Question

I’m trying to set up a property binding (WPF) in code. The code compiles fine, but the property I bind is never set. Below follows a minimal example:

The view-model:

public class FooViewModel: INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private string _value;
    public string Value 
    { 
        get { return _value; }
        set
        {
            _value = value;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Value"));
        }
    }
}

The view:

public class FooView: Window
{
    public string Value
    {
        get { return Title; }
        set
        {
            // Breakpoint here never hits!
            Title = value;
        }
    }

    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register("Value", typeof(string), typeof(FooView));

    public FooView()
    {
        Binding valueBinding = new Binding("Value");
        valueBinding.Mode = BindingMode.OneWay;
        SetBinding(ValueProperty, valueBinding);
    }
}

The “main()”:

        FooView view = new FooView();
        FooViewModel model = new FooViewModel();
        view.DataContext = model;
        view.Show();
        model.Value = "ABC";

I expected the FooView.Value-setter to be invoked when model.Value is set. I’ve also tried explicitly setting the Binding.Source property to the model. How should the binding be set up?

Answer 1

The problem here is that the Value property is completely unrelated to the ValueProperty dependency property.

The CLR wrapper for a dependency property needs to call the DependencyObject’s GetValue and SetValue methods like below:

public string Value
{
    get { return (string)GetValue(ValueProperty); }
    set { SetValue(ValueProperty, value); }
}

In order to get notified about property changes, you would have to register a PropertyChangedCallback with the PropertyMetadata:

public static readonly DependencyProperty ValueProperty =
    DependencyProperty.Register(
        "Value", typeof(string), typeof(FooView),
        new PropertyMetadata(ValuePropertyChanged));

private static ValuePropertyChanged(DependencyObject obj,
    DependencyPropertyChangedEventArgs e)
{
    // obj is your FooView instance
    // get new property value from e.NewValue
}

Get more information in Custom Dependency Properties.