linq中first() firstordefault() last() lastOrDefault() single() singleOrDeafult

一、firstordefault() 和 first()

    class Program
    {
        static void Main(string[] args)
        {
            List<Emp> list = new List<Emp> {
             new Emp{ Id="", Name="张三", Age="", Address="河北石家庄"},
             new Emp{ Id="", Name="李四", Age="", Address="山东济南"},
             new Emp{ Id="", Name="王五", Age="", Address="河南郑州"}
            };
            //firstordefault 如果找不到返回默认值  first如果找不到会发生异常
            var emp = list.Where(m => m.Name == "张三").FirstOrDefault();
            Console.WriteLine(emp);
            var emp2 = list.Where(m => m.Name == "").First();
            Console.WriteLine(emp2);
        }
    }
    class Emp
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public string Age { get; set; }
        public string Address { get; set; }

    }

二、last() lastordefault()

  class Program
    {
        static void Main(string[] args)
        {
            List<Emp> list = new List<Emp> {
             new Emp{ Id="", Name="张三", Age="", Address="河北石家庄"},
             new Emp{ Id="", Name="李四", Age="", Address="山东济南"},
             new Emp{ Id="", Name="王五", Age="", Address="河南郑州"},
             new Emp{ Id="", Name="王五", Age="", Address="天津武清"}
            };
            //lastOrdefult 和last  如果找不到返回默认值  last如果找不到会发生异常
            var emp = list.Where(m => m.Name == "王五").LastOrDefault();
            Console.WriteLine(emp.Address);
            var emp2 = list.Where(m => m.Name == "").Last();
            Console.WriteLine(emp2);
        }
    }
    class Emp
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public string Age { get; set; }
        public string Address { get; set; }

    }
}

三、singe() singleordefault 唯一结果查询，超过一个结果报异常

     static void Main(string[] args)
        {
            List<Emp> list = new List<Emp> {
             new Emp{ Id="", Name="张三", Age="", Address="河北石家庄"},
             new Emp{ Id="", Name="李四", Age="", Address="山东济南"},
             new Emp{ Id="", Name="王五", Age="", Address="河南郑州"},
             new Emp{ Id="", Name="王五", Age="", Address="天津武清"}
            };
           // single 和singleordefault 如果序列超过一个会报异常，常用唯一结果查询
            var emp = list.Where(m => m.Name == "王五").SingleOrDefault();
            Console.WriteLine(emp.Address);
            var emp2 = list.Where(m => m.Name == "王五").Single();
            Console.WriteLine(emp2);
        }
    }
    class Emp
    {
        public string Id { get; set; }
        public string Name { get; set; }
        public string Age { get; set; }
        public string Address { get; set; }

    }