboboxItem.Age</p>
  
  <p>Let me know if my explanation is too weird :-)</p>
  
  <p>Thank you,
  Cheers</p>