Question

What are the key differences between IEnumerable Count() and Length?

Answer 1

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.