相识LINQ二之方法语法操作

上一篇对LINQ的基础有了一个初步的认识，本博客主要就是为了实用，所以直接一起学习查询。

一、LINQ标准查询操作符

1、p投影操作符：将序列中的元素转换为一个由开发人员定义的形式的操作。

　　1）Select ：对单个序列或集合中的值进行投影。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };

 var SelectDevices = Devices.Select(device=>device);

            foreach (string dev in SelectDevices1)

            {

              Console.WriteLine(“所有的设备：{0}", dev);

            }

　　2）SelectMany：将序列的每个元素投影到 IEnumerable<(Of <(T>)>) 并将结果序列合并为一个序列。

   class Devices
    {
        public string Name
        { get; set; }
        public List<double> Price
        { get; set; }
    }
Devices[] DeviceGroup = { new Devices { Name = "电视", Price =new List<double>{ 3000,2000} }, new Devices { Name = "电冰箱", Price =new List<double> { 4000,5000 }}, new Devices { Name = "洗衣机", Price =new List<double> { 1500,1000 }} };
            var devices = DeviceGroup.SelectMany(device => device.Price  );
            foreach (var  v in devices)
            {
                Console.WriteLine(v);
            }
        } 

　2、限制操作符：按照一定的限制，对序列进行过滤。

　　1）Where：对单个序列或集合中的值进行投影。

       string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };
var SelectDevices = Devices.Where(device=>device.StartsWith ("电"));
            foreach (string dev in SelectDevices)
            {
                Console.WriteLine("带电的设备：{0}", dev);
            }

3、排序操作符：按照升序和降序的方式对结果进行排序的功能。

1）OrderBy：将序列中的返回值按照升序的顺序进行排列。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };
var SelectDevices = Devices.OrderBy (device => device);
            foreach (string dev in SelectDevices)
            {
                Console.WriteLine(“所有的设备（排序后）：{0}", dev);
            }

2）OrderByDescending：将序列中的返回值按照降序的顺序进行排列。　

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };
 var SelectDevices = Devices.OrderByDescending(device => device);
            foreach (string dev in SelectDevices)
            {
                Console.WriteLine(“所有的设备（倒序后）：{0}", dev);
            }

3) ThenBy：排序依据的次关键字。

Devices[] DeviceGroup = { new Devices { Name = "电视", Price = 10000 },     new Devices { Name = "电话", Price = 240 }, new Devices { Name = "电视",     Price = 3000 } };
var devices = DeviceGroup.AsQueryable().OrderBy(device =>     device.Name).ThenBy(device => device.Price);
            foreach (var dev in devices)
            {
                Console.WriteLine("品名：{0}，价格：{1}",dev.Name,dev.Price );
            }

4)ThenByDescending：倒排序依据的次关键字。　

Devices[] DeviceGroup = { new Devices { Name = "电视", Price = 10000 },     new Devices { Name = "电话", Price = 240 }, new Devices { Name = "电视",     Price = 3000 } };
var devices = DeviceGroup.AsQueryable().OrderBy(device =>     device.Name).ThenByDescending(device => device.Price);
            foreach (var dev in devices)
            {
                Console.WriteLine("品名：{0}，价格：{1}",dev.Name,dev.Price );
            }

5)Reverse：反转序列。

string[] Devices = { "电视", "电冰箱", "洗衣机", "电话", "微波炉" };
var SelectDevices = Devices.Select(device=>device).Reverse ();
            foreach (string dev in SelectDevices)
            {
                Console.WriteLine("带电的设备：{0}", dev);
            }

4、联接操作符：将两个或多个数据源对象进行关联或联合。

1）Join：按照一定的条件，将两个数据源关联起来。

class StudentMessage//学生信息类
    {
        public int ID//编号
        { get; set; }
        public string Name//姓名
        { get; set; }
        public bool Sex//姓别
        { get; set; }
        public int Age//年龄
        { get; set; }
    }
class StudentScort//学生成绩类
    {
        public int ID//编号
        { get; set; }
        public float MathsScort//数学分数
        { get; set; }
        public float ChineseScort//语文分数
        { get; set; }
    }

这两个表是通过学生ID关联起来

接下来在程序入口中实例化出一些对象

static void Main()
        {
            List<StudentMessage> A_Class = new List<StudentMessage>();                          //声名一个A班，来存放学员
            A_Class.Add(new StudentMessage { ID = 1001, Name = "张三", Sex = true, Age = 20 }); //添加学员
            A_Class.Add(new StudentMessage { ID = 1002, Name = "李四", Sex = false, Age = 21 });//添加学员
            A_Class.Add(new StudentMessage { ID = 1003, Name = "王五", Sex = true, Age = 19 }); //添加学员

            List<StudentScort> A_ScortSheet = new List<StudentScort>();                              //声名一个成绩表
            A_ScortSheet.Add(new StudentScort { ID = 1001, ChineseScort = 90f, MathsScort = 88f });  //添加学员成绩
            A_ScortSheet.Add(new StudentScort { ID = 1002, ChineseScort = 80f, MathsScort = 68f });  //添加学员成绩
            A_ScortSheet.Add(new StudentScort { ID = 1003, ChineseScort = 60f, MathsScort = 98f });  //添加学员成绩

var messages=A_Class.Join (A_ScortSheet ,mess=>mess. ID ,scos =>scos.ID ,(mes,sco)=>new {mes.ID, mes.Name, sco.ChineseScort, sco.MathsScort });            
            Console.WriteLine("{0,4}　{1,10}　{2,3}　{3,3}", "学号","姓名","语文","数学");            
            foreach (var scort in messages)
            {
                Console.WriteLine("{0,6}　{1,10}　{2,4}　{3,4}",scort .ID ,scort .Name ,scort.ChineseScort ,scort .MathsScort );
            }
        } 　　　

2）GroupJoin：将主数据源中的每一个值或元素与次数据源中相应的值联接起来

interface Scort //成绩接口
    {
        int ID         //编号
        { get; set; }
        float Scort  //分数
        { get; set; }
        string Subject //科目
        { get; set; }
    }
class StudentMessage//学生信息类
    {
        public int ID//编号
        { get; set; }
        public string Name//姓名
        { get; set; }
        public bool Sex//姓别
        { get; set; }
        public int Age//年龄
        { get; set; }
    }
class ChineaseScort : Scort //语文成绩类
    {
        public int ID//编号
        { get; set; }
        public float Scort
        { get; set; }//语文分数  
        public string Subject//科目
        { get; set; }
    }
class MathsScort : Scort//数学成绩类
    {
        public int ID//编号
        { get; set; }
        public float Scort
        { get; set; }//数学分数  
        public string Subject//科目
        { get; set; }
    }

成绩接口ID和语文成绩和数学成绩ID关联，成绩接口和学生成绩类通过ID关联

static void Main()
        {
           List<StudentMessage> A_Class = new List<StudentMessage>(); //声名一个A班，来存放学员
            A_Class.Add(new StudentMessage { ID = 1001, Name = "张三", Sex = true, Age = 20 }); //添加学员
            A_Class.Add(new StudentMessage { ID = 1002, Name = "李四", Sex = false, Age = 21 });//添加学员

            List<Scort> A_ScortSheet = new List<Scort>(); //声名一个成绩表
            A_ScortSheet.Add(new ChineaseScort { ID = 1001, Scort = 100f, Subject = "语文" }); //添加语文成绩
            A_ScortSheet.Add(new ChineaseScort { ID = 1002, Scort = 80f, Subject = "语文" });  //添加语文成绩
            A_ScortSheet.Add(new MathsScort { ID = 1001, Scort = 90f, Subject = "数学" });     //添加数学成绩
            A_ScortSheet.Add(new MathsScort { ID = 1002, Scort = 60f, Subject = "数学" });     //添加数学成绩

            var messages = A_Class.GroupJoin(A_ScortSheet, mes => mes.ID, sco => sco.ID, (mess, scor) => new { Mess = mess.ID, Mename = mess.Name, Scor = scor.Select(Scor => Scor) });
            foreach (var scort in messages)
            {
                Console.WriteLine("学号：{0,4} 姓名：{1}", scort.Mess, scort.Mename);
                foreach (var sc in scort.Scor)
                {
                    Console.WriteLine("  　{0}：{1}分", sc.Subject, sc.Scort);
                }
            }

5、分组操作符：按照一定的值将序列中的值或元素进行分组。

1）GroupBy：分组

class Student
    {
        public string ClassName
        { get; set; }
        public string Name
        { get; set; }
    }
 static void Main()
        {
            Student[] students = new Student[] {
             new Student{ClassName="A", Name="张三"},
             new Student{ClassName ="B", Name ="李四"},
             new Student{ClassName ="A" ,Name ="王五"},
             new Student {ClassName ="B", Name ="赵六"},
             new Student{ClassName ="B" ,Name ="钱七"},
             new Student {ClassName ="B", Name ="孙八"}
            };

var stus = students.GroupBy(st => st.ClassName);
            foreach (var v in stus)
            {
                Console.WriteLine("{0}班学生：", v.Key);
                foreach (var va in v)
                {
                    Console.WriteLine("  姓名：{0}", va.Name);
                }
            }
        } 　

6、合并操作符：将两个对象合并在一起。

Concat：　

string[] Citys = new string[] { "北京","上海","东京"};
            string[] Mobiles = new string[] {"诺基亚","摩托罗拉","三星" };
            var Cont = Citys.Select(city => city).Concat(Mobiles.Select(mobil => mobil));
            foreach (var v in Cont)
            {
                Console.WriteLine(v);            
            }

对于LINQ这些查询已基本满足平时操作的需求，对于其他的一些比如生成操作符，转换操作符等查询，当用到时在网上搜索一下，就可以很快理解！

好了，今天就到这里吧！