Gracefully handling expected BindingExpression errors

Question

I have a ComboBox bound to an ObservableCollection where the DisplayMemberPath references an object property Url.

My CompositeCollection contains a null item, allowing the user to have an optional selection:

<ComboBox Grid.Row="7" Grid.Column="1" Height="24" VerticalAlignment="Top"
          IsSynchronizedWithCurrentItem="True"
          SelectedItem="{Binding SelectedNotificationServer, Mode=TwoWay}"
          DisplayMemberPath="Url">
    <ComboBox.Resources>
        <CollectionViewSource x:Key="comboBoxSource" Source="{Binding Path=NotificationServers}" />
    </ComboBox.Resources>
    <ComboBox.ItemsSource>
        <CompositeCollection>
            <ComboBoxItem Content="None"/>
            <CollectionContainer Collection="{Binding Source={StaticResource comboBoxSource}}" />
        </CompositeCollection>
    </ComboBox.ItemsSource>
</ComboBox>

This works fine (it produces a dropdown of URLs and a default "None" item), but while debugging, I have an error because the Url property is not available on the null item:

System.Windows.Data Error: 40 : BindingExpression path error: 'Url' property not found on 'object' ''ComboBoxItem' (Name='')'. BindingExpression:Path=Url; DataItem='ComboBoxItem' (Name=''); target element is 'ComboBox' (Name=''); target property is 'NoTarget' (type 'Object')

How do I gracefully handle this, even though it doesn't cause any runtime issues. It's probably just debugging noise, but a nuisance nonetheless.

Answer 1

Your 'null' item should simply be an item in your collection (of the same type as the rest of the items) that has no properties set, or at least no properties that are displayed. That way, you can avoid the Binding errors all together, because your 'null' item will have a Url property to data bind to.

---

UPDATE >>>

You can set the 'null' item like this when you access your other data items:

NotificationServers.Add(new NotificationServer());
NotificationServers.AddRange(GetNotificationServers());

That will make it the first item. You could also do something like this:

NotificationServers.Add(new NotificationServer() { DisplayedProperty = "Please Select" });