Interface Statement (Visual Basic)
Declares the name of an interface and introduces the definitions of the members that the interface comprises.
[ <attributelist> ] [ accessmodifier ] [ Shadows ] _ Interface name [ ( Of typelist ) ] [ Inherits interfacenames ] [ [ modifiers ] Property membername ] [ [ modifiers ] Function membername ] [ [ modifiers ] Sub membername ] [ [ modifiers ] Event membername ] [ [ modifiers ] Interface membername ] [ [ modifiers ] Class membername ] [ [ modifiers ] Structure membername ] End Interface
An interface defines a set of members, such as properties and procedures, that classes and structures can implement. The interface defines only the signatures of the members and not their internal workings.
A class or structure implements the interface by supplying code for every member defined by the interface. Finally, when the application creates an instance from that class or structure, an object exists and runs in memory. For more information, see Object-Oriented Programming in Visual Basic and Interfaces in Visual Basic.
For a comparison of interfaces and classes, see Interfaces Overview.
You can use Interface only at namespace or module level. This means the declaration context for an interface must be a source file, namespace, class, structure, module, or interface, and cannot be a procedure or block. For more information, see Declaration Contexts and Default Access Levels.
Nesting Interfaces. You can define one interface within another. The outer interface is called the containing interface, and the inner interface is called a nested interface.
Member Declaration. When you declare a property or procedure as a member of an interface, you are defining only the signature of that property or procedure. This includes the element type (property or procedure), its parameters and parameter types, and its return type. Because of this, the member definition uses only one line of code, and terminating statements such as End Function or End Property are not valid in an interface.
In contrast, when you define an enumeration or structure, or a nested class or interface, it is necessary to include their data members.
Member Modifiers. You cannot use any access modifiers when defining module members, nor can you specify Shared (Visual Basic) or any procedure modifier except Overloads. You can declare any member with Shadows, and you can use Default (Visual Basic) when defining a property, as well as ReadOnly (Visual Basic) or WriteOnly.
Inheritance. If the interface uses the Inherits Statement, you can specify one or more base interfaces. You can inherit from two interfaces even if they each define a member with the same name. If you do so, the implementing code must use name qualification to specify which member it is implementing.
An interface cannot inherit from another interface with a more restrictive access level. For example, a Public interface cannot inherit from a Friend interface.
An interface cannot inherit from an interface nested within it.
Implementation. When a class uses the Implements (Visual Basic) statement to implement this interface, it must implement every member defined within the interface. Furthermore, each signature in the implementing code must exactly match the corresponding signature defined in this interface. However, the name of the member in the implementing code does not have to match the member name as defined in the interface.
When a class is implementing a procedure, it cannot designate the procedure as Shared.
Default Property. An interface can specify at most one property as its default property, which can be referenced without using the property name. You specify such a property by declaring it with the Default (Visual Basic) modifier. For more information, see Default Properties.
Notice that this means that an interface can define a default property only if it inherits none.
Access Level. All interface members implicitly have Public (Visual Basic) access. You cannot use any access modifier when defining a member. However, a class implementing the interface can declare an access level for each implemented member.
If you assign a class instance to a variable, the access level of its members can depend on whether the data type of the variable is the underlying interface or the implementing class. The following example illustrates this.
If you access class members through varAsInterface, they all have public access. However, if you access members through varAsClass, the Sub procedure doSomething has private access.
Scope. An interface is in scope throughout its namespace, class, structure, or module.
The scope of every interface member is the entire interface.
Lifetime. An interface does not itself have a lifetime, nor do its members. When a class implements an interface and an object is created as an instance of that class, the object has a lifetime within the application in which it is running. For more information, see "Lifetime" in Class Statement (Visual Basic).
The following example uses the Interface statement to define an interface named thisInterface, which must be implemented with a Property statement and a Function statement.
Note that the Property and Function statements do not introduce blocks ending with End Property and End Function within the interface. The interface defines only the signatures of its members. The full Property and Function blocks appear in a class that implements thisInterface.