Question

The following example is taken from C# in Depth: What you need to master C# 2 and 3, and seems to only only cause a breaking change as jskeet has identified, but be wrong. Please explain:

delegate void SampleDelegate(string x);

 public void CandidateAction (string x)
 {
     Console.WriteLine("Snippet.CandidateAction")
 }

public class Derived: Snippet
{
   public void CandidateAction (object x)
   {
     Console.WriteLine("Derived.CandidateAction")
   }
}
....

Derived x = new Derived();
SampleDelegate factory = new SampleDelegate (x.CandidateAction);
factory ("test");

Now, why should it work altogether as SampleDelegate accept string not the object. And to my knowledge, object doesn’t derive from the string. It is the other way around. That’s what contravariance permits under c# 2.0. The seems to demonstrate the opposite effect.

Answer 1

Conceptually, Derived.CandidateAction’s signature is saying, “I can handle any object you want to throw at me.” SampleDelegate’s contract is “You have to be able to handle a string.” Now if a method can handle any object, it can certainly handle a string. So Derived.CandidateAction is capable of fulfilling what SampleDelegate needs, and can therefore be assigned to a SampleDelegate variable.

I wrote a more detailed discussion of this (admittedly from a C# 4 point of view) at http://hestia.typepad.com/flatlander/2008/12/c-covariance-and-contravariance-by-example.html.