In this post I am investigating the difference between Count property and Count() extension method of the .NET enumerable.
Below code snippet sets the scene of what we trying to answer.
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static void Main(string[] args) { IList<int> ages = ReadAges(); int ageCount = ages.Count; //Interface Property //OR int ageLinqCount = ages.Count(); //Linq extension IEnumerable<string> names = ReadNames(); var nameCount = names.Count(); //Linq extension; only option } private static IEnumerable<string> ReadNames() { //... } private static IList<int> ReadAges() { //... } |
As in the previous post will start by decompiling the System.Core 4.0.0.0 dll.
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//Count() extension method implementation public static int Count<TSource>(this IEnumerable<TSource> source) { if (source == null) { throw Error.ArgumentNull("source"); } ICollection<TSource> collection = source as ICollection<TSource>; if (collection != null) { return collection.Count; } ICollection collection2 = source as ICollection; if (collection2 != null) { return collection2.Count; } int num = 0; checked { using (IEnumerator<TSource> enumerator = source.GetEnumerator()) { while (enumerator.MoveNext()) { num++; } } return num; } } |
The Linq Count() extension method behaves differently based on the source implementation. If the source extends ICollection/ICollection<T> interface, then it relies on the underneath implementation Count property of the source. For instance, List<T>, IReadOnlyList<T>, IDictionary<TKey, TValue>, Queue, Stack, or HashSet<T> they extend ICollection, and the Count property is backed by a private field int size as shown below. So using Count property whenever is available is the most efficient.
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//mscorlib (4.0.0.0) [Serializable] public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>, IEnumerable, IList, ICollection, IReadOnlyList<T>, IReadOnlyCollection<T> { //... [__DynamicallyInvokable] public int Count { [__DynamicallyInvokable] get { return this._size; } } //... } |
Thanks, great article.