LINQ |元素操作员|单个

原文:https://www.geeksforgeeks.org/linq-element-operator-single/

元素运算符用于从序列或集合中返回单个元素或特定元素。比如在一所学校当我们问,谁是校长?那么只有一个人会成为学校的校长。所以学生的数量是一个集合,校长是这个集合的唯一结果。

LINQ 标准查询运算符支持 8 种类型的元素运算符:

  1. 元素
  2. 元素默认值
  3. 第一
  4. 一阶违约
  5. 最后
  6. 货物订货标准
  7. 单字错误

单一操作员

单个运算符用于返回集合或序列的单个元素。或者它返回指定给定条件的单个元素。该方法可以通过两种不同的方式重载:

  • Single(IEnumerable,Func < TSource,Boolean > ): 此方法返回集合或序列中指定给定条件的唯一元素,如果存在多个指定给定条件的元素,将引发异常。
  • Single(IEnumerable):此方法返回给定序列或集合中的唯一元素,如果序列或集合中没有恰好一个元素,则抛出异常。

要点:

  • 它不支持 C# 和 VB.Net 语言中的查询语法。
  • 它支持 C# 和 VB.Net 语言中方法语法。
  • 它同时出现在可查询类和可枚举类中。
  • 如果序列不包含指定给定条件的元素,或者包含多个指定给定条件的元素,或者序列为空,则抛出invalidOperationException

例 1:

// C# program to illustrate the
// use of Single operator
using System;
using System.Linq;
using System.Collections.Generic;

class GFG {

    static public void Main()
    {

        // Data source
        int[] sequence1 = {112, 44, 55, 66, 77, 777, 56};
        int[] sequence2 = {2};

        // Get the element which specifies the given 
        // condition Using Single(Condition) function
        var result1 = sequence1.Single(seq => seq == 112);

        Console.WriteLine("Element: {0}", result1);

        // Get the only element of the 
        // sequence Using Single() function
        var result2 = sequence2.Single();

        Console.WriteLine("Element: {0}", result2);

        // This commented part gives you exception
        // because the given sequence does not 
        // contain elements

        /*
            int[] sequence3 = {};
            var result3 = sequence3.Single();
            Console.WriteLine("Element: {0}", result3);
        */
    }
}

Output:

Element: 112
Element: 2

例 2:

// C# program to find the 
// name of the employee
using System;
using System.Linq;
using System.Collections.Generic;

// Employee details
public class Employee {

    public int emp_id
    {
        get;
        set;
    }

    public string emp_name
    {
        get;
        set;
    }

    public string emp_gender
    {
        get;
        set;
    }

    public string emp_hire_date
    {
        get;
        set;
    }

    public int emp_salary
    {
        get;
        set;
    }
}

class GFG {

    // Main method
    static public void Main()
    {
        List<Employee> emp = new List<Employee>() {

            new Employee() {emp_id = 209, emp_name = "Anjita", emp_gender = "Female",
                                    emp_hire_date = "12/3/2017", emp_salary = 20000},

            new Employee() {emp_id = 210, emp_name = "Soniya", emp_gender = "Female",
                                    emp_hire_date = "22/4/2018", emp_salary = 30000},

            new Employee() {emp_id = 211, emp_name = "Rohit", emp_gender = "Male",
                                  emp_hire_date = "3/5/2016", emp_salary = 40000},

            new Employee() {emp_id = 212, emp_name = "Supriya", emp_gender = "Female",
                                      emp_hire_date = "4/8/2017", emp_salary = 40000},

            new Employee() {emp_id = 213, emp_name = "Anil", emp_gender = "Male",
                                emp_hire_date = "12/1/2016", emp_salary = 40000},

            new Employee() {emp_id = 214, emp_name = "Anupriya", emp_gender = "Female",
                                         emp_hire_date = "17/6/2015", emp_salary = 50000},
        };

        // Query to find the name the 
        // employee Using Single method
        var res = emp.Single(e => e.emp_name == "Anjita");

        string val = res.emp_name;

        Console.WriteLine("Employee name: {0}", val);
    }
}

Output:

Employee name: Anjita