Programming Visual Basic .NET, Second Edition

Using Shared Members

The properties and methods of a class can be either instance members or shared members. Instance members are associated with instances of a type, while shared members are associated with the class, and not with any particular instance. Methods are instance methods unless you explicitly mark them with the keyword Shared.

The vast majority of methods will be instance methods. The semantics of an instance method are that you are taking an action on a specific object. From time to time, however, it is convenient to be able to invoke a method without having an instance of the class, and for that you will use a shared method.

You can access a shared member through the name of the class in which it is declared. For example, suppose you have a class named Button and have instantiated objects of that class named btnUpdate and btnDelete.

Suppose that the Button class has an instance method Draw( ) and a shared method GetButtonCount( ). The job of Draw( ) is to draw the current button; the job of GetButtonCount( ) is to return the number of buttons currently visible on the form.

You access an instance method through an instance of the class; that is, through an object:

btnUpdate.SomeMethod( )

You can access a shared method in the same way:

btnUpdate.GetButtonCount( )

You can also access a shared method through the class name (rather than through an instance):

Button.GetButtonCount( )

This allows you to access the shared method without having an instance of the class. ...