Question

I’m using EntityFramework, WPF and MVVM in my application and got some problems with updating the databinding of relationships between EntityObjects. I was able to downsize my problem to only a few lines of XAML and I hope someone can help me as I’m still not very confident with EF and MVVM.

Anyway, here we go with the simplified XAML:

    <DatePicker Grid.Row="2" Grid.Column="1" 
                    SelectedDate="{Binding Path=File.SentDate, 
StringFormat={}{0:dd/MM/yyyy}, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                    VerticalAlignment="Center" IsEnabled="{Binding Path=IsEnabled}"/>

        <ComboBox Grid.Row="3" Grid.Column="1" ItemsSource="{Binding Contacts}" DisplayMemberPath="Name" 
                  SelectedItem="{Binding Path=File.Sender, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" IsEditable="True"
                  VerticalAlignment="Center">
        </ComboBox>

        <Label Content="{Binding Path=File.SenderId}" Grid.Row="4"/>
        <Label Content="{Binding Path=File.Sender.Name}" Grid.Row="5"/>
        <Label Content="{Binding Path=File.SentDate}" Grid.Row="6"/>

I’m using the last 3 Labels to test my databinding. Changing the File.SentDate using the DatePicker updates the databinding to the last Label without problem.

Now File is of type EntityObject and has a SenderId property of type GUID. It also has a relationship to my Contacts through the Sender property. Obvisouly, SenderId is the GUID of the corresponding Contact EntityObject which is related to File through the Sender relationship. A File can have only 1 single Sender of type Contact.

Anyway, what happens is that when I select another sender using the combobox, the Label displaying the File.SenderId property get properly updated. However, the one with the File.Sender.Name property i.e. the one using the reléationship does not get updated.

So I’m guessing that there is something special about updating the databinding of relationships in EF.

Can someone please suggest a solution to this?

Answer 1

Unfortunately, the Entity Framework doesn’t notify when an association property changes. That’s the reason why your Binding didn’t work.

The issue is reported to Microsoft: http://connect.microsoft.com/VisualStudio/feedback/details/532257/entity-framework-navigation-properties-don-t-raise-the-propertychanged-event

Another workaround is shown by the BookLibrary sample application of the WPF Application Framework (WAF). The Book class listens to the AssociationChanged event and raises the appropriate PropertyChanged event.

public Book()
{
    …
    LendToReference.AssociationChanged += LendToReferenceAssociationChanged;
}

private void LendToReferenceAssociationChanged(object sender, 
        CollectionChangeEventArgs e)
{
    // The navigation property LendTo doesn't support the PropertyChanged event. 
    // We have to raise it ourselves.
    OnPropertyChanged("LendTo");
}