`Count`

This C# extension method enumerates elements. In many cases, the Count() extension is not useful, but in others it is appropriate.

Most collections in C# have a Length or Count property that is more efficient. For these situations, use the property directly.

Lambda example

This is a complex example of Count(). Count() can be used with an argument of type Func. The Func can be specified with a lambda.

Here In this example we count only int elements greater than 2. The values 3, 4 and 5—three elements—are greater than 2.

using System;
using System.Linq;

int[] array = { 1, 2, 3, 4, 5 };
// ... Count only elements greater than 2.
int greaterThanTwo = array.Count(element => element > 2);
Console.WriteLine(greaterThanTwo);
3

Query example

To continue, the Count() extension is found in System.Linq. It acts upon IEnumerable—so it works on Lists and arrays, even when those types have other counting properties.

Part 1 When Count() is called on Lists or arrays, you lose some performance over calling the direct properties available.

Part 2 If you use a LINQ query expression (or have an IEnumerable instance) Count is an effective option.

using System;
using System.Collections.Generic;
using System.Linq;

int[] array = { 1, 2, 3 };

// Part 1: don't use Count() like this.
// ... Use Length instead.
Console.WriteLine(array.Count());

List<int> list = new List<int>() { 1, 2, 3 };
Console.WriteLine(list.Count());

// Part 2: use Count on query expression.
var result = from element in array
             orderby element descending
             select element;
Console.WriteLine(result.Count());3
3
3

In benchmarks, the Length and Count properties are compared to the Count extension method. Using a property on a collection is much faster than Count.

A summary

The Count() method gets the number of elements in an IEnumerable collection. It enumerates the elements. This often is an inefficient way to get the element count of collections.