Covariant and contravariant generic interface C#

Covariance

If the type T is marked as "out" in a generic interface, then that means the interface will only output T. It doesnot have any method that takes T as an input. An example of a Covariant interface in .net is IEnumerable<T>. This interface is defined as

public interface IEnumerable<out T> : IEnumerable

  {

     

    IEnumerator<T> GetEnumerator();

  }

Significance:

What this practically means is that, in place of T, you can give it a child class of T.

For example if its expecting a List of objects you can give a List of strings.

using System;

using System.Collections.Generic;

namespace ConsoleApplication5

{

  

    internal class Program

    {

        private static void Main(string[] args)

        {

            var s = "";

            var o=new object();

            IEnumerable<string> strings;

            IEnumerable<object> objects;

            objects=new List<string>();//allowed 

            strings = new List<object>();//Throws Compile time error

        }

    }

    

}

Contravariance

This is the opposite of Covariance. If a type T is marked "in" in a generic interface, then that means that the interface has methods that accept T as an input and no methods in this interface output T. In the .net framework IComparer<T> is contravariant.

  public interface IComparer<in T>

  {     

    int Compare(T x, T y);

  }

Significance

 In place of T you can give it a Parent of T.

For example 

using System;

using System.Collections.Generic;

namespace ConsoleApplication5

{

    internal class Program

    {

        private static void Main(string[] args)

        {

            var s = "";

            var o = new object();

            var strings = new List<string>();

            IComparer<Animal> a1 = new ObjectComparer();//allowed

            IComparer<Animal> a2 = new PersonComparer();//throws compile time error

        }

    }

    public class Animal

    {

    }

    public class Person : Animal

    {

    }

    public class ObjectComparer : IComparer<object>

    {

        public int Compare(object x, object y)

        {

            return 0;

        }

    }

   

    public class  PersonComparer : IComparer<Person>

    {

        public int Compare(Person x, Person y)

        {

            return 0;

        }

    }

}