Implements Statement

Implements Statement

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Syntax

 Implements   InterfaceName   [,   InterfaceName   ][,...] 

InterfaceName (required; String literal)

The name of the interface that a class implements

Description

The Implements statement specifies that you will implement an interface within the class in which the Implements statement appears.

Rules at a Glance

• Implementing an interface or class means that the implementing class will provide code to implement every Public member of the implemented interface or class. If you fail to implement even a single Public member, an error will result.

• The Implements statement cannot be used in a standard module; it is used only in class modules.

• By convention, interface names begin with a capital I , as in IMyInterface .

• For more information on this topic, see Chapter 4.

Example

 Friend Interface IAnimal    ReadOnly Property Name(  ) As String    Function Eat(  ) As String    Function SoundNoise(  ) As String End Interface Public Class CWolf    Implements IAnimal    Public ReadOnly Property Name(  ) As String _                              Implements IAnimal.Name       Get          Return "Wolf"       End Get    End Property    Public Function Eat(  ) As String Implements IAnimal.Eat       Eat = "caribou, salmon, other fish"    End Function    Public Function Sound(  ) As String Implements IAnimal.SoundNoise       Sound = "howl"    End Function End Class Module modMain Public Sub Main    Dim oWolf As New CWolf    Console.WriteLine(oWolf.Sound)    oWolf = Nothing End Sub End Module 

Programming Tips and Gotchas

• If you do not wish to support a procedure from the implemented class, you must still create a procedure declaration for the implemented procedure. However, you can simply raise an error using the special error constant Const E_ NOTIMPL = &H80004001 so a user will know that the member is not implemented in any meaningful way. Alternately, you can also raise a NotImplementedException exception.

• Interfaces, or abstract base classes, allow for greater coherence when developing in teams . For example, all developers could use a set of interfaces to produce controls and objects of a particular type without being constrained by implementation. That is, each developer would be free to implement a particular property or method in the way that he saw fit.

• Maintaining compatibility across multiple versions dictates that interfaces should not change once they have been written and distributed. Any additional functionality required should be provided by defining additional interfaces.

• VB.NET provides only single inheritance using the Inherits statement. However, by using interface-based inheritance with the Implements statement, you can in effect implement multiple inheritance.

VB.NET/VB 6 Differences

• In VB 6, the Implements statement does not support events; any events publicly declared in an interface are ignored. VB.NET, on the other hand, allows derived classes to trap the events defined in interfaces.

See Also

Implements Statement, Interface Statement