Menggunakan Varian untuk Delegat Generik Func dan Action (Visual Basic)

Contoh-contoh ini menunjukkan cara menggunakan kovariansi dan kontravariansi dalam Func delegasi generik dan Action untuk mengaktifkan penggunaan kembali metode dan memberikan lebih banyak fleksibilitas dalam kode Anda.

Untuk informasi selengkapnya tentang kovariansi dan kontravariansi, lihat Variansi pada Delegasi (Visual Basic).

Menggunakan Delegate dengan Parameter Jenis Kovarian

Contoh berikut mengilustrasikan manfaat dukungan kovarians dalam delegasi generik Func . Metode ini FindByTitle mengambil parameter jenis String dan mengembalikan objek jenis .Employee Namun, Anda dapat menetapkan metode ini ke Func(Of String, Person) delegasi karena Employee mewarisi 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

Menggunakan Delegasi dengan Parameter Jenis Kontravarian

Manfaat dukungan kontravariansi dalam delegasi generik Action diilustrasikan oleh contoh berikut. Metode AddToContacts menerima parameter dengan tipe Person. Namun, Anda dapat menetapkan metode ini ke Action(Of Employee) delegasi karena Employee mewarisi 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

Lihat juga

• Kovariansi dan Kontravariansi (Visual Basic)
• Generics