LINQ |元素运算符| ElementAtOrDefault

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

ElementAtOrDefault运算符

ElementAtOrDefault运算符用于从给定的集合或序列中的特定索引返回元素，如果索引超出范围，它将提供默认值。它克服了ElementAt运算符的ArgumentOutOfRangeException问题。

• 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。
• 当索引超出范围时，它不会引发ArgumentOutOfRangeException。
• 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 ElementAtOrDefault operator
using System;
using System.Linq;
  
class GFG {
  
    static public void Main()
    {
  
        // Data source
        string[] sequence1 = {"Dog", "Cat", "Goat", "Parrot"};
        int[] sequence2 = {112, 44, 55, 66, 77, 777, 56};
  
        // Get the element at the given index
        // Using ElementAtOrDefault function
        var result1 = sequence1.ElementAtOrDefault(2);
        Console.WriteLine("Element at index 2 in sequence 1: {0}", result1);
          
        var result2 = sequence1.ElementAtOrDefault(4);
        Console.WriteLine("Element at index 4 in sequence 1: {0}", result2);
          
        var result3 = sequence1.ElementAtOrDefault(6);
        Console.WriteLine("Element at index 6 in sequence 1: {0}", result3);
          
        var result4 = sequence2.ElementAtOrDefault(2);
        Console.WriteLine("Element at index 2 in sequence 2: {0}", result4);
          
        var result5 = sequence2.ElementAtOrDefault(5);
        Console.WriteLine("Element at index 5 in sequence 2: {0}", result5);
          
        var result6 = sequence2.ElementAtOrDefault(8);
        Console.WriteLine("Element at index 8 in sequence 2: {0}", result6);
    }
}

输出：

Element at index 2 in sequence 1: Goat
Element at index 4 in sequence 1: 
Element at index 6 in sequence 1: 
Element at index 2 in sequence 2: 55
Element at index 5 in sequence 2: 777
Element at index 8 in sequence 2: 0

范例2：

// C# program to find the
// ID 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 = "Anju", emp_gender = "Female",
                                  emp_hire_date = "17/6/2015", emp_salary = 50000},
        };
  
        // Query to find the ID of
        // the employee at index 3
        // Using ElementAtOrDefault method
        var res = emp.Select(e => e.emp_id).ElementAtOrDefault(3);
          
        Console.WriteLine("Employee ID: {0}", res);
    }
}

输出：

Employee ID: 212