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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: DelegatesMulticast Delegates (Multicast Delegates)Anonymous Methods

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