Interfaces define the properties, methods, and events that classes can implement. Interfaces allow you to define features as small groups of closely related properties, methods, and events; this reduces compatibility problems because you can develop enhanced implementations for your interfaces without jeopardizing existing code. You can add new features at any time by developing additional interfaces and implementations.
There are several other reasons why you might want to use interfaces instead of class inheritance:
Interfaces are better suited to situations in which your applications require many possibly unrelated object types to provide certain functionality.
Interfaces are more flexible than base classes because you can define a single implementation that can implement multiple interfaces.
Interfaces are better in situations in which you do not have to inherit implementation from a base class.
Interfaces are useful when you cannot use class inheritance. For example, structures cannot inherit from classes, but they can implement interfaces.
Interface definitions are enclosed within the
End Interface statements. Following the
Interface statement, you can add an optional
Inherits statement that lists one or more inherited interfaces. The
Inherits statements must precede all other statements in the declaration except comments. The remaining statements in the interface definition should be
Enum statements. Interfaces cannot contain any implementation code or statements associated with implementation code, such as
End Sub or
In a namespace, interface statements are
Friend by default, but they can also be explicitly declared as
Friend. Interfaces defined within classes, modules, interfaces, and structures are
Public by default, but they can also be explicitly declared as
Shadows keyword can be applied to all interface members. The
Overloads keyword can be applied to
Property statements declared in an interface definition. In addition,
Property statements can have the
WriteOnly modifiers. None of the other modifiers—
Overridable—are allowed. For more information, see Declaration Contexts and Default Access Levels.
For example, the following code defines an interface with one function, one property, and one event.
Interface IAsset Event ComittedChange(ByVal Success As Boolean) Property Division() As String Function GetID() As Integer End Interface
The Visual Basic reserved word
Implements is used in two ways. The
Implements statement signifies that a class or structure implements an interface. The
Implements keyword signifies that a class member or structure member implements a specific interface member.
If a class or structure implements one or more interfaces, it must include the
Implements statement immediately after the
Structure statement. The
Implements statement requires a comma-separated list of interfaces to be implemented by a class. The class or structure must implement all interface members using the
Implements keyword requires a comma-separated list of interface members to be implemented. Generally, only a single interface member is specified, but you can specify multiple members. The specification of an interface member consists of the interface name, which must be specified in an implements statement within the class; a period; and the name of the member function, property, or event to be implemented. The name of a member that implements an interface member can use any legal identifier, and it is not limited to the
InterfaceName_MethodName convention used in earlier versions of Visual Basic.
For example, the following code shows how to declare a subroutine named
Sub1 that implements a method of an interface:
Class Class1 Implements interfaceclass.interface2 Sub Sub1(ByVal i As Integer) Implements interfaceclass.interface2.Sub1 End Sub End Class
The parameter types and return types of the implementing member must match the interface property or member declaration in the interface. The most common way to implement an element of an interface is with a member that has the same name as the interface, as shown in the previous example.
To declare the implementation of an interface method, you can use any attributes that are legal on instance method declarations, including
Shared attribute is not legal since it defines a class rather than an instance method.
Implements, you can also write a single method that implements multiple methods defined in an interface, as in the following example:
Class Class2 Implements I1, I2 Protected Sub M1() Implements I1.M1, I1.M2, I2.M3, I2.M4 End Sub End Class
You can use a private member to implement an interface member. When a private member implements a member of an interface, that member becomes available by way of the interface even though it is not available directly on object variables for the class.
Interface Implementation Examples
Classes that implement an interface must implement all its properties, methods, and events.
The following example defines two interfaces. The second interface,
Interface1 and defines an additional property and method.
Interface Interface1 Sub sub1(ByVal i As Integer) End Interface ' Demonstrates interface inheritance. Interface Interface2 Inherits Interface1 Sub M1(ByVal y As Integer) ReadOnly Property Num() As Integer End Interface
The next example implements
Interface1, the interface defined in the previous example:
Public Class ImplementationClass1 Implements Interface1 Sub Sub1(ByVal i As Integer) Implements Interface1.sub1 ' Insert code here to implement this method. End Sub End Class
The final example implements
Interface2, including a method inherited from
Public Class ImplementationClass2 Implements Interface2 Dim INum As Integer = 0 Sub sub1(ByVal i As Integer) Implements Interface2.sub1 ' Insert code here that implements this method. End Sub Sub M1(ByVal x As Integer) Implements Interface2.M1 ' Insert code here to implement this method. End Sub ReadOnly Property Num() As Integer Implements Interface2.Num Get Num = INum End Get End Property End Class
You can implement a readonly property with a readwrite property (that is, you do not have to declare it readonly in the implementing class). Implementing an interface promises to implement at least the members that the interface declares, but you can offer more functionality, such as allowing your property to be writable.
|Walkthrough: Creating and Implementing Interfaces||Provides a detailed procedure that takes you through the process of defining and implementing your own interface.|
|Variance in Generic Interfaces||Discusses covariance and contravariance in generic interfaces and provides a list of variant generic interfaces in the .NET Framework.|