linq

概述：
    C# 3.0中加入的最为复杂的特性就是Linq查询表达式了，这使我们可直接采用类似于SQL的语法对集合进行查询，这就使我们可以享受到关系数据查询的强大功能
实例代码：

  public static void Main()
            {
                // 定义匿名数组persons, 并为其赋初值
                 var persons = new[] {
                     new { Name="Terry", Sex=false, Age=22 },
                    new { Name="Martin", Sex=true, Age=30 },
                     new { Name="Jerry", Sex=false, Age=24 },
                    new { Name="Brog", Sex=false, Age=25 },
                    new { Name="Tom", Sex=true, Age=20 }
               };
     
     
              /**//*
                   执行简单Linq查询
     
                    检索所有年龄在24岁以内的人
                    查询结果放在results变量中
    
                    results变量的类型与数组persons相同                
               */
               var results = from p in persons
                              where p.Age <= 24
                            select p;
     
               foreach (var person in results)
                {
                  Console.WriteLine(person.Name);
                }
     
                Console.WriteLine();
     
   
                // 定义匿名数组customers, 并为其赋初值
                // 该数组是匿名类型的
                var customers = new[] {
                    new {
                        Name="Terry", City="China", Orders=new[] {
                           new {
                               OrderNo=0,
                               OrderName="C# Programming Language(Second Edition)",
                                OrderDate=new DateTime(2007,9, 5)
                           },
                           new {
                                OrderNo=1,
                                OrderName="Head First Design Patterns(Chinese Edition)",
                               OrderDate=new DateTime(2007,9,15)
                            },
                          new {
                                OrderNo=2,
                               OrderName="ASP.NET Unleashed 2.0(Chinese Edition)",
                               OrderDate=new DateTime(2007,09,18)
                         },
                         new {
                              OrderNo=3,
                               OrderName="The C++ Programming Langauge(Special Edition)",
                                OrderDate=new DateTime(2002, 9, 20)
                           }
                       }
                  },
                   new {
                        Name="Brog", City="China", Orders=new[] {
                            new {
                               OrderNo=0,
                                OrderName="C# Programming Language(Second Edition)",
                               OrderDate=new DateTime(2007, 9, 15)
                            }
                        }
                   },
                   new {
                       Name="Vicky", City="London", Orders=new[] {
                            new { OrderNo=0,
                                OrderName="C++ Programming Language(Special Edition)",
                               OrderDate=new DateTime(2007, 9, 20)
                            }
                        }
                    }
                };
     
     
                /**//*
                    执行多重Linq查询
     
                    检索所在城市为中国, 且订单日期为2007年以后的所有记录
                    查询结果是一个匿名类型的数组
                    其中包含客户名, 订单号, 订单日期, 订单名四个字段
               */
               var someCustomers = from c in customers
                                    where c.City == "China"
                                    from o in c.Orders
                                    where o.OrderDate.Year >= 2007
                                    select new { c.Name, o.OrderNo, o.OrderDate, o.OrderName };
     
    
                foreach (var customer in someCustomers)
                {
                    Console.WriteLine(
                       customer.Name + ", " + customer.OrderName + ",  " +
                      customer.OrderDate.ToString("D")
                   );
               }
           }

运行结果：