ICollection not Covariant?

Question

The purpose of this is to synchronize two collections, sender-side & receiver-side, containing a graph edge, so that when something happens (remove edge, add edge, etc)

Answer 1

Eric Lippert said that C# will only support type-safe covariance and contravariance. If you would think of it, making ICollection covariant is not type-safe.

Let's say you have

ICollection dogList = new List();
ICollection mammalList = dogList; //illegal but for the sake of showing, do it
mammalList.Add(new Cat());

Your mammalList (which is actually a dogList) would now then contain a Cat.

IEnumerable is covariant because you cannot Add to it... you can only read from it -- which, in turn, preserves type-safety.