Learning C# 3.0

Static and Instance Members

The fields, properties, and methods of a class can be either instance members or static members. Instance members are associated with instances of a type, whereas static members are associated with the class itself, and not with any particular instance. All methods are instance methods unless you explicitly mark them with the keyword static.

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 static method.

You access a static 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 static method GetButtonCount( ). The job of Draw( ) is to draw the current button, and the job of GetButtonCount( ) is to return the number of buttons currently visible on the form. Since GetButtonCount( ) applies to more than just the one button, it wouldn’t make sense to call it on a specific instance of Button; therefore, it’s static.

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

btnUpdate.SomeMethod( );

You access a static method through the class name, not through an instance:

Button.GetButtonCount( ); ...