private static void OnValueChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
// Note: This callback only purpose is to validate the value change.
// The value change is not acceptable if one the the two conditions
// below is met :
// 1 - the new value is DependencyProperty.UnsetValue.
// 2 - the setter is sealed.
// We reset the value to the old value and throw an exception if one
// of these conditions is verified.
Setter setter = (Setter)d;
if (setter._throwOnNextValueChange)
{
setter._throwOnNextValueChange = false;
setter.CheckSealed();
throw new InvalidOperationException("Cannot unset a Setter value.");
}
if (setter.IsSealed || e.NewValue == DependencyProperty.UnsetValue)
{
setter._throwOnNextValueChange = true;
setter.Value = e.OldValue;
}
}