C＃依赖项属性-是否在CLR包装器中检查值

Question

On writing custom dependency proerties in C#, a fairly common wrapper goes something like:

    public string Surname
    {
        get
        {
            return this.GetValue(SurnameProperty) as string;
        }

        set
        {
            this.SetValue(SurnameProperty, value);
        }
    }

Now, when using the NotifyPropertyChanged system, I would normally check whether a value has changed in the 'set' block before actually committing the value and calling OnPropertyChanged. Should I do the same for dependency properties? ie:

        set
        {
            if(this.GetValue(SurnameProperty) != value)
               this.SetValue(SurnameProperty, value);
        }

...or is this something that is completely unnecessary and already taken care of by the CLR? All examples on MSDN that I have seen to not bother to do any checking before calling SetValue. Many thanks.

Answer 1

The short answer is, no it's taken care by the framework already.

Actually, according to Adam Nathan's "Windows Presentation Foundation", XAML compiler depends on the property warpper at compile time. However, .NET property wrappers are actually bypassed at run-time in XAML. Therefore, you should actually avoid adding any kinds of logic in addtion to GetValue/SetValue. Whatever logic you added in the setter is only executed if you call the property explicitly. However, if you bind that property in XAML, the runtime will skip it. If you have a bug in your setter, this may take you some time to figure out. Please see page 53 if you got that book in your hand.

Answer 2

不，它由依赖项属性系统处理。

Answer 3

It is taken care by the property changed callback. The PropertyChangedCallback is called only when there is a change in the property value.

public static readonly DependencyProperty MyDependencyProperty =
                   DependencyProperty.Register(
                         "Dependency",
                          typeof(string),
                          typeof(MyUserControl),
                          new FrameworkPropertyMetadata("Initializing...", new PropertyChangedCallback(OnMyDependencyChangedCallBack)));