Enumerable.CountBy<TSource,TKey> 方法

定义

命名空间:
System.Linq
程序集:
netstandard.dll, System.Linq.dll
Source:
CountBy.cs
Source:
CountBy.cs
Source:
CountBy.cs

返回按键分组的源序列中的元素计数。

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))

类型参数

TSource

的元素 source类型。

TKey

返回 keySelector的键的类型。

参数

source
IEnumerable<TSource>

包含要计数的元素的序列。

keySelector
Func<TSource,TKey>

用于提取每个元素的键的函数。

keyComparer
IEqualityComparer<TKey>

与键进行比较的一个 IEqualityComparer<T> 。

返回

IEnumerable<KeyValuePair<TKey,Int32>>

一个枚举，其中包含每个键出现 source的频率。

示例

下面的代码示例演示如何用于 CountBy 计算每个部门的员工数。

(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
*/

适用于