How do I bind combobox text to legacy data not in the drop down list?

Question

The drop-down list (itemssource) of my combobox contains new product request items. I want to bind to legacy data that is not in the drop-down list. To complicate things I'm using multibinding with an IMultiValueConverter to combine fields for display. Also, the names of bound fields do not match the names of the properties I'm bound to.

The combobox itemssource is a list of NewProductRequests. From this NPR object NewProdNumber and NewProdName are combined for display in the drop-down list by my type converter. The ConvertBack method returns the values NewProdNumber and NewProdNumberCombinedWithName. These two values will be saved to database fields with slightly different names. For this example I'll call them DBProdRequestNumber and DBProdRequestTitle.

I've succeeded in displaying and saving new items. The problem is I haven't figured out how to display legacy data that is not in the list. It's not in the list because it no longer qualifies as a new product request.

Here is the problem XAML (the itemssource is set in code-behind):

<ComboBox x:Name="NPRComboBox" IsSynchronizedWithCurrentItem="False" IsEditable="False">
    <ComboBox.SelectedItem>
        <MultiBinding Converter="{StaticResource combineNPRStuffMultiConverter}">
            <Binding Path="DBProdRequestNumber" UpdateSourceTrigger="PropertyChanged"/>
            <Binding Path="DBProdRequestTitle" UpdateSourceTrigger="PropertyChanged"/>
        </MultiBinding>
    </ComboBox.SelectedItem>
    <ComboBox.ItemTemplate>
        <DataTemplate>
            <TextBlock DataContext="{Binding}">
                <TextBlock.Text>
                    <MultiBinding  Converter="{StaticResource combineNPRStuffMultiConverter}">                             
                        <Binding Path="NewProdNumber" UpdateSourceTrigger="PropertyChanged"/>                              
                        <Binding Path="NewProdNumberCombinedWithName" UpdateSourceTrigger="PropertyChanged"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
        </DataTemplate>
    </ComboBox.ItemTemplate>
</ComboBox>

A similar problem with a datagrid and combobox I solved using a DataGridTemplateColumn.CellEditingTemplate based on this MSDN Magazine example from Julie Lerman. Of course, in this case I'm not using a datagrid.

Thanks in advance for any help.

Answer 1

This answer (to my own question) was pulled from a comment in the NathanAW answer:

Unfortunately I can't include legacy items in the ItemsSource. The list is from a web service that is out of my control. I devised a kludgy solution which I don't really like (but it works)...Since I know the combobox is needed only for new records it is visible only when the user clicks "Add". In the same location I placed a textbox bound to the legacy data that is visible when NOT in add mode. So, I toggle the visiblity of each control as the app switches in and out of add mode. I'm sure there is a better way!