Control in an ItemTemplate of a ComboBox loses its binding

Question

I have a ComboBox that uses an ItemTemplate as shown below. Somehow the Text property of the text box defined in the item template gets disconnected from the binding and stops being updated when the selected item changes.

The ComboBox.ItemsSource is bound to a DependencyProperty that is list of CatheterDefinition objects. The ComboBox.SelectedItem is bound to a DependencyProperty that is a single CatheterDefinition object.

<ComboBox
  AutomationProperties.AutomationId="CatheterInfoModelFieldID"
  VerticalAlignment="Center" HorizontalAlignment="Stretch"
  ItemsSource="{x:Static PumpAndCatheter:CatheterInfoViewModel.CatheterModelDefinitions}"
  SelectedItem="{Binding ElementName=UserControl, Path=ViewModel.SelectedCatheterModel, Mode=TwoWay, NotifyOnSourceUpdated=True}"
  SourceUpdated="HandleModelSourceUpdated">
  <ComboBox.ItemContainerStyle>
    <!-- A style used to set the AutomationID based on the item goes here -->
  </ComboBox.ItemContainerStyle>
  <ComboBox.ItemTemplate>
    <DataTemplate>
      <!-- This line below is the location of the problem -->
      <TextBlock Text="{Binding Converter={StaticResource CatheterModelDefinitionToStringConverter}}">
        <!-- A style used to set the AutomationID based on the item goes here -->
      </TextBlock>
    </DataTemplate>
  </ComboBox.ItemTemplate>
</ComboBox>

I have an automated test that produces a very strange behavior (I saw the same behavior a few time during the initial development of the code, but was unable to reproduce it manually) - The test that reproduces this selects an item form the ComboBox, then goes to another part of the application and takes some actions that end up saving this change in a data model. When the test returns to the screen with this ComboBox, it tries to select another item from the ComboBox. The SelectedItem changes, and the values that it is bound to change, BUT the text in the ComboBox does not change - somehow the binding to the Text property of the text box gets broken (or something)... The binding still executes (the converter still runs when the selection changes and it converts to the correct value), but the text property is never updated.

Thoughts? (I can't provide an example of this because it is a huge application and it is only reproducible under one test that I know of)

Answer 1

Broken bindings are most of the time caused by not calling (or not correctly calling) the OnPropertyChanged("PropName") method.

Without seeing your underlying implementation, I would say that this is most likely the source of the problem.