Dependency property error

Question

I am learning dependency properties. I read many posts & books but still I am not clear.

The program shown below is which I wrote to learn. Some error in that, please help in resolving. I have questions.

1. The main use of custom Dependency property element is for the notification of change?
2. I found an 'IsDefaultProperty' code for Button in a WPF text book. It means 'IsDefault' property is a dependency property?
3. Why they shown that code? It means, internally, in Button class, its defined like that? (They showed internal code?) or they showed how to define as custom?

Here is my code:

namespace DependencyProperties
{
    public class Contact
    {
        private int id=100;
        private string name="shri";
        public static readonly DependencyProperty IsPresentProperty;

        public int ID
        {
            get { return id; }
        }
        public string NAME
        {
            get { return name; }
        }

        static Contact()
        {
            IsPresentProperty = DependencyProperty.Register("IsPresent", typeof(bool),typeof(Contact),new FrameworkPropertyMetadata(false,new PropertyChangedCallback(OnIsPresentChanged)));
        }

        public bool Present
        {
            get { return (bool)GetValue(Contact.IsPresentProperty); }
            set { SetValue(Contact.IsPresentProperty, value); }
        }

        private static void OnIsPresentChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
        {  

        }
    }
}

And I see the error:

> Error: GetValue and SetValue does not exist in the current context

Answer 1

DependencyProperty instances must be defined on instances of DependencyObject. So, your class must derive from DependencyObject, or one of its subclasses. Many types in WPF derive from this, including Button.

So to get your code working in this case, you must use:

public class Contact : DependencyObject
{
    // ...

This is why you are receiving an error on GetValue and SetValue -- they are defined on DependencyObject.

Answer 2

The main use of custom Dependency property element is for the notification of change?

No, that can also be arranged by having the class implement INotifyPropertyChanged as well. Dependency properties to provide change notifications, but the real rationale is something different. See Why dependency properties? and How is the WPF property system economical?

I found an 'IsDefaultProperty' code for Button in a WPF text book. It means 'IsDefault' property is a dependency property?

Yes. Naming a field "FooBarProperty" is the WPF convention for defining dependency properties; you can check the docs for IsDefaultProperty to see that it's indeed a dependency property, and even the IsDefault documentation has a section called "dependency property information".

Why they shown that code? It means, internally, in Button class, its defined like that? (They showed internal code?) or they showed how to define as custom?

I 'm not sure what "that" code is, but yes, the property is almost certainly defined like that in Button ("almost" because I 'm not sure what you are referring to).

Error: GetValue and SetValue does not exist in the current context

That's because you are not deriving from DependencyObject.