What are the key differences between IEnumerable
Count()
and Length
?
By calling Count on IEnumerable<T>
I'm assuming you're referring to the extension method Count
on System.Linq.Enumerable
. Length
is not a method on IEnumerable<T>
but rather a property on array types in .Net such as int[]
.
The difference is performance. TheLength
property is guaranteed to be a O(1) operation. The complexity of the Count
extension method differs based on runtime type of the object. It will attempt to cast to several types which support O(1) length lookup like ICollection<T>
via a Count
property. If none are available then it will enumerate all items and count them which has a complexity of O(N).
For example
int[] list = CreateSomeList();
Console.WriteLine(list.Length); // O(1)
IEnumerable<int> e1 = list;
Console.WriteLine(e1.Count()); // O(1)
IEnumerable<int> e2 = list.Where(x => x <> 42);
Console.WriteLine(e2.Count()); // O(N)
The value e2
is implemented as a C# iterator which does not support O(1) counting and hence the method Count
must enumerate the entire collection to determine how long it is.