LINQ SingleOrDefault方法

本文概述

• LINQ SingleOrDefault()方法的语法
• LINQ SingleOrDefault()方法的示例

在LINQ中, SingleOrDefault()方法用于返回单个元素。如果列表/集合中没有元素, 则它将返回多个元素, 并会引发异常, 如Single()方法。

LINQ SingleOrDefault()方法的语法

这是使用LINQ SingleOrDefault()方法从集合中获取单个元素的语法。

int a = objList.SingleOrDefault();

在以上语法中, 我们将从集合” objList”中返回单个元素。并且, 如果集合不包含任何元素, 则它将返回默认值。

LINQ SingleOrDefault()方法的示例

这是LINQ SingleOrDefault方法的示例, 当元素不存在于集合中时, 该方法可从集合中获取单个元素。

using System;
using System. Collections;
using System.Collections.Generic;
using System. Linq;
using System. Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Programme2
    {
        static void Main(string[] args)
        {
//create the object 'objStudnet' of the class 'Student' added the record to the list 
            List<Student> objStudent = new List<Student>()
            {
                new Student() { Name = "Akshay Tyagi", Gender = "Male", Location="Chennai" }, new Student() { Name = "Vaishali Tyagi", Gender = "Female", Location="Chennai" }, new Student() { Name = "Arpita Rai", Gender = "Male", Location="Bangalore" }, new Student() { Name = "Shubham Rastogi", Gender = "Male", Location ="Vizag"}, new Student() { Name = "Aman Singhal", Gender = "Male", Location="Nagpur"}
            };
    //initialize the array 'vs' from 1 to 5
                int[] vs = { 1, 2, 3, 4, 5 };
    //objStudent.SingleOrDefault() method will return the information of the student
                var user = objStudent.SingleOrDefault(i => i.Name == "Akshay Tyagi");
                string result = user.Name;
                int val = vs.SingleOrDefault(j => j > 5);
                Console.WriteLine("Element from objStudent: {0}", result);
                Console.WriteLine("Element from objList: {0}", val);
                Console.ReadLine();
        }
    }
      class Student
        {
            public string Name { get; set; }
            public string Gender { get; set; }
            public string Location { get; set; }
        }
}

在上面的示例中, 我们尝试使用LINQ SingleOrDefault()方法从两个列表(objStudent, objList)对象中获取单个元素

输出

如果列表/集合返回多个元素, 则LINQ SingleOrDefault()方法将引发InvalidOperationException错误。

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
{
    class Programme2
    {
        static void Main(string[] args)
        {
           List<Student> objStudent = new List<Student>()
           {
                new Student() { Name = "Akshay Tyagi", Gender = "Male", Location="Chennai" }, new Student() { Name = "Vaishali Tyagi", Gender = "Female", Location="Chennai" }, new Student() { Name = "Arpita Rai", Gender = "Male", Location="Bangalore" }, new Student() { Name = "Shubham Rastogi", Gender = "Male", Location ="Vizag"}, new Student() { Name = "Aman Singhal", Gender = "Male", Location="Nagpur"}
            };
    //initialize the objList array from 1 to 5
            int[] objList = { 1, 2, 3, 4, 5 };
    //here SingleOrDefault()method will return the default value
            int val = objList.SingleOrDefault();

            Console.WriteLine("Element from objList: {0}", val);

            Console.ReadLine();

            //int[] vs = { 1, 2, 3, 4, 5 };

            //var user = objStudent.SingleOrDefault(i => i.Name == "Akshay Tyagi");

            //string result = user.Name;

            //int val = vs.SingleOrDefault(j => j > 5);

            //Console.WriteLine("Element from objStudent: {0}", result);

            //Console.WriteLine("Element from objList: {0}", val);

            Console.ReadLine();

        }

    }

    class Student

    {

        public string Name { get; set; }

        public string Gender { get; set; }

        public string Location { get; set; }

    }
}

如果我们运行上面的代码, 它将抛出InvalidOperationException错误, 因为” objlist”返回了多个值。

输出