Section 13.1. Defining an Interface

13.1 Defining an Interface

The syntax for defining an interface is very similar to the syntax for defining a class or a structure:

 [  attributes  ] [  access-modifier  ]  Interface   identifier    [  interface-bases  ]  interface-body   End Interface  

The optional attributes are not discussed in this book. Access modifiers ( Public , Private , etc.) work just as they do with classes. (See Chapter 8 for more about access modifiers.) The Interface keyword is followed by an identifier (the interface name). It is common (but not required) to begin the name of your interface with a capital I. Thus, IStorable, ICloneable, IAndThou, etc. The optional list of interface-bases is discussed in Section 13.5, later in this chapter.

The body of the interface is terminated with the keywords End Interface .

Suppose you want to create an interface to define the contract for data being stored to a database or file. Your interface will define the methods and properties a class will need to implement in order to be stored. You decide to call this interface IStorable.

In this interface, you might specify two methods, Read( ) and Write( ), and a property, Status, which appear in the interface body:

 Interface IStorable    Sub Read( )    Sub Write(object)    Property Status( ) As Integer End Interface 

Note that when declaring the methods of the interface, you provide a prototype:

 Sub Read( ) 

but no implementation and no End Function , End Sub , or End Property statement. Notice also that the IStorable method declarations do not include access modifiers (e.g., Public , Private , Protected , Friend ). In fact, providing an access modifier generates a compile error. Interface methods are implicitly public because an interface is a contract meant to be used by other classes.