wpf Dependency property before changing event?

Question

I have a dependency property on a control which is a custom class.

Now is there an event that gets raised BEFORE the value is being changed?

I know that OnPropertyChanged is raised after the property has already changed.

I need some event before so that I can cancel the changing....in order to preserve the state of the control.

I cannot set back the dependency property back to its old value as that will mean that I lose state in the control.

Thanks!

Answer 1

If its your DependencyProperty , you can use the ValidateValueCallback to validate the incoming value and reject it, if its not as you desire.

In the following example, only values greater than 0 will be accepted:

public int Test {
    get { return (int)GetValue(TestProperty); }
    set { SetValue(TestProperty, value); }
}


public static readonly DependencyProperty TestProperty =
    DependencyProperty.Register("Test", typeof(int), typeof(YourClass), 
    new UIPropertyMetadata(0), delegate(object v) { 
      return ((int)v) > 0; // Here you can check the value set to the dp
    });

Answer 2

If your data objects implement INotifyPropertyChanging, then you can handle the PropertyChanging event which is raised before the property value changes.

INotifyPropertyChanging was introduced in .NET 3.5

Answer 3

You may check value of property in property declaration set section. Suppose we have CustomColor dep property:

public Color CustomColor
{
   get { return GetValue(CustomColorProperty) as Color;}

   set 
   { 
      //check value before setting
      SetValue(CustomColorProperty, value);
   }
}

Also there will be helpfull for you PropertyChangedCallback , ValidateValueCallback , CoerceValueCallback delegates.