Is there any difference between Count () (linq extension) and List <T> .Count

Question

List<string> list = new List<string>() {"a", "b", "c"}; IEnumerable<string> enumerable = list; int c1 = list.Count; int c2 = list.Count(); int c3 = enumerable.Count(); 

Are there differences in performance and implementation between these last 3 statements? Will list.Count() work worse or the same as list.Count , and it matters if the link is of type IEnumerable<string> ?

Answer 1

Take a look at Reflector:

 public static int Count<TSource>(this IEnumerable<TSource> source) { if (source == null) { throw Error.ArgumentNull("source"); } ICollection<TSource> is2 = source as ICollection<TSource>; if (is2 != null) { return is2.Count; } ICollection is3 = source as ICollection; if (is3 != null) { return is3.Count; } int num = 0; using (IEnumerator<TSource> enumerator = source.GetEnumerator()) { while (enumerator.MoveNext()) { num++; } } return num; } 

So, if your IEnumerable<T> implements ICollection<T> or ICollection , it will return the Count property.

Answer 2

The Linq Count method is smart enough not to iterate over the base collection if it implements the ICollection interface and therefore already has the Count property.

Answer 3

Performing a count on IEnumerable first checks to see if the enumerated list implements ICollection<T> , where T is the general parameter of the enumerated list.

If so, it returns an ICollection<T>.Count .

If not, it checks to see if it implements ICollection . If it returns, it returns ICollection.Count .

If he does not implement any of them, he must iterate over the whole list and count, and this can be an expensive operation for a large list.

List<string> however implements ICollection<string> , and therefore the performance will be the same.

Answer 4

I assume that it works as follows: List stores its own score in a variable. Count () iterates through IEnumerable to count the number of elements. This would make List.Count more efficient.