WPF: TwoWay Binding doesn't update back

Question

I have the following custom control:

<abc:MyControl MyProperty="{Binding FieldInMyModel, Mode=TwoWay}">

And in my custom control I have

public static readonly DependencyProperty MyPropertyProperty =
     DependencyProperty.Register("MyProperty", typeof(double), typeof(MyControl),
     new PropertyMetadata(0.0, OnMyPropertyChanged));

public double MyProperty
{
     get { return (double)GetValue(MyPropertyProperty ); }
     set { SetValue(MyPropertyProperty , value); }
}

private static void OnMyPropertyChanged(DependencyObject d, 
     DependencyPropertyChangedEventArgs e)
{
     if (d is MyControl mc)
     {
          // here I check if the new value is valid for my scope.
          // if it is not I update it here to be valid
          var v = (double)e.NewValue;
          if (!mc.IsValidValue(v))
          {
               v = mc.MakeValidValue(v);
               mc.MyProperty = v;
          }
     }
}

In my model I change the value of FieldInMyModel to be not valid for my scope. OnMyPropertyChanged is called, and after making a valid value from received invalid value I expect that the FieldInMyModel will be updated to have the new value of MyProperty but actually nothing happens. Any thought?

Do you expect the validated, i.e. new property value to be passed back to the view model while the view model is currently updating its FieldInMyModel property? — Clemens
@Clemens yes, that is exactly what I want. btw, coerceValueCallback also didn't work for it. — L_A_Hooper
Afaik this will not work, because the framework is not designed to pass a value back to a binding source while this source is currently changing. You may try to do this asynchronously somehow. — Clemens
Yea, this was what I also was thinking of and afraid of at the same time :) . Will try to workaround by introducing a new property instead and do the validation in the model before passing it to my control. Thanks! — L_A_Hooper

o_w o_w · Accepted Answer · 2020-04-20T10:32:57

Setting MyProperty from code will break your TwoWay Binding.

Instead you can use SetCurrentValue method.

WPF: TwoWay Binding doesn't update back

1 Answers