Question

Is it possible to bind data in the “wrong” direction? I want a value in a custom control to be bound to my ViewModel. I’ve tried binding with mode “OneWayToSource” but I can’t get it to work.

Scenario (simplified):

I have a custom control (MyCustomControl) that has a dependency property that is a list of strings:

public class MyCustomControl : Control
{
    static MyCustomControl()
    {
        //Make sure the template in Themes/Generic.xaml is used.
        DefaultStyleKeyProperty.OverrideMetadata(typeof (MyCustomControl),  new FrameworkPropertyMetadata(typeof (MyCustomControl)));

        //Create/Register the dependency properties.
        CheckedItemsProperty = DependencyProperty.Register("MyStringList", typeof (List<string>),  typeof (MyCustomControl), new FrameworkPropertyMetadata(new List<string>()));
    }

    public List<string> MyStringList
    {
        get
        {
            return (List<string>)GetValue(MyCustomControl.MyStringListProperty);
        }
        set
        {
            var oldValue = (List<string>)GetValue(MyCustomControl.MyStringListProperty);
            var newValue = value;
            SetValue(MyCustomControl.MyStringListProperty, newValue);
            OnPropertyChanged(new DependencyPropertyChangedEventArgs(MyCustomControl.MyStringListProperty, oldValue, newValue));
        }
    }
    public static readonly DependencyProperty MyStringListProperty;
}

The control also contains code to manipulate this list.

I use this custom control in a UserControl that has a ViewModel. The ViewModel has a property that is also a list of strings:

public List<string> MyStringsInTheViewModel
{
    get
    {
        return _myStringsInTheViewModel;
    }
    set
    {
        if (value != _myStringsInTheViewModel)
        {
            _myStringsInTheViewModel = value;
            OnPropertyChanged("MyStringsInTheViewModel");
        }
    }
}
private List<string> _myStringsInTheViewModel;

Now I want to bind the list in my custom control (MyStringList) to the list in my ViewModel (MyStringsInTheViewModel) so that when the list is changed in the custom control it is also changed in the ViewModel. I’ve tried this but can’t get it to work…

<myns:MyCustomControl MyStringList="{Binding Path=MyStringsInTheViewModel, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}">

How can I make such a binding?

Answer 1

Use ObservableCollection<T> instead of List<T>. It implements INotifyCollectionChanged Interface.