C# LINQ笔记
from子句
- foreach语句命令式指定了按顺序一个个访问集合中的项。from子句只是声明式地规定集合中的每个项都要访问，并没有指定顺序。
- foreach在遇到代码时就执行其主体。from子句什么也不执行，只有在遇到访问查询变量的语句时才会执行。
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp2
{
class Test
{
public static void Main(string[] args)
{
int[] ints = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
var nums = from n in ints
where n > 7
select n;
foreach (var n in nums)
{
Console.WriteLine(n);
}
Console.ReadKey();
}
}
}
运行结果
join子句
使用联结来结合两个或更多集合中的数据。
联结对象接受两个集合，然后创建一个临时的对象集合，每一个对象包含原始集合对象中的所有字段。
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp2
{
class student
{
public int id;
public string name;
}
class courseStudent
{
public string course;
public int id;
}
class Test
{
static student[] students = new student[]
{
new student{ id = 1, name = "熊1" },
new student{ id = 2, name = "熊2" },
};
static courseStudent[] courseStudents = new courseStudent[]
{
new courseStudent{ id = 1, course = "体育" },
new courseStudent{ id = 2, course = "历史" },
new courseStudent{ id = 3, course = "语文" },
};
public static void Main(string[] args)
{
var names = from s in students
join c in courseStudents on s.id equals c.id
where c.course == "历史"
select s.name;
foreach (var item in names)
{
Console.WriteLine(item);
}
Console.ReadKey();
}
}
}
运行结果
因为选修“历史”课程的是熊2，所以输出的是熊2：
orderby
using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp2
{
class student
{
public int id;
public string name;
}
class courseStudent
{
public string course;
public int id;
}
class Test
{
static student[] students = new student[]
{
new student{ id = 1, name = "熊1" },
new student{ id = 2, name = "熊2" },
};
static courseStudent[] courseStudents = new courseStudent[]
{
new courseStudent{ id = 1, course = "体育" },
new courseStudent{ id = 2, course = "历史" },
new courseStudent{ id = 3, course = "语文" },
};
public static void Main(string[] args)
{
//匿名类型的对象数组
var students2 = new[]
{
new { Name = "student1", Age = 1, Course = "体育" },
new { Name = "student2", Age = 2, Course = "美术" },
new { Name = "student3", Age = 3, Course = "历史" },
new { Name = "student4", Age = 4, Course = "历史" },
new { Name = "student5", Age = 5, Course = "历史" },
new { Name = "student6", Age = 1, Course = "历史" },
};
var stu = from s in students2
orderby s.Age
select s;
foreach (var student2 in stu)
Console.WriteLine("{0},{1},{2}", student2.Name, student2.Age, student2.Course);
Console.ReadKey();
}
}
}
运行结果
