LINQ(语言集成查询)使用案例

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LinkConsole
{
    class Program
    {
        static void Main(string[] args)
        {
              //-------------最基本的简单查询----------------//
            List<int> numbers = new List<int>() { 1,2,3,4,5,6,7,8,9,10};
            var numQuery = from num in numbers
                          where num % 2 == 0
                           select num;
            foreach (var num in numQuery)
            {
               Console.WriteLine("{0,1}", num);
            }

             //-------------读取List<>中的句子----------------//
             FormExpDemo2();
            //-------------复合form子句----------------//
            FormExpDemo();
            //-------------多个from句子---------------//
            FormExpDemo3();
            //-------------where-------------------//
             WhereExpDemo();
            //-------------select------------------//
            SelectDemo();
            //-------------group--------------------//
            GroupDemo();
            //-------------into------------------------//
            IntoDemo();
            //--------------OrderBy--------------------//
            ThenByDemo();
            //--------------let----------------------//
            LetDemo();
            //--------------join--------------------//
            JoinDemo();
            Console.ReadLine();
        }
        public class CustomerInfo
        {
            public string Name { get; set; }
            public int Age { get; set; }
            public string Tel { get; set; }
            public List<string> telTable { get; set; }
        }
        public static void FormExpDemo2()
        {
            //
            List<CustomerInfo> customers = new List<CustomerInfo>
            {
                new CustomerInfo{ Name = "欧阳晓晓",Age = 35,Tel = "123"},
                new CustomerInfo{ Name = "上官飘飘",Age = 17,Tel = "456"},
                new CustomerInfo{ Name = "诸葛菲菲",Age = 23,Tel = "789"}
            };
            var query = from ci in customers
                        where ci.Age > 20
                        select ci;
            foreach (var ci in query)
            {
                Console.WriteLine("姓名：{0}年龄：{1}电话：{2}", ci.Name, ci.Age, ci.Tel);
            }
        }

        //复合from子句    // 相当于两个for循环而已
        private static void FormExpDemo()
        {
            List<CustomerInfo> customers = new List<CustomerInfo>
            {
                new CustomerInfo { Name = "欧阳小小",Age= 35,telTable = new List<string> {"123","234"} },
                new CustomerInfo { Name = "上官飘飘",Age= 35,telTable = new List<string> {"456","567"} },
                new CustomerInfo { Name = "诸葛菲菲",Age= 35,telTable = new List<string> {"789","456"} },
            };
            //查询包含电话号码456的客户
            var query = from ci in customers
                        from tel in ci.telTable
                        where tel.IndexOf("456") > -1
                        select ci;
            foreach (var ci in query)
            {
                Console.WriteLine("姓名：{0}年龄：{1}", ci.Name, ci.Age);
                foreach (var tel in ci.telTable)
                {
                    Console.WriteLine("          电话：{0}", tel);
                }

            }
        }

        //多个from子句，和复合子句看起来是一样的，其实不一样，一个是单个数据源中的子元素的集合，一个是对多个数据源进行查询
        private static void FormExpDemo3()
        {
            List<CustomerInfo> customers = new List<CustomerInfo>
            {
                new CustomerInfo{ Name = "欧阳晓晓",Age = 35,Tel = "123"},
                new CustomerInfo{ Name = "上官飘飘",Age = 77,Tel = "456"},
                new CustomerInfo{ Name = "诸葛菲菲",Age = 23,Tel = "789"}
            };
            List<CustomerInfo> customers2 = new List<CustomerInfo>
            {
                new CustomerInfo{ Name = "令狐冲",Age = 25,Tel = "123"},
                new CustomerInfo{ Name = "东方不败",Age = 15,Tel = "456"},
                new CustomerInfo{ Name = "任盈盈",Age = 13,Tel = "789"}
            };
            //在customers 中寻找年龄大于20的客户
            //在customenrs中寻找年龄小于30岁的客户
            var query = from custo in customers
                        where custo.Age > 20
                        from custo2 in customers2
                        where custo2.Age < 30
                        select new { custo, custo2 };
            foreach (var ci in query)
            {
                Console.WriteLine("{0},{1}", ci.custo.Name, ci.custo2.Name);//这样得到的是一个交叉联结表，有点类似于SQL中的笛卡尔沉积
            }
        }

        //where子句查询
        //where就是用来筛选元素的，除了开始和结束位置，where可以在任意位置使用，
        //一个LIKQ语句中可以有where子句，也可以没有，可以有一个，也可以有多个。
        //多个where子句之间的关系相当于逻辑“与”，每个子句中又可以包含多个用“谓词”链接的逻辑表达式，&&，或者||
        private static void WhereExpDemo()
        {
            List<CustomerInfo> clist = new List<CustomerInfo>
            {
                 new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708****"},
                 new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842****"},
                 new CustomerInfo{ Name="令狐冲", Age=23, Tel ="1380524****"}
            };
            //可以查询符合多个条件的人(名字是三个字或者姓令的，但年龄必须大于20)
            var query = from custo in clist
                        where (custo.Name.Length == 3 || custo.Name.Substring(0, 1) == "令") && custo.Age > 20
                        select custo;//select 也可以改成，比如custo.Name。或者用一个函数，把变量传出去
            foreach (var ci in query)
            {
                Console.WriteLine("姓名：{0}年龄：{1}电话：{2}", ci.Name, ci.Age, ci.Tel);

            }

            //where中使用自定义函数,查询三个字并且姓令的客户
            var query2 = from custo in clist
                         where (custo.Name.Length == 3 && ChechName(custo.Name))
                         select custo;
            foreach (var ci in query2)
            {
                Console.WriteLine("姓名：{0}年龄：{1}电话：{2}", ci.Name, ci.Age, ci.Tel);
            }


        }
        private static bool ChechName(string name)
        {
            if (name.Substring(0, 1) == "令")
                return true;
            else
                return false;
        }

        //select 用法举例
        private static void SelectDemo()
        {
            List<CustomerInfo> clist = new List<CustomerInfo>
            {
                 new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708****"},
                 new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842****"},
                 new CustomerInfo{ Name="令狐冲", Age=23, Tel ="1380524****"}
            };
            string[] names = { "令狐冲", "任盈盈", "杨过", "小龙女", "欧阳小夏", "欧阳晓晓" };
            //查询在给定谓词数组里存在的客户
            var query = from custo in clist
                        where custo.Age < 30
                        select new MyCustomerInfo { Name = custo.Name, Tel = custo.Tel };
            foreach (var ci in query)
            {
                Console.WriteLine("姓名：{0}电话：{1}类型{2}", ci.Name, ci.Tel, ci.GetType().FullName);
            }
        }
        public class MyCustomerInfo
        {
            public string Name { get; set; }
            public string Tel { get; set; }
        }

        //-------------------Group----------------------//
        static List<CustomerInfo> clist = new List<CustomerInfo>
        {
             new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708"},
             new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842"},
             new CustomerInfo{ Name="欧阳锦鹏", Age=35, Tel ="1330708"},
             new CustomerInfo{ Name="上官无忌", Age=23, Tel ="1380524"}
        };
        private static void GroupDemo()
        {
            //按照名字的前两个字进行分组
            var query = from custo in clist
                        group custo by custo.Name.Substring(0, 2);
            foreach (IGrouping<string, CustomerInfo> group in query)
            {
                Console.WriteLine("分组键：{0}", group.Key);
                foreach (var ci in group)
                {
                    Console.WriteLine("姓名：{0}电话：{1}", ci.Name, ci.Tel);
                }
                Console.WriteLine("*********************");
            }
            //可以知道group子句返回的是一个IGrouping<TKey,TElement>泛型接口的对象集合
            //TKey是键的对象类型，在用于group子句的时候，编译器会识别数据类型，用于存储分组的键值,也就是根据什么分的组
            //TElement是指的对象类型用于分配储存结果，变量基于这个接口的类型就是遍历这个值，也就是分组的对象
        }

        //----------------into子句---------------//
        private static void IntoDemo()
        {
            //into提供了一个临时标识符，它储存了into子句前面的查询内容，使他后面的子句可以方便使用，再次查询投影
            var query = from custo in clist
                        group custo by custo.Name.Substring(0, 2) into gpcustomer
                        orderby gpcustomer.Key descending //排序，
                        select gpcustomer;
            Console.WriteLine("into用于group子句");
            foreach (var group in query)
            {
                Console.WriteLine("分组见：{0}", group.Key);
                foreach (var ci in group)
                {
                    Console.WriteLine("姓名：{0}电话：{1}", ci.Name, ci.Tel);
                }
                Console.WriteLine("***********************");
            }
            var query2 = from custo in clist
                         select new { NewName = custo.Name, NewAge = custo.Age } into newCustomer
                         orderby newCustomer.NewAge
                         select newCustomer;
            Console.WriteLine("into用于select子句");
            foreach (var ci in query2)
            {
                Console.WriteLine("{0}年龄：{1}", ci.NewName, ci.NewAge);
            }
        }

        //---------------排序子句--------------------//
        //LINQ可以按元素的一个或者多个属性对元素进行排序，表达式的排序方式分为OrderBy、OrderByDescending、ThenBy、ThenByDescending
        //加了Descending的就是降序，没有加的就是升序

        private static void ThenByDemo()
        {
            List<CustomerInfo> clist = new List<CustomerInfo>
            {
                 new CustomerInfo{ Name="欧阳晓晓 ", Age=35, Tel ="1330708****"},
                 new CustomerInfo{ Name="上官飘飘 ", Age=17, Tel ="1592842****"},
                 new CustomerInfo{ Name="郭靖 ", Age=17, Tel ="1330708****"},
                 new CustomerInfo{ Name="黄蓉 ", Age=17, Tel ="1300524****"}
            };
            //按照年龄升序，再按照名字的字数次要排序
            var query = from customer in clist
                        orderby customer.Age, customer.Name.Length
                        select customer;
            Console.WriteLine("按年龄排列，按名字字数进行次要排序");
            foreach (var ci in query)
            {
                Console.WriteLine("姓名:{0} 年龄：{1} 电话：{2}",ci.Name, ci.Age, ci.Tel);

            }
            //按年龄降序，再按名字的字数降序次要排列
            var query2 = from customer in clist
                         orderby customer.Age descending , customer.Name.Length descending
                         select customer;
            Console.WriteLine("\n按年龄排列，按名字字数进行降序次要排列");
            foreach (var ci in query2)
            {
                Console.WriteLine("姓名：{0} 年龄：{1} 电话：{2}", ci.Name, ci.Age, ci.Tel);
            }

        }

        //--------------let子句---------------------//

        private static void LetDemo()
        {
            var query = from custo in clist
                        let g = custo.Name.Substring(0, 1)//let建立一个范围变量，在where中使用
                        where g == "欧" || g == "上"//也可以不写，写成customer.Name.Substring(0, 1) == "郭" || customer.Name.Substring(0, 1) == "黄"
                        select custo;
            foreach (var ci in query)
            {
                Console.WriteLine("姓名：{0} 年龄：{1} 电话：{2}", ci.Name, ci.Age, ci.Tel);
            }
        }

        //-------------join子句-------------------//
        private static void JoinDemo()
        {
            //如果两个数据源中的属性可以进行相等比较，那么两个句子可以用join进行关联，比较的符号为equal，而不是==
            List<CustomerInfo> clist = new List<CustomerInfo>
            {
               new CustomerInfo{ Name="欧阳晓晓", Age=35, Tel ="1330708****"},
               new CustomerInfo{ Name="上官飘飘", Age=17, Tel ="1592842****"},
               new CustomerInfo{ Name="郭靖", Age=17, Tel ="1330708****"},
               new CustomerInfo{ Name="黄蓉", Age=17, Tel ="1300524****"}
            };
            List<CustomerTitle> titleList = new List<CustomerTitle>
            {
               new CustomerTitle{ Name="欧阳晓晓", Title="歌手"},
               new CustomerTitle{ Name="郭靖", Title="大侠"},
               new CustomerTitle{ Name="郭靖", Title="洪七公徒弟"},
               new CustomerTitle{ Name="黄蓉", Title="才女"},
               new CustomerTitle{ Name="黄蓉", Title="丐帮帮主"}
            };
            //根据姓名进行内部联结
            var query = from customer in clist
                        join title in titleList
                        on customer.Name equals title.Name
                        select new { Name = customer.Name, Age = customer.Age, Title = title.Title };
            foreach (var ci in query)
            {
                Console.WriteLine("姓名：{0} 年龄：{1}{2}", ci.Name, ci.Age, ci.Title);
            }
            //根据姓名进行分组联结
            Console.WriteLine("\n根据姓名进行分组联结");
            var query2 = from customer in clist
                         join title in titleList
                         on customer.Name equals title.Name into tgroup
                         select new { Name = customer.Name, Titles = tgroup };
            foreach (var g in query2)
            {
                Console.WriteLine(g.Name);
                foreach (var g2 in g.Titles)
                {
                    Console.WriteLine("     {0}", g2.Title);
                }
            }
            //根据姓名进行 左外部联结
            Console.WriteLine("\n左外部联结");
            var query3 = from customer in clist
                         join title in titleList
                         on customer.Name equals title.Name into tgroup
                         from subTitle in tgroup.DefaultIfEmpty()
                         select new { Name = customer.Name, Title = (subTitle == null ? "空缺" : subTitle.Title) };
            foreach (var ci in query3)
            {
                Console.WriteLine("姓名：{0} ", ci.Name, ci.Title);
            }
        }
        public class CustomerTitle
        {
            public string Name { get; set; }
            public string Title { get; set; }
        }
    }
}