Linq

一、LINQ查询表达式基础

  1、简单的 LINQ 查询

    static class Program
    {
        static void Main(string[] args)
        {
            List<Student> list = new List<Student>()
            {
                new Student(){Id=1,Name="AA",score=60},
                new Student(){Id=1,Name="BB",score=82},
                new Student(){Id=1,Name="CC",score=90},
                new Student(){Id=1,Name="DD",score=98},
                new Student(){Id=1,Name="EE",score=76},
            };
            var itemList = from l in list
                           where l.score > 80
                           orderby l.score ascending
                           select l;
            foreach (var item in itemList)
            {
                Console.WriteLine("{0}-{1}", item.Name, item.score);
            }
        }
    }
    /// <summary>
    /// Student Model
    /// </summary>
    public class Student
    {
        public int Id { set; get; }

        public string Name { set; get; }

        public int score { set; get; }
    }

  2、Where 的使用

        static void Main(string[] args)
        {
            List<Student> list = new List<Student>()
            {
                new Student(){Id=1,Name="AA",score=60},
                new Student(){Id=1,Name="BB",score=82},
                new Student(){Id=1,Name="CC",score=90},
                new Student(){Id=1,Name="DD",score=84},
                new Student(){Id=1,Name="EE",score=76},
            };
            var itemList = list.Where(p => p.score > 80);
            foreach (var item in itemList)
            {
                Console.WriteLine("{0}-{1}", item.Name, item.score);
            }
        }

  3、First，FirstOrDefault 和 Last，LastOrDefault；尽量使用 FirstOrDefault 和 LastOrDefault ，可避免 InvalidOperationException 异常

            var item1 = list.Where(p => p.score > 180).First();
            var item2 = list.Where(p => p.score > 180).FirstOrDefault();
            var item3 = list.Where(p => p.score > 180).Last();
            var item4 = list.Where(p => p.score > 180).LastOrDefault();

二、分组联接

 static void Main(string[] args)
        {
            List<Student> studentlist = new List<Student>()
            {
                new Student(){Id=1,Name="AA",score=60},
                new Student(){Id=2,Name="BB",score=82},
                new Student(){Id=3,Name="CC",score=90},
                new Student(){Id=4,Name="DD",score=84},
                new Student(){Id=5,Name="EE",score=76},
            };
            List<Book> bookList = new List<Book>() 
            {
                new Book(){Id=1,BokkName="C++"},
                new Book(){Id=1,BokkName="C"},
                new Book(){Id=2,BokkName="JAVA"},
                new Book(){Id=2,BokkName="C#"},
                new Book(){Id=3,BokkName="ASP.NET"},
            };
            var doubleModel = from s in studentlist
                              join b in bookList on s.Id equals b.Id
                              where s.Id == 1
                              select new { Name = s.Name, bookName = b.BokkName };

            foreach (var item in doubleModel)
            {
                Console.WriteLine("{0}-{1}",item.Name,item.bookName);
            }
        }

 三、自定义

　　list.GroupBy(n => new { n.StudentName, n.StudentCode }).Select(x => x.First())

未完待续。。。