TommyCarlier said:
One thing I sometimes miss is an interface between IEnumerable<T> and ICollection<T> that represents a read-only collection (implements IEnumerable<T>) with a Count-property. Something like this:
public interface IReadOnlyCollection<T>
 : IEnumerable<T>
{
    int Count { get; }
}
If it supports count would it not also support indexed based access? and I agree.. personally readonlycollection makes me cringe.. I don't like exposing it, and it can only ever then act as runtime protection.. I would love some readonly interfaces for data structures.. and perhaps even some immutable data structure interfaces and implementations (surely the implementations already exist in f# for example).