Interfaces (Visual Basic)

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.

Declaring Interfaces

Interface definitions are enclosed within the Interface and 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 Event, Sub, Function, Property, Interface, Class, Structure, and Enum statements. Interfaces cannot contain any implementation code or statements associated with implementation code, such as End Sub or End Property.

In a namespace, interface statements are Friend by default, but they can also be explicitly declared as Public or Friend. Interfaces defined within classes, modules, interfaces, and structures are Public by default, but they can also be explicitly declared as Public, Friend, Protected, or Private.


The Shadows keyword can be applied to all interface members. The Overloads keyword can be applied to Sub, Function, and Property statements declared in an interface definition. In addition, Property statements can have the Default, ReadOnly, or WriteOnly modifiers. None of the other modifiers—Public, Private, Friend, Protected, Shared, Overrides, MustOverride, or 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

Implementing Interfaces

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.

Implements Statement

If a class or structure implements one or more interfaces, it must include the Implements statement immediately after the Class or 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.

Implements Keyword

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 Overloads, Overrides, Overridable, Public, Private, Protected, Friend, Protected Friend, MustOverride, Default, and Static. The Shared attribute is not legal since it defines a class rather than an instance method.

Using 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, Interface2, inherits 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 Interface1:

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
            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.

Title Description
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.