Verwenden von Varianz für die generischen Delegaten Func und Action (C# und Visual Basic)
In diesen Beispielen wird veranschaulicht, wie Kovarianz und Kontravarianz in den generischen Delegaten Func und Action verwendet werden, um die Wiederverwendung von Methoden zu ermöglichen und Code flexibler zu gestalten.
Weitere Informationen zu Ko- und Kontravarianz Sie unter Varianz in Delegaten (C# und Visual Basic).
Verwenden von Delegaten mit kovarianten Typparametern
Im folgenden Beispiel werden die Vorteile der Kovarianzunterstützung in den generischen Func-Delegaten veranschaulicht. Die FindByTitle-Methode akzeptiert einen Parameter vom Typ String und gibt ein Objekt vom Typ Employee zurück. Sie können diese Methode jedoch dem Func<String, Person>-Delegaten (Func(Of String, Person) in Visual Basic) zuweisen, da Employee den Typ Person erbt.
' 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
// Simple hierarchy of classes.
public class Person { }
public class Employee : Person { }
class Program
{
static Employee FindByTitle(String title)
{
// This is a stub for a method that returns
// an employee that has the specified title.
return new Employee();
}
static void Test()
{
// Create an instance of the delegate without using variance.
Func<String, Employee> findEmployee = FindByTitle;
// The delegate expects a method to return Person,
// but you can assign it a method that returns Employee.
Func<String, Person> findPerson = FindByTitle;
// You can also assign a delegate
// that returns a more derived type
// to a delegate that returns a less derived type.
findPerson = findEmployee;
}
}
Verwenden von Delegaten mit kontravarianten Typparametern
Im folgenden Beispiel werden die Vorteile der Kontravarianzunterstützung in den generischen Action-Delegaten veranschaulicht. Die AddToContacts-Methode akzeptiert einen Parameter vom Typ Person. Sie können diese Methode jedoch dem Action<Employee>-Delegaten ((Action(Of Employee) in Visual Basic) zuweisen, da Employee den Typ Person erbt.
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
public class Person { }
public class Employee : Person { }
class Program
{
static void AddToContacts(Person person)
{
// This method adds a Person object
// to a contact list.
}
static void Test()
{
// Create an instance of the delegate without using variance.
Action<Person> addPersonToContacts = 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.
Action<Employee> addEmployeeToContacts = AddToContacts;
// You can also assign a delegate
// that accepts a less derived parameter to a delegate
// that accepts a more derived parameter.
addEmployeeToContacts = addPersonToContacts;
}
}