# Using Variance in Delegates (Visual Basic)

When you assign a method to a delegate, covariance and contravariance provide flexibility for matching a delegate type with a method signature. Covariance permits a method to have return type that is more derived than that defined in the delegate. Contravariance permits a method that has parameter types that are less derived than those in the delegate type.

## Example 1: Covariance

### Description

This example demonstrates how delegates can be used with methods that have return types that are derived from the return type in the delegate signature. The data type returned by DogsHandler is of type Dogs, which derives from the Mammals type that is defined in the delegate.

### Code

Class Mammals
End Class

Class Dogs
Inherits Mammals
End Class
Class Test
Public Delegate Function HandlerMethod() As Mammals
Public Shared Function MammalsHandler() As Mammals
Return Nothing
End Function
Public Shared Function DogsHandler() As Dogs
Return Nothing
End Function
Sub Test()
Dim handlerMammals As HandlerMethod = AddressOf MammalsHandler
' Covariance enables this assignment.
Dim handlerDogs As HandlerMethod = AddressOf DogsHandler
End Sub
End Class


## Example 2: Contravariance

### Description

This example demonstrates how delegates can be used with methods that have parameters whose types are base types of the delegate signature parameter type. With contravariance, you can use one event handler instead of separate handlers. The following example makes use of two delegates:

• A KeyEventHandler delegate that defines the signature of the Button.KeyDown event. Its signature is:

Public Delegate Sub KeyEventHandler(sender As Object, e As KeyEventArgs)

• A MouseEventHandler delegate that defines the signature of the Button.MouseClick event. Its signature is:

Public Delegate Sub MouseEventHandler(sender As Object, e As MouseEventArgs)


The example defines an event handler with an EventArgs parameter and uses it to handle both the Button.KeyDown and Button.MouseClick events. It can do this because EventArgs is a base type of both KeyEventArgs and MouseEventArgs.

### Code

' Event handler that accepts a parameter of the EventArgs type.
Private Sub MultiHandler(ByVal sender As Object,
ByVal e As System.EventArgs)
Label1.Text = DateTime.Now
End Sub

Private Sub Form1_Load(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles MyBase.Load

' You can use a method that has an EventArgs parameter,
' although the event expects the KeyEventArgs parameter.