LINQ之let关键字

let子句用于在LINQ表达式中存储子表达式的计算结果。let子句创建一个范围变量来存储结果，变量被创建后，不能修改或把其他表达式的结果重新赋值给它。
此范围变量可以再后续的LINQ子句中使用。

实例1

 int[] numbers = new[] { , , , , , , , , ,  };

 //传统下的子查询做法
 //var query = from num in numbers
 //            select num * (from n in numbers
 //                          where n % 2 == 0
 //                          select n).Count();

 //使用LET关键字的做法
 var query = from num in numbers
          let evenNumbers = from n in numbers
                     where n %  ==
                          select n
         select num * evenNumbers.Count();

 foreach (var item in query)
 {
     Console.WriteLine(item);
 } 

运行结果：0 5 10 15 20 25 30 35 40 45

实例2

 string[] strings = { "A penny saved is a penny earned.", "The aaxly sdj", "the pa is no" };

 var query = from sentence in strings
             let words = sentence.Split(' ')//用空格分割成数组
             from word in words
             let w = word.ToLower()//把每个字母小写
             where w[] == 'a' || w[] == 'e'
             select word;

 foreach (var s in query)
 {
     Console.WriteLine(s);
 }

运行结果：

实例3

 List<Person> persons = new List<Person>
 {
     new Person { ID = "", Name = "张三" ,Gender = "男" , Age = },
     new Person { ID = "", Name = "李四" ,Gender = "女" , Age = },
     new Person { ID = "", Name = "王五" ,Gender = "女" , Age = },
     new Person { ID = "", Name = "赵六" ,Gender = "男", Age = }
 };
 var query = from p in persons
             let friendlyName = p.Gender == "男" ? "Mr" + p.Name : "Ms" + p.Name
             select new
             {
                 UserID = p.ID,
                 FriendName = friendlyName
             };
 foreach (var item in query)
 {
     Console.WriteLine("No:{0},Friendly Name:{1}", item.UserID, item.FriendName);
 }

运行结果：

实例4

 List<Person> persons = new List<Person>
 {
     new Person { ID = "", Name = "张三" ,Gender = "男" , Age = },
     new Person { ID = "", Name = "李四" ,Gender = "女" , Age = },
     new Person { ID = "", Name = "王五" ,Gender = "女" , Age = },
     new Person { ID = "", Name = "赵六" ,Gender = "男", Age = }
 };

 var query = from p in persons
             let gender = p.Gender
             let age = p.Age
             where age >  && gender == "女"
             select p;
 foreach (var item in query)
 {
     Console.WriteLine("姓名: " + item.Name + "  年龄:" + item.Age);
 }

运行结果：