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針對 Func 與 Action 泛型委派使用變異數 (Visual Basic)

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下列範例示範如何在 Func 和 Action 泛型委派中使用共變數和反變數，以便在您的程式碼中重複使用方法並提供更多彈性。

如需共變數與反變數的詳細資訊，請參閱委派中的變異數 (Visual Basic)。

使用具有 Covariant 型別參數的委派

下列範例說明在泛型 Func 委派中支援共變數的好處。 FindByTitle 方法使用 String 類型的參數，並傳回 Employee 類型的物件。 不過，您可以將此方法指派給 Func(Of String, Person) 委派，因為 Employee 會繼承 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

使用具有 Contravariant 型別參數的委派

下列範例說明在泛型 Action 委派中支援反變數的好處。 AddToContacts 方法使用 Person 類型的參數。 不過，您可以將此方法指派給 Action(Of Employee) 委派，因為 Employee 會繼承 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

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