Count (LINQ)

Enumerable.Count is extension method from System.Linq namespace. It counts number of items in collection.

Count Examples

Counts number of items in the IEnumerable collection.

IEnumerable<string> items = new List<string> { "A", "B", "C" }; int count = items.Count(); // count: 3

Returns 0 if there is no item in the collection.

IEnumerable<string> items = new List<string> { }; int count = items.Count(); // count: 0

Count with Predicate

Counts number of items which match specified predicate.

IEnumerable<int> items = new List<int> { 8, 3, 2 }; // these two lines do the same int count = items.Where(x => x < 5).Count(); // count: 2 int count = items.Count(x => x < 5); // count: 2

Count with Query Syntax

LINQ query expression to count number of all items in the collection.

IEnumerable<int> items = new List<int> { 8, 3, 2 }; int count = (from x in items select x).Count(); // count: 3

LINQ query expression to count number of items which match specified predicate.

IEnumerable<int> items = new List<int> { 8, 3, 2 }; int count = (from x in items where x < 5 select x).Count(); // count: 2

Count with Group By

This example shows how to count number of items per group. Lets count players in each team.

var players = new List<Player> { new Player { Name = "Alex", Team = "A" }, new Player { Name = "Anna", Team = "A" }, new Player { Name = "Luke", Team = "L" }, new Player { Name = "Lucy", Team = "L" }, new Player { Name = "Mike", Team = "M" }, }; var playersPerTeam = from player in players group player by player.Team into playerGroup select new { Team = playerGroup.Key, Count = playerGroup.Count(), }; // playersPerTeam is collection of anonymous objects: // { Team = "A", Count = 2 } // { Team = "L", Count = 2 } // { Team = "M", Count = 1 }

Count Implementation

This is .NET Framework implementation of Enumerable.Count method. Note that there is optimization. Before counting items one by one, it first checks whether the IEnumerable<T> can be cast to ICollection<T> or ICollection. If so, it simply returns value of its Count property. ICollection<T> is implemented by List<T>, Dictionary<T>, HashSet<T>, array T[] and others.

public static int Count<TSource>(this IEnumerable<TSource> source) { if (source == null) throw Error.ArgumentNull("source"); ICollection<TSource> collectionOfT = source as ICollection<TSource>; if (collectionOfT != null) return collectionOfT.Count; ICollection collection = source as ICollection; if (collection != null) return collection.Count; int count = 0; using (IEnumerator<TSource> e = source.GetEnumerator()) { checked { while (e.MoveNext()) count++; } } return count; }

Menu: