Covariance Vs Contravariance

Covariance and Contravariance are the two new features introduced by C# 2.0 in which, delegate objects support the notion of covariance on their return type and contravariance on their arguments.

Covariance enables a method to be assigned to a delegate when the method's return type is a class derived from the class specified by the return type of the delegate.

Contravariance enables a method to be assigned to a delegate when a method's parameter type is a base class specified by the delagate's declaration.

Example:

// Demonstrate covariance and contravariance.

using System;

class ReturnX

{

  public int iValue;

}

// ArgumentY is derived from ReturnX.

class ArgumentY : ReturnX { }

// This delegate returns ReturnX and takes an ArgumentY argument.

delegate ReturnX ChangeIt(ArgumentY obj);

class CoContraVariance

{

  // This method returns ReturnX and has an ReturnX parameter.

  static ReturnX IncrA(ReturnX obj)

  {

    ReturnX oReturnX = new ReturnX();

    oReturnX.iValue = obj.iValue + 1;

    return oReturnX;

  }

  // This method returns ArgumentY and has a ArgumentY parameter.

  static ArgumentY IncrB(ArgumentY obj)

  {

    ArgumentY oArgumentY = new ArgumentY();

    oArgumentY.iValue = obj.iValue + 1;

    return oArgumentY;

  }

  static void Main()

  {

    ArgumentY oArgumentY = new ArgumentY();

    // In this case, the parameter to IncrA is ReturnX and the parameter to ChangeIt is 
    // ArgumentY.

    // Because of contravariance, the following line is OK.

    ChangeIt oChangeIt = IncrA;

    ReturnX oReturnX = oChangeIt(oArgumentY);

    Console.WriteLine("oReturnX: " + oReturnX.iValue);

    // In the next case, the return type of IncrB is ArgumentY and the return type of 
    // ChangeIt is ReturnX.

    // Because of covariance, the following line is OK.

    oChangeIt = IncrB;

    oArgumentY = (ArgumentY) oChangeIt(oArgumentY);

    Console.WriteLine("oArgumentY: " + oArgumentY.iValue);

  }

}

The output from the program is shown here:

oReturnX: 1

oArgumentY: 1

Covariance and Contravariance – VS2010 C# - Part 1

Covariance and Contravariance – VS2010 C# - Part 2

Covariance and Contravariance – VS2010 C# - Part 3

Related Post: Delegates, Multicast Delegates (Multicast Delegates), Anonymous Methods