Linq

一、LINQ查询表达式基础

1、简单的 LINQ 查询

 1     static class Program
 2     {
 3         static void Main(string[] args)
 4         {
 5             List<Student> list = new List<Student>()
 6             {
 7                 new Student(){Id=1,Name="AA",score=60},
 8                 new Student(){Id=1,Name="BB",score=82},
 9                 new Student(){Id=1,Name="CC",score=90},
10                 new Student(){Id=1,Name="DD",score=98},
11                 new Student(){Id=1,Name="EE",score=76},
12             };
13             var itemList = from l in list
14                            where l.score > 80
15                            orderby l.score ascending
16                            select l;
17             foreach (var item in itemList)
18             {
19                 Console.WriteLine("{0}-{1}", item.Name, item.score);
20             }
21         }
22     }
23     /// <summary>
24     /// Student Model
25     /// </summary>
26     public class Student
27     {
28         public int Id { set; get; }
29 
30         public string Name { set; get; }
31 
32         public int score { set; get; }
33     }

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2、Where 的使用

 1         static void Main(string[] args)
 2         {
 3             List<Student> list = new List<Student>()
 4             {
 5                 new Student(){Id=1,Name="AA",score=60},
 6                 new Student(){Id=1,Name="BB",score=82},
 7                 new Student(){Id=1,Name="CC",score=90},
 8                 new Student(){Id=1,Name="DD",score=84},
 9                 new Student(){Id=1,Name="EE",score=76},
10             };
11             var itemList = list.Where(p => p.score > 80);
12             foreach (var item in itemList)
13             {
14                 Console.WriteLine("{0}-{1}", item.Name, item.score);
15             }
16         }

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

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

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二、分组联接

 1  static void Main(string[] args)
 2         {
 3             List<Student> studentlist = new List<Student>()
 4             {
 5                 new Student(){Id=1,Name="AA",score=60},
 6                 new Student(){Id=2,Name="BB",score=82},
 7                 new Student(){Id=3,Name="CC",score=90},
 8                 new Student(){Id=4,Name="DD",score=84},
 9                 new Student(){Id=5,Name="EE",score=76},
10             };
11             List<Book> bookList = new List<Book>() 
12             {
13                 new Book(){Id=1,BokkName="C++"},
14                 new Book(){Id=1,BokkName="C"},
15                 new Book(){Id=2,BokkName="JAVA"},
16                 new Book(){Id=2,BokkName="C#"},
17                 new Book(){Id=3,BokkName="ASP.NET"},
18             };
19             var doubleModel = from s in studentlist
20                               join b in bookList on s.Id equals b.Id
21                               where s.Id == 1
22                               select new { Name = s.Name, bookName = b.BokkName };
23 
24             foreach (var item in doubleModel)
25             {
26                 Console.WriteLine("{0}-{1}",item.Name,item.bookName);
27             }
28         }

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三、自定义

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

未完待续。。。