`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.

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.