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Using Variance for Func and Action Generic Delegates (Visual Basic)

  • Artikkeli

These examples demonstrate how to use covariance and contravariance in the Func and Action generic delegates to enable reuse of methods and provide more flexibility in your code.

For more information about covariance and contravariance, see Variance in Delegates (Visual Basic).

Using Delegates with Covariant Type Parameters

The following example illustrates the benefits of covariance support in the generic Func delegates. The FindByTitle method takes a parameter of the String type and returns an object of the Employee type. However, you can assign this method to the Func(Of String, Person) delegate because Employee inherits Person.

' Simple hierarchy of classes.
Public Class Person
End Class

Public Class Employee
    Inherits Person
End Class

Class Finder
    Public Shared Function FindByTitle(
        ByVal title As String) As Employee
        ' This is a stub for a method that returns
        ' an employee that has the specified title.
        Return New Employee
    End Function

    Sub Test()
        ' Create an instance of the delegate without using variance.
        Dim findEmployee As Func(Of String, Employee) =
            AddressOf FindByTitle

        ' The delegate expects a method to return Person,
        ' but you can assign it a method that returns Employee.
        Dim findPerson As Func(Of String, Person) =
            AddressOf FindByTitle

        ' You can also assign a delegate
        ' that returns a more derived type to a delegate
        ' that returns a less derived type.
        findPerson = findEmployee
    End Sub
End Class

Using Delegates with Contravariant Type Parameters

The following example illustrates the benefits of contravariance support in the generic Action delegates. The AddToContacts method takes a parameter of the Person type. However, you can assign this method to the Action(Of Employee) delegate because Employee inherits Person.

Public Class Person
End Class

Public Class Employee
    Inherits Person
End Class

Class AddressBook
    Shared Sub AddToContacts(ByVal person As Person)
        ' This method adds a Person object
        ' to a contact list.
    End Sub

    Sub Test()
        ' Create an instance of the delegate without using variance.
        Dim addPersonToContacts As Action(Of Person) =
            AddressOf AddToContacts

        ' The Action delegate expects
        ' a method that has an Employee parameter,
        ' but you can assign it a method that has a Person parameter
        ' because Employee derives from Person.
        Dim addEmployeeToContacts As Action(Of Employee) =
            AddressOf AddToContacts

        ' You can also assign a delegate
        ' that accepts a less derived parameter
        ' to a delegate that accepts a more derived parameter.
        addEmployeeToContacts = addPersonToContacts
    End Sub
End Class

See also