Enumerable.CountBy<TSource,TKey> Method

Definition

Namespace:

System.Linq

Assemblies:

netstandard.dll, System.Linq.dll

Source:

CountBy.cs

Returns the count of elements in the source sequence grouped by key.

public static System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<TKey,int>> CountBy<TSource,TKey>(this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,TKey> keySelector, System.Collections.Generic.IEqualityComparer<TKey>? keyComparer = default);

static member CountBy : seq<'Source> * Func<'Source, 'Key> * System.Collections.Generic.IEqualityComparer<'Key> -> seq<System.Collections.Generic.KeyValuePair<'Key, int>>

<Extension()>
Public Function CountBy(Of TSource, TKey) (source As IEnumerable(Of TSource), keySelector As Func(Of TSource, TKey), Optional keyComparer As IEqualityComparer(Of TKey) = Nothing) As IEnumerable(Of KeyValuePair(Of TKey, Integer))

Type Parameters

TSource

The type of elements of source.

TKey

The type of the key returned by keySelector.

Parameters

source

IEnumerable<TSource>

A sequence that contains elements to be counted.

keySelector

Func<TSource,TKey>

A function to extract the key for each element.

keyComparer

IEqualityComparer<TKey>

An IEqualityComparer<T> to compare keys with.

Returns

IEnumerable<KeyValuePair<TKey,Int32>>

An enumerable containing the frequencies of each key occurrence in source.

Examples

The following code example demonstrates how to use CountBy to calculate the number of employees in each department.

(string Name, int Age, string Department)[] employees =
{
    ("Saly", 23, "IT"),
    ("David", 25, "Sales"),
    ("Mahmoud", 22, "IT"),
    ("Qamar", 22, "HR"),
    ("Sara", 25, "IT"),
    ("John", 26, "HR"),
    ("Jaffar", 32, "Sales")
};

// Count the number of employees per department
var countPerDepartment = employees.CountBy(employee => employee.Department);

foreach (var item in countPerDepartment)
{
    Console.WriteLine($"Department: {item.Key} - Employees Count: {item.Value}");
}

/*
This code produces the following output:

Department: IT - Employees Count: 3
Department: Sales - Employees Count: 2
Department: HR - Employees Count: 2
*/