How come WPF ComboBox ItemsSource Binding doesn't update when set?

Question

I have the following inside a user control called UserInputOutput :

<ComboBox Grid.Column="1" Background="White" Visibility="{Binding InputEnumVisibility}"     
          FontSize="{Binding FontSizeValue}" Width="Auto" Padding="10,0,5,0"     
          ItemsSource="{Binding EnumItems}"     
          SelectedIndex="{Binding EnumSelectedIndex}"/>    

I have several bindings here which all work great except ItemsSource. Here is my Dependency Property and public variable.

public ObservableCollection<String> EnumItems
{
    get { return (ObservableCollection<String>)GetValue(EnumItemsProperty); }
    set { SetValue(EnumItemsProperty, value); }
}

public static readonly DependencyProperty EnumItemsProperty =
    DependencyProperty.Register("EnumItems", typeof(ObservableCollection<string>),typeof(UserInputOutput)

All the Bindings are set in XAML except the ComboBox's ItemSource. This has to be set at runtime. In my code I use the following:

ObservableCollection<string> enumItems = new ObservableCollection<string>();
UserInputOutput.getEnumItems(enumItems, enumSelectedIndex, ui.ID, ui.SubmodeID);
instanceOfUserInputOutput.EnumItems = enumItems;

I run this code after the XAML is loaded from a file. The instaceOfUserInputOutput.EnumItems contains the correct items after I set it equal to enumItems, but it doesn't show up in the combo box in my program.

Not sure where I'm going wrong here. Any thoughts?

Thank you!

Answer 1

I assume your ViewModel class (the one that is used as a source of binding) implements INotifyPropertyChanged interface. Otherwise update won't work.

Then, in your setter method, do this:

set
{
     // set whatever internal properties you like
     ...

     // signal to bound view object which properties need to be refreshed
     OnPropertyChanged("EnumItems");
}

where OnProperyChanged method is like this:

protected void OnPropertyChanged(string propertyName)
{
    PropertyChangedEventHandler handler = this.PropertyChanged;
    if (handler != null)
    {
        var e = new PropertyChangedEventArgs(propertyName);
        handler(this, e);
    }
}

BTW, I don't know why you need to declare EnumItems as a dependency property. Having it as a class field would work fine, unless you want to use it as a target for binding (right now it is used as a binding source).