LINQ |元素运算符|最后的

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

最后的操作员

Last运算符用于返回集合或序列的最后一个元素。或返回指定给定条件的最后一个元素。可以通过两种不同的方式重载此方法：

• Last (IEnumerable )：此方法返回给定集合或序列的最后一个元素。
• Last (IEnumerable ，Func )：此方法根据给定条件返回最后一个元素。

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。
• 如果序列不包含指定给定条件的元素，或者序列为空，则抛出InvalidOperationException 。

范例1：

// C# program to illustrate the 
// use of Last 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};
        string[] sequence2 = {"Geeks", "GeeksforGeeks"};
  
        // Get the element which specifies
        // the given condition Using 
        // Last(Condition) function
        var result1 = sequence1.Last(seq => seq < 777);
          
        Console.WriteLine("Element: {0}", result1);
  
        // Get the only element of the 
        // sequence Using Last() function
        var result2 = sequence2.Last();
          
        Console.WriteLine("Element: {0}", result2);
  
        // This commented part will give 
        // InvalidOperationException because
        // the sequence does not contain element
        /*
            int[] sequence3 = {};
            var result3 = sequence3.Last();
            Console.WriteLine("Element: {0}", result3);
        */
    }
}

输出：

Element: 56
Element: GeeksforGeeks

范例2：

// C# program to find the last 
// 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 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
        // of the last employee
        // Using Last method
        var res = emp.Select(e => e.emp_name).Last();
          
        Console.WriteLine("Employee name: {0}", res);
    }
}

输出：

Employee name: Anupriya