DataGrid column sorts on lookup value not the displayed value when using a binding converter

Hi,

Does anyone know how to get a WPF datagrid to sort using the content it is actually displaying and not its lookup values??

I have a master / detail type database and have written some converters to look up field ID numbers and display them as full text.  ie I have a field which stores ContactID numbers and use these to look up a the full contact name which is then displayed rather than the ContactID.

If the user clicks the column headers the DataGrid sorts correctly for all the columns that are not converted. The columns that are converted are sorted by the ID numbers and NOT what the user sees.

Looking at various forum Q & As it seems as if I might be able to set the <DataGridTextColumn>  SortPathMember property to "Content" or "Text" ie the property that it is displaying. Does anyone know a way to do this? I can't find any examples that show me the syntax needed. This would be my preferred way to do this as it is easy - if it is possible.

If not has anyone got any examples of Icomparer code so I can learn and understand how to link them between xaml and the code behind.

Some snippets of code which might help you see what I am doing are:-

<UserControl.Resources><CollectionViewSource x:Key="ContactsViewSource" Source="{Binding Path=Contacts, Source={StaticResource StockDataSet}}" /><CollectionViewSource x:Key="AddressViewSource" Source="{Binding Path=Address, Source={StaticResource StockDataSet}}" /><my:LookupCityConverter x:Key="LookupCity"/></UserControl.Resources><DataGrid AutoGenerateColumns="False" Margin="12,0,12,12" Name="DataGd1" VerticalAlignment="Stretch" Foreground="Black" ItemsSource="{Binding}" 
 SelectionMode="Extended" IsReadOnly="True" IsSynchronizedWithCurrentItem="True" ><DataGrid.Columns><!-- Next two columns sort perfectly --><DataGridTextColumn Header="Customer" Visibility="Visible" Binding="{Binding Customer}"/><DataGridTextColumn Header="Telephone"  Binding="{Binding Telephone}"/><!-- Next column is converted and sorts by ID number but displays city name so sort looks odd! --><DataGridTextColumn Header="Delivery City"   Binding="{Binding Path=DelAddID, Converter={StaticResource LookupCity}, ConverterParameter={StaticResource AddressViewSource } }"/></DataGrid.Columns></DataGrid>

And for completeness here is the VB converter class code:-

Public Class LookupCityConverter
    Implements IValueConverter
    Public Function Convert(value As Object, targetType As System.Type, parameter As Object, culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IValueConverter.Convert
        Dim IDNum As Integer = CType(value, Integer)
        Dim CVS As CollectionViewSource = CType(parameter, CollectionViewSource)   'Rows.Find(IDNum)
        Dim Addr As String = "*"
        Try
            If CVS.Source.ToString = "Address" Then Addr = StockDS.Address.FindByAddressID(IDNum).City.ToString 
        Catch ex As Exception
        End Try
        Return Addr
    End Function
    Public Function ConvertBack(value As Object, targetType As System.Type, parameter As Object, culture As System.Globalization.CultureInfo) As Object Implements System.Windows.Data.IValueConverter.ConvertBack
        Return value
    End Function
End Class ' this converter looks up the city for a contact

For completeness this is using two database tables one stores contacts info with AddressID numbers, the other stores address info indexed by the AddressID number. So the customer and telephone columns are in the ItemsSource whereas the City is a converted column. (hope that makes sense!)