为什么ValidateValueCallback被调用两次？

Question

Suppose I've the following DependencyObject type,

public class Test : DependencyObject
{


    public int Order
    {
        get { return (int)GetValue(OrderProperty); }
        set { SetValue(OrderProperty, value); }
    }

    public static readonly DependencyProperty OrderProperty =
        DependencyProperty.Register("OrderProperty",
        typeof(int),
        typeof(Test),
        new FrameworkPropertyMetadata(6,
            new PropertyChangedCallback(OnOrderPropertyChanged),
            new CoerceValueCallback(OnCoerceValueCallBack)),
        new ValidateValueCallback(OnValidateValueCallBack));

    static void OnOrderPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        Console.WriteLine("OnOrderPropertyChanged");
    }

    static object OnCoerceValueCallBack(DependencyObject d, object baseValue)
    {
        Console.WriteLine("OnCoerceValueCallBack");
       return 200;
    }

    static bool OnValidateValueCallBack(object value)
    {
        Console.WriteLine("OnValidateValueCallBack");
        int iValue = (int)value;
        return iValue > 5;
    }
}

When I create an instance of Test , I see OnValidateValueCallBack is called twice and OnCoerceValueCallBack isn't called at all. Based on what I saw, I guess when I create the instance, WPF will call OnValidateValueCallBack to check whether the default value is valid, if so, it will use the default value and won't call CoerceValueCallback at all, so where does the second call of OnValidateValueCallBack come from?

Answer 1

I can confirm this behavior. Upon registering a dependency property, the validate callback is called by the internals of WPF. One call always occurs , regardless of the metadata passed. A second call only occurs when a default value is passed with the metadata.

Originally I thought the first call was due to internal initialization of WPF, so it would always be the default value of the type. This is not the case! The object passed in the first call is either the default value of the type, or the default value which is specified in the metadata . The second call seems more logical, and just passes the default value.

No coerce or changed callbacks are called upon registering.

What the use of the first call is is a mystery to me.

Relevant (unanswered) question on msdn.