• 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 (""));
                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!

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  • 原文地址:https://www.cnblogs.com/nyzhai/p/2817945.html
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