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Asked: 2026-05-11T10:57:04+00:00

I have a control with a DependencyProperty with a CoerceValueCallback. This property is bound

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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;         }     } }
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1 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.

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