First_FirstOrDefault_Single_SingleOrDefault 使用

• First 默认取第一个数据，如果没有数据会产生异常
• FirstOrDefault 默认取第一个数据，如果没有数据则为null
• Last 默认取最后一个数据，如果没有数据会产生异常
• LastOrDefault 默认取最后一个数据，如果没有数据则为null
• Single 只取一个数据，若集合中包含多个或者没有数据，抛异常
• SingleOrDefault 只取一个数据，若集合中包含多个数据，抛异常；若没有数据则为null

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Demo:

public class Program
    {
        static void Main(string[] args)
        {
            List<People> peoples = new List<People>();
            //获取数据
            peoples = GetData();
            
            //下面使用 lamda 表达式演示
            People p00 = peoples.First(p => p.ID > 1);          // 取 ID=2 的数据
            People p01 = peoples.First(p => p.ID > 6);          // 异常
            People p02 = peoples.FirstOrDefault(p => p.ID > 1); // 取 ID=2 的数据
            People p03 = peoples.FirstOrDefault(p => p.ID > 6); // null
    
            People p10 = peoples.Single(p => p.ID > 5);         // 取 ID=6 的数据 ,此时搜索结果中只有1条数据
            People p11 = peoples.Single(p => p.ID > 1);         // 异常 ,此时搜索结果中有5条数据
            People p12 = peoples.SingleOrDefault(p => p.ID > 5);// 取 ID=6 的数据 ,此时搜索结果中只有1条数据
            People p13 = peoples.SingleOrDefault(p => p.ID > 1);// null ,此时搜索结果中有5条数据
            
            People p20 = peoples.Last(p => p.ID > 1);           // 取 ID=6 的数据
            People p21 = peoples.Last(p => p.ID > 6);           // 异常 ,此时搜索结果无数据
            People p22 = peoples.LastOrDefault(p => p.ID > 1);  // 取 ID=6 的数据
            People p23 = peoples.LastOrDefault(p => p.ID > 6);  // null ,此时搜索结果无数据
        }


        //Insert Data
        public static List<People> GetData()
        {
            List<People> peoples = new List<People>();
            peoples.Add(new People
            {
                ID = 1,
                Name = "zs",
                Age = 20,
                Address = "上海"
            });
            peoples.Add(new People
            {
                ID = 2,
                Name = "xm",
                Age = 25,
                Address = "苏州"
            });
            peoples.Add(new People
            {
                ID = 3,
                Name = "ls",
                Age = 23,
                Address = "无锡"
            });
            peoples.Add(new People
            {
                ID = 4,
                Name = "wt",
                Age = 28,
                Address = "南京"
            });
            peoples.Add(new People
            {
                ID = 5,
                Name = "ll",
                Age = 26,
                Address = "镇江"
            });
            peoples.Add(new People
            {
                ID = 6,
                Name = "ql",
                Age = 30,
                Address = "合肥"
            });
            return peoples;
        }
    }

People类：

public class People
    {
        public int ID { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        public string Address { get; set; }

        public string ToString()
        {
            return "ID:"+ID+" ,Name:"+Name+" ,Age:"+Age+" ,"+Age+" ,Address:"+Address;
        }
    }