And now for Contravariance

Take the classes in the last post (First, Second, Third) and assume they are exactly the same in this example.

In Covariance, we learned that whatever variable to you set equal to the return of a method has to be equal in type of larger. Or in other words, it has to have equal or less functionality.

  Second second = ReturnThird();  //OK since second has less functionality

  Third third = ReturnSecond(); //BAD since third has more functionality

Now I think you can guess what Contravariance is, but if you can’t it’s ok. Most likely you’re a tool just like me. Contravariance is the movement from small to large meaning that the type must be equal to or larger than. Following the “in other words” manor, it has to have equal or more functionality.

Now small note before I go on, saying that it has to have more/less functionality can be somewhat dangerous. After all, Third could inherit from Second and add no functionality, but I find this is an easier way to think of it. I suppose another way of thinking of it is that with Covariance the return type has to have equal or more knowledge. Meaning, Second has full knowledge of what First is, but Second has no idea what Third is.

Anywho, onto some examples. Say we take the FillX methods and add something to it.

  private void FillFirst(First firstToFill)
    firstToFill.FirstOutput = "";

  private void FillSecond(Second secondToFill)
    secondToFill.SecondOutput = "";

  private void FillThird(Third thirdToFill)
    thirdToFill.ThirdOutput = "";

Right off the bat you might notice that if methods allowed Covariance with parameters, you’d be in trouble. After all, if FillThird allowed parameter covariance, you could pass in a First object. What what that object do with ThirdOutPut? As things are, you would have a bad day. Lucky for you, at least if you aren’t adamant about wanting Covariance in parameters, this can’t happen.

Well shoot, I just gave away the fun of this post. Oh well, I’ll keep going in case you just have more time.

  Action<First> fillFirstAction = FillFirst;
  //No problems here since FillFirst expects a First

  Action<Second> fillSecondAction = FillFirst;
  //Still no problems although this may look odd.  But remember, FillFirst
  //just needs an object that : First, it doesn't care if the object
  //has more functionality than first.
  //The FillFirst method uses the FirstOutput property and by inheritance
  //the Second being passed in has said property

  Action<Second> fillThirdAction = FillThird;
  //Not gonna happen.  The FillThird expects a third or smaller object.  Since
  //Third : Second, third is smaller than second.  Implications?  Look in the 
  //The method expects the object to have the ThirdOutput property which means
  //Second has to inherit from Third.  We know this to be untrue.

So basically Contravariance is used with parameters in methods to guarantee the object being passed in has at least the functionality used within the method.

Apparently there was a problem in the lobby so there will be no refreshments served tonight.