对 Func 和 Action 泛型委托使用变体（C# 和 Visual Basic）
下例演示如何在 Func 和 Action 泛型委托中使用协变和逆变，以便能够在代码中重用方法及提供更大的灵活性。
有关协变和逆变的更多信息，请参见委托中的变体（C# 和 Visual Basic）。
使用具有协变类型参数的委托
下例展示了 Func 泛型委托中的协变支持带来的益处。 FindByTitle 方法获取 String 类型的参数，返回 Employee 类型的对象。 但是，您可以将此方法指派给 Func<String, Person> 委托（在 Visual Basic 中为 Func(Of String, Person)），因为 Employee 继承 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
// 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;
}
}
使用具有逆变类型参数的委托
下例展示了 Action 泛型委托中的逆变支持带来的益处。 AddToContacts 方法获取 Person 类型的参数。 但是，您可以将此方法指派给 Action<Employee> 委托（在 Visual Basic 中为 (Action(Of Employee)），因为 Employee 继承 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
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;
}
}