Can a WPF ComboBox display alternative text when its selection is null?
- by Garth T Kidd
G'day!
I want my WPF ComboBox to display some alternative text when its data-bound selection is null.
The view model has the expected properties:
public ThingoSelectionViewModel : INotifyPropertyChanged {
public ThingoSelectionViewModel(IProvideThingos) {
this.Thingos = IProvideThingos.GetThingos();
}
public ObservableCollection<Thingo> Thingos { get; set; }
public Thingo SelectedThingo {
get { return this.selectedThingo; }
set { // set this.selectedThingo and raise the property change notification
}
// ...
}
The view has XAML binding to the view model in the expected way:
<ComboBox x:Name="ComboboxDrive" SelectedItem="{Binding Path=SelectedThingo}"
IsEditable="false" HorizontalAlignment="Left" MinWidth="100"
IsReadOnly="false" Style="{StaticResource ComboboxStyle}"
Grid.Column="1" Grid.Row="1" Margin="5" SelectedIndex="0">
<ComboBox.ItemsSource>
<CompositeCollection>
<ComboBoxItem IsEnabled="False">Select a thingo</ComboBoxItem>
<CollectionContainer
Collection="{Binding Source={StaticResource Thingos}}" />
</CompositeCollection>
</ComboBox.ItemsSource>
</ComboBox>
The ComboBoxItem wedged into the top is a way to get an extra item at the top. It's pure chrome: the view model stays pure and simple. There's just one problem: the users want "Select a thingo" displayed whenever the ComboBox' selection is null.
The users do not want a thingo selected by default. They want to see a message telling them to select a thingo.
I'd like to avoid having to pollute the viewmodel with a ThingoWrapper class with a ToString method returning "Select a thingo" if its .ActualThingo property is null, wrapping each Thingo as I populate Thingos, and figuring out some way to prevent the user from selecting the nulled Thingo.
Is there a way to display "Select a thingo" within the ComboBox' boundaries using pure XAML, or pure XAML and a few lines of code in the view's code-behind class?
