WPF binding to combobox in DataGridTemplateColumn - wpf

Question

I have a class in a WPF MVVM application which I would like to display in a popup form and manage some of its members.
public class Route
{
public string Name { get; set; }
public List<RouteSegment> MandatorySegments { get; set; }
}
public class RouteSegment
{
public decimal Id { get; set; }
public string Name { get; set; }
}
I've only added a few of the member fields, the rest are not relevant.
What I would like to do is to display all the items found in MandatorySegments list in a combobx in a datagrid so I can add and remove new members easily.
This is the code I have so far:
In my ViewModel
public RouteTest SelectedRoute { get; set; }
public ObservableCollection<RouteSegment> AllRouteSegments { get; private set; }
public RouteSegment SelectedMandatorySegment { get; set; }
In my View
<DataGrid Grid.RowSpan="2"
AutoGenerateColumns="False" HorizontalAlignment="Left" Margin="5,6,0,5" Name="dgMandatorySegments" VerticalAlignment="Stretch" Width="306"
ItemsSource="{Binding SelectedRouteTest.MandatorySegments, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" CanUserResizeRows="False" CanUserSortColumns="False" CanUserResizeColumns="False" CanUserReorderColumns="False"
CanUserAddRows="{StaticResource False}" SelectionMode="Single" SelectedItem="{Binding SelectedMandatorySegment, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
<DataGrid.Columns>
<DataGridTemplateColumn Header="Route Segment">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<ComboBox ItemsSource="{Binding DataContext.AllRouteSegments, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}"
ItemTemplate="{StaticResource CboxItemTemplate}"/>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
</DataGrid.Columns>
</DataGrid>
What I'm missing is the SelectedValue or SelectedItem on the combobox in the templated grid column. Now, in the grid I have the same number of rows as elements in my list and the ItemsSource of the combobox binds properly, because I can choose from all the RouteSegment elements. But the initial value is always empty for all the rows in the datagrid.
What should the SelectedValue of the combobox bind to?

Answer 1

This depends on whether you want the same default value for all rows - or row-independent initial values. I would recommend you use the SelectedItem to avoid issues with SelectedValue not working reliably. The current VM you have only allows for one selected mandatory segment - given this assumption, you are only setup for one default value for all grid rows.
<ComboBox ItemsSource="{Binding DataContext.AllRouteSegments, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}"
SelectedItem="{Binding DataContext.SelectedMandatorySegment, UpdateSourceTrigger=PropertyChanged, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}}"
ItemTemplate="{StaticResource CboxItemTemplate}"/>

Answer 2

DisplayMemberPath is probably what you want. The following ComboBox works nicely in a DataGrid in my WPF application. (Please note that you need to update the binding path to make it work in your application.)
<ComboBox ItemsSource="{Binding Path=AllRouteSegments}" DisplayMemberPath="Name" SelectedItem="{Binding Path=SelectedMandatorySegment, UpdateSourceTrigger=PropertyChanged}"/>