The breaking changes are usually minor and side-effects of the other language changes. For example, IEnumerable<T> was made covariant in .Net 4.0. Consider what

The breaking changes are usually minor and side-effects of the other language changes. For example, IEnumerable<T> was made covariant in .Net 4.0. Consider what happened to code passing an IEnumerable<List<int>> to a method with two overloads taking IEnumerable<Object> and Object respectively.