IDictionary<,> contravariance?

Question

I have the following method in an external class

public static void DoStuffWithAnimals(IDictionary animals)

In my cal

Answer 1

1. I believe what you want is called covariance, not contravariance.
2. Classes in .Net can't be co- or contravariant, only interfaces and delegates can.

3. IDictionary can't be covariant, because that would allow you to do something like:

   IDictionary sheep = new Dictionary();
   IDictionary animals = sheep;
   animals.Add("another innocent sheep", new Wolf());
   

4. There is IReadOnlyDictionary in .Net 4.5. At first sight it could be covariant (the other new interface, IReadOnlyList is covariant). Unfortunately, it isn't, because it also implements IEnumerable> and KeyValuePair is not covariant.

5. To work around this, you could make your method generic with a constraint of the type parameter:

   void DoStuffWithAnimals(IDictionary animals) where T : Animal