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What are the key differences between IEnumerable Count() and Length?
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Length is a property returning the number of elements in an Array . Count() is a LINQ extension that does the same on an IEnumerable . Optionally, it can take a predicate as parameter, and will return the number of elements that satisfy it.
IEnumerable has not Count function or property. To get this, you can store count variable (with foreach, for example) or solve using Linq to get count.
IEnumerable doesn't have a Count method.
Count() is there as an extension method from LINQ - Count is a property on List s, actual . NET collection objects. As such, Count() will almost always be slower, since it will enumerate the collection / queryable object. On a list, queue, stack etc, use Count .
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.
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