📜  LINQ |元素运算符|单一或默认

📅  最后修改于: 2021-05-29 19:04:25             🧑  作者: Mango

元素运算符用于从序列或集合中返回单个或特定元素。例如,当我们问学校时,校长是谁?这样一来,只有一人成为学校的校长。因此,学生人数是一个集合,而校长是唯一来自集合的结果。

LINQ标准查询运算符支持8种类型的元素运算符:

  1. ElementAt
  2. ElementAtOrDefault
  3. 第一的
  4. 第一或默认
  5. 最后的
  6. LastOrDefault
  7. 单身的
  8. 单一或默认

SingleOrDefault运算子

SingleOrDefault运算符用于返回集合或序列的单个元素。基本上,它返回指定给定条件的单个元素。如果给定的集合或序列不包含任何值,则返回默认值。可以通过两种不同的方式重载此方法:

  • SingleOrDefault (IEnumerable ,Func ):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,则将引发异常。
  • SingleOrDefault (IEnumerable ):此方法返回给定序列或集合中的唯一元素,如果序列或集合中存在多个元素,则引发异常。

重要事项:

  • 它不支持C#和VB.Net语言的查询语法。
  • 它支持C#和VB.Net语言的方法语法。
  • 它同时存在于Queryable和Enumerable类中。
  • 如果它包含指定给定条件的多个元素或给定序列或集合中存在的多个元素,则抛出InvalidOperationException。
  • 引用类型和可为空的类型的默认值为null。

范例1:

// C# program to illustrate the use
// of SingleOrDefault operator
using System;
using System.Linq;
using System.Collections.Generic;
  
class GFG {
  
    // Main Method
    static public void Main()
    {
  
        // Data source
        int[] sequence1 = {112, 44, 55, 66, 77, 777, 56};
        int[] sequence2 = {2 };
        int[] sequence3 = {};
  
        // Get the element which specifies
        // the given condition Using the 
        // SingleOrDefault(Condition)
        var result1 = sequence1.SingleOrDefault(seq => seq == 777);
          
        Console.WriteLine("Element: {0}", result1);
  
        // Get the only element of the sequence
        // Using SingleOrDefault()
        var result2 = sequence2.SingleOrDefault();
          
        Console.WriteLine("Element: {0}", result2);
  
        // Get the default value
        // Using SingleOrDefault() 
        var result3 = sequence3.SingleOrDefault();
          
        Console.WriteLine("Element: {0}", result3);
    }
}
输出:
Element: 777
Element: 2
Element: 0

范例2:

// C# program to find the name the employee
// whose salary is equal to 50000
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 emp = new List() {
              
            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
        // whose salary is equal to 50000
        // Using SingleOrDefault method
        var res = emp.SingleOrDefault(e => e.emp_salary == 50000);
          
        string val = res.emp_name;
          
        Console.WriteLine("Employee name: {0}", val);
    }
}
输出:
Employee name: Anupriya