How do I make Binding respect DependencyProperty value coercion?

Question

I have a control with a DependencyProperty with a CoerceValueCallback. This property is bound to a property on a model object.

When setting the control property to a value that causes coercion the Binding pushes the uncoerced value to the model object. The property value on the control is coerced correctly.

How do I get the Binding to push the coerced value to the model object?

void Initialize()
{
    UIObject ui = new UIObject();
    ModelObject m = new ModelObject();
    m.P = 4;

    Binding b = new Binding("P");
    b.Source = m;
    b.Mode = BindingMode.TwoWay;
    Debug.WriteLine("SetBinding");
    // setting the binding will push the model value to the UI
    ui.SetBinding(UIObject.PProperty, b);

    // Setting the UI value will result in coercion but only in the UI.
    // The value pushed to the model through the binding is not coerced.
    Debug.WriteLine("Set to -4");
    ui.P = -4;

    Debug.Assert(ui.P == 0);
    // The binding is TwoWay, the DP value is coerced to 0.
    Debug.Assert(m.P == 0); // Not true. This will be -4. Why???
}

class UIObject : FrameworkElement
{
    public static readonly DependencyProperty PProperty =
        DependencyProperty.Register("P", typeof(int), typeof(UIObject), 
        new FrameworkPropertyMetadata(
            new PropertyChangedCallback(OnPChanged), 
            new CoerceValueCallback(CoerceP)));

    public int P
    {
        get { return (int)GetValue(PProperty); }
        set { SetValue(PProperty, value); }
    }

    private static void OnPChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Debug.WriteLine(typeof(UIObject) + ".P changed from " + e.OldValue + " to " + e.NewValue);
    }

    private static object CoerceP(DependencyObject sender, object value)
    {
        int p = (int)value;
        if (p < 0)
        {
            Debug.WriteLine(typeof(UIObject) + ".P coerced from " + p + " to 0");
            p = 0;
        }
        return p;
    }
}

class ModelObject
{
    private int p;
    public int P
    {
        get
        {
            Debug.WriteLine(this + ".P returned " + this.p);
            return this.p;
        }
        set
        {
            Debug.WriteLine(this + ".P changed from +" + this.p + " to " + value);
            this.p = value;
        }
    }
}

Answer 1

I don't think the coerce callback is meant to be a two-way street. One workaround would be to update the model's value inside of the coerce callback.

Answer 2

I think that that's the whole idea of the coercion - correct value on the fly without triggering modification of any other dependencies. You can use the code below instead of native coercion mechanisms:

OnPChanged(/* ... */)
{
    // ...
    var coercedP = CoerceP(P);
    if (P != coercedP)
        P = coercedP;
    // ...
}

HTH.

Answer 3

Here is extension method where you set value on target object

public static void SetTargetValue<T>(this FrameworkElement element, DependencyProperty dp, T value)
    {
        var binding = BindingOperations.GetBinding(element, dp);
        if (binding == null) return;
        var name = binding.Path.Path;
        var splits = name.Split('.');
        var target = element.DataContext;
        for (var i = 0; i < splits.Length; i++)
        {
            PropertyInfo property;
            if (i == splits.Length - 1)
            {
                property = target.GetType().GetProperty(splits[i]);
                property.SetValue(target, value);
            }
            else
            {
                property = target.GetType().GetProperty(splits[i]);
                target = property.GetValue(target);
            }
        }
    }

So, in this method, using binding, you can set value to source. Of cource source Path can have a lot of names - Property1.Property2.Property3 and etc. In coerce method you need only call this method:

private static object CoerceProperty(DependencyObject d, object baseValue)
    {
        if (!Check)
        {
            var sender = (FrameworkElement)d;
            sender.SetTargetValue(MyPropertyProperty, myValue);
            return needValue;
        }
        return baseValue;
    }