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針對 Func 和 Action 泛型委派使用變數 （Visual Basic）

這些範例示範如何在 和 Func 泛型委派中使用Action共變數和反變數，以重複使用方法，並在程序代碼中提供更多彈性。

如需共變數和反變數的詳細資訊，請參閱委派中的變數（Visual Basic）。

運用委派與協變型別參數

下列範例說明泛型 Func 委派中協變支持的優點。 FindByTitle 方法會採用一個 String 型別的參數，並傳回 Employee 型別的物件。 然而，您可以將這個方法指派給Func(Of String, Person)委派，因為Employee繼承自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

使用具逆變型別參數的委派

下列範例說明泛型 Action 委派中反變數支持的優點。 AddToContacts 方法接受 Person 類型的參數。 然而，您可以將這個方法指派給Action(Of Employee)委派，因為Employee繼承自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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