In IEnumerable extensions - why is only Count () optimized for ICollection?

Question

After decompiling the Linq IEnumerable extension methods, I was glad to see that
The Count() method, before trying to iterate through all the enumerable, tries to reset it to an ICollection or ICollection<T> for example:

  public static int Count<TSource>(this IEnumerable<TSource> source) { if (source == null) throw Error.ArgumentNull("source"); ICollection<TSource> collectionoft = source as ICollection<TSource>; if (collectionoft != null) return collectionoft.Count; ICollection collection = source as ICollection; if (collection != null) return collection.Count; int count = 0; using (IEnumerator<TSource> e = source.GetEnumerator()) { checked { while (e.MoveNext()) count++; } } return count; } 

Why doesn't this happen in Any() ? wouldn't it be useful to use .Count > 0 instead of creating an array counter?

Answer 1

Not all collections provide O (1) access for the Count property. For example, access to the count property ConcurrentQueue<T> is O (n). Thus, optimization will make it worse and therefore should not be called optimization.

Not only ConcurrentQueue<T> , almost all concurrent collections ( ConcurrentDictionary<TKey,TValue> , ConcurrentStack<T> , etc.) fall into this category

Perhaps that’s why they decided not to.

Answer 2

Nope. The cost of the code will always be more than any possible gain from growth, which is substantially equal to zero. Do not forget that the whole LINQ point is a chain of such expressions, so you are most likely using Any either with a predicate or with its own set of filters, etc. Before.

In which case, when would you use Any as an alternative to Count ? Even the optimization of Count has a very subtle use of IMO - if you do Count() inside some logic that expects IEnumerable as a parameter, you are probably mistaken - at least you tend to list the enumerated several times, which is likely to break that or will kill your performance: D

Do not forget that as soon as you do something in this enumerated array, you have lost the "array", for example. categories.Where(i => i.Name.StartsWith("Hello")).Count() will not use the abbreviation.

And, of course, the cost of an enumerator on any empty enumerable is likely to be quite free. An enumerator is just a simple structure, and there will also be no costly initialization in the array.