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Usando variação para delegações genéricas Func e Action (C# e Visual Basic)

Esses exemplos demonstram como usar o covariância/contravariância na Func e Action representantes genéricos para permitir a reutilização dos métodos e fornecem maior flexibilidade no seu código.

Para obter mais informações sobre covariância e contravariância, consulte Variação em delegações (C# e Visual Basic).

Usando representantes com parâmetros de tipo Covariant

O exemplo a seguir ilustra os benefícios do suporte a covariância no genérico Func delegados. O FindByTitle método aceita um parâmetro da String digite e retorna um objeto da Employee tipo. No entanto, você pode atribuir a esse método para o Func<String, Person> delegar (Func(Of String, Person) em Visual Basic) porque Employee herda 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;

    }
}

Usando representantes com parâmetros de tipo Contravariant

O exemplo a seguir ilustra os benefícios do suporte/contravariância no genérico Action delegados. O AddToContacts método aceita um parâmetro da Person tipo. No entanto, você pode atribuir a esse método para o Action<Employee> delegar (Action(Of Employee) em Visual Basic) porque Employee herda 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;
    }
}

Consulte também

Outros recursos

Covariância e contravariância (C# e Visual Basic)

Genéricos no .NET Framework