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Using Variance for Func and Action Generic Delegates (C# and Visual Basic)

These examples demonstrate how to use covariance and contravariance in the Func and Action generic delegates to enable reuse of methods and provide more flexibility in your code.

For more information about covariance and contravariance, see Variance in Delegates (C# and Visual Basic).

The following example illustrates the benefits of covariance support in the generic Func delegates. The FindByTitle method takes a parameter of the String type and returns an object of the Employee type. However, you can assign this method to the Func<String, Person> delegate (Func(Of String, Person) in Visual Basic) because Employee inherits Person.

// 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;

    }
}

The following example illustrates the benefits of contravariance support in the generic Action delegates. The AddToContacts method takes a parameter of the Person type. However, you can assign this method to the Action<Employee> delegate (Action(Of Employee) in Visual Basic) because Employee inherits Person.

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;
    }
}
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