Question

I’m fairly new to Data binding & XAML, so this probably is fairly simple thing but I’ve been stumped on it for days now (and frustrated with more googling than i can track at this point) and would appreciate any pointers in the right direction. My only preference is to keep it in pure XAML if possible.

In my RIA SL4 project, I have two Entities PackageOS and OS where PackageOS has an association to OS through PackageOS.OS (associating through PackageOS.OSID <-> OS.ID – and [Include] + .Include() setup properly on relevant sections)

This is the template (defined in Page.Resource section along with all other involved DDS) I’m using in DataForm to get OSEntities List to bind into PackageOS Entity (coming from RIA GetOSEntities() using DDS):

<DataTemplate x:Key="POSItemTemplate">
    <StackPanel>
        <toolkit:DataField Label="PackageOS.OS">
            <TextBlock Text="{Binding Source={StaticResource packageOSEntityDomainDataSource}, Path=Data.CurrentItem.OS}" />
        </toolkit:DataField>
        <toolkit:DataField Label="OS">
            <ComboBox ItemsSource="{Binding Path=Data, Source={StaticResource osEntityDomainDataSource}}"
                      SelectedItem="{Binding Path=Data.CurrentItem.OS, Source={StaticResource packageOSEntityDomainDataSource}}"/>
        </toolkit:DataField>
    </StackPanel>
</DataTemplate>

The core problem is SelectedItem of ComboBox is not working. All the bindings are reachable from IDE Binding wizard so it’s not a problem of me typing incorrect path. I can see packageOSEntityDomainDataSource.Data.CurrentItem to be of type PackageOS.

If i create a manual entry in backend database, the result is shown in PackageOS.OS textblock so I know it is properly being returned but SelectedItem refuses to pick it up (it ends up selecting the first value in dropdown list regardless of OS item in PackageOS).

Many thanks in advance!

Answer 1

Finally figured this out. Leaving my answer in hopes that it saves somebody else the time that I spent on this.

First Lesson
The problem was in the fact that I didn’t have a custom Equality implementation for generated entities and default reference equality didn’t work as I was using two different instances. Once I implemented IEquatable on my generated entities (through .shared.cs partial classes on server side) everything started working like a charm.

For details please see Silverlight ComboBox Control Population by Manishdalal

Second lesson
Do not use multiple DDS controls if you can help it. Especially once you use a write operation on a DDS, you cannot load/refresh any other DDS that is sharing the DomainContext until changes are committed. The link above shows how to avoid multiple DDS by using list generators when all you want is to pick up list of entities to fill ComboBox up.

My new code looks like this:

<DataTemplate x:Key="POSItemTemplate">
    <StackPanel d:DataContext="{Binding Source=packageOSDomainDataSource, Path=Data.CurrentItem}">
        <toolkit:DataField Label="OS">
            <ComboBox DisplayMemberPath="Name"
                      ItemsSource="{Binding Path=OSList, Source={StaticResource OSListGenerator}}"
                      SelectedItem="{Binding Path=OS, Mode=TwoWay}" />
        </toolkit:DataField>
    </StackPanel>
</DataTemplate>

Where OSListGenerator is returning an IEnumerable<OSEntity> through its OSList property after loading it from DomainContext

Third Lesson
In DDS DataTemplate you have to be explicit with TwoWay Binding. This is the new behaviour; something that took me days to figure as most of the tutorials I referred to were using SL3 and I didn’t realize that this was a breaking change in DDS DataTemplate behaviour in SL4.