Question

I’m trying to replace usages of T[] or List<T> as function parameters and return values with more appropriate types such as IEnumerable<T>, ICollection<T> and IList<T>.

ICollection<T> from my understanding is preferrable to IList<T> where you are only needing basic/simple collection functionality (eg an enumerator and count functionality) as it provides this with the least restriction. From reading on here one of the main differentiators I thought was that ICollection<T> doesn’t require that the underlying collection to be index based where IList<T> does?

In switching my List<T> references over I needed to replace a List<T>.GetRange() call and I was very surprised to find the ICollection<T>.TakeWhile() extension method which has an overload supporting selection based on index?! (msdn link)

I’m confused why this method exists on ICollection where there is nothing index based on this interface? Have I misunderstood or how can this method actually work if the underlying collection is eg a Hashset or something?

Answer 1

The method, like most of LINQ, is on IEnumerable<T>. Any features that just pass the indexer to the consumer (such as TakeWhile) only need to loop while incrementing a counter. Some APIs may be able to optimize using an indexer, and then it is up to them to decide whether to do that, or just use IEnumerable<T> and simply skip (etc) unwanted data.

For example:

int i = 0;
foreach(var item in source) {
    if(!predicate(i++, item)) break;
    yield return item;
}