Covariance & Contravariance

Covariance 

Before .Net 4.0 it was not possible to assign a reference variable of List of parent type to List of its child type.

IEnumerable<Employee> empList = new List<PermanentEmployee>(); X (not possible)

Where PermanentEmployee is child type of Employee type.

But possible in 4.0 on-words by introducing out keyword.

Contravariance

A base type of function pointer (delegate) reference variable was not able to get assign to reference variable of its child type.

delegate void MyPtr<T>(T o);

MyPtr<Employee> oEmp = Print;

MyPtr<PermanentEmployee> oPEmp = oEmp; X (not possible)

But possible in 4.0 on-words by introducing in keyword.

delegate void MyPtr<in T>(T o);

Below is the complete code demonstrating both.

using System;

using System.Collections.Generic;

namespace CovarianceContravariance

{

    class Program

    {

        delegate void MyPtr<in T>(T o);

        static public void Contraveriance()

        {

            MyPtr<Employee> oEmp = Print;

            MyPtr<PermanentEmployee> oPEmp = oEmp; //Backword campatability

        }

        static void Print(Employee emp)

        {

            Console.WriteLine(emp.Name);

        }

        //-------------------------------------------------------------

        public void Coveriance()

        {

            Employee emp = new PermanentEmployee(); //Polymorphism - Valid Statement

            emp = new ContractEmployee(); //Polymorphism - Valid Statement

            //Valid Statement, Invalid below 4.0

            IEnumerable<Employee> empList = new List<PermanentEmployee>();

        }

        //-------------------------------------------------------------

        static void Main(string[] args)

        {

           

        }

    }

    public class Employee

    {

        public string Name = "Animal";

    }

    public class PermanentEmployee : Employee

    {

    }

    public class ContractEmployee : Employee

    {

    }

}