MultipleBinding for a WPF Converter… when does ConvertBack get called?

Question

I have a ComboBox with a set of values. The selected value is determined by a Converter with a multiple two-way binding. When the user changes the value in the ComboBox, I expected the ConvertBack method to get called and change the values of the two properties used in the Multiple Binding, but this doesn't happen, even when the ComboBox loses focus. I tried to change the UpdateSourceTrigger property to "LosesFocus" but this is apparently not valid for a MultipleBinding. How do I get the ConvertBack method to fire, either when the value changes or when the control loses focus? Either will work for my purposes.

XAML:

<ComboBox ItemsSource="{Binding DescriptionList}" DisplayMemberPath="Description" SelectedValuePath="Description" IsEnabled="{Binding EditMode}">
    <ComboBox.SelectedValue>
        <MultiBinding Converter="{StaticResource DescriptionConverter}">
            <Binding Path="PersonRow.DescriptionType" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" />
            <Binding Path="PersonRow.DescriptionSuccessful" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged" />
        </MultiBinding>
    </ComboBox.SelectedValue>
</ComboBox>

Answer 1

You probably need to add Mode="TwoWay" to your <MultiBinding>.

Answer 2

Jay's suggestion hinted me in the right direction. I also added an UpdateSourceTrigger="LostFocus" to the MultiBinding tag and was able to fire the event.