Edit

Share via

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

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#).

Using Delegates with Covariant Type Parameters

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 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;  
  
    }  
}

Using Delegates with Contravariant Type Parameters

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 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;  
    }  
}

Contravariance and anonymous functions

When working with anonymous functions (lambda expressions), you might encounter counterintuitive behavior related to contravariance. Consider the following example:

public class Person
{
    public virtual void ReadContact() { /*...*/ }
}

public class Employee : Person
{
    public override void ReadContact() { /*...*/ }
}

class Program
{
    private static void Main()
    {
        var personReadContact = (Person p) => p.ReadContact();

        // This works - contravariance allows assignment.
        Action<Employee> employeeReadContact = personReadContact;

        // This causes a compile error: CS1661.
        // Action<Employee> employeeReadContact2 = (Person p) => p.ReadContact();
    }
}

This behavior seems contradictory: if contravariance allows assigning a delegate that accepts a base type (Person) to a delegate variable expecting a derived type (Employee), why does direct assignment of the lambda expression fail?

The key difference is type inference. In the first case, the lambda expression is first assigned to a variable with type var, which causes the compiler to infer the lambda's type as Action<Person>. The subsequent assignment to Action<Employee> succeeds because of contravariance rules for delegates.

In the second case, the compiler cannot directly infer that the lambda expression (Person p) => p.ReadContact() should have type Action<Person> when it's being assigned to Action<Employee>. The type inference rules for anonymous functions don't automatically apply contravariance during the initial type determination.

Workarounds

To make direct assignment work, you can use explicit casting:

// Explicit cast to the desired delegate type.
Action<Employee> employeeReadContact = (Action<Person>)((Person p) => p.ReadContact());

// Or specify the lambda parameter type that matches the target delegate.
Action<Employee> employeeReadContact2 = (Employee e) => e.ReadContact();

This behavior illustrates the difference between delegate contravariance (which works after types are established) and lambda expression type inference (which occurs during compilation).

See also