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[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

//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 % 2 == 0
                         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[0] == 'a' || w[0] == '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 = "0001", Name = "张三" ,Gender = "男" , Age = 18},
    new Person { ID = "0002", Name = "李四" ,Gender = "女" , Age = 17},
    new Person { ID = "0003", Name = "王五" ,Gender = "女" , Age = 19},
    new Person { ID = "0004", Name = "赵六" ,Gender = "男", Age = 20}
};
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 = "0001", Name = "张三" ,Gender = "男" , Age = 18},
    new Person { ID = "0002", Name = "李四" ,Gender = "女" , Age = 17},
    new Person { ID = "0003", Name = "王五" ,Gender = "女" , Age = 19},
    new Person { ID = "0004", Name = "赵六" ,Gender = "男", Age = 20}
};

var query = from p in persons
            let gender = p.Gender
            let age = p.Age
            where age > 18 && gender == "女"
            select p;
foreach (var item in query)
{
    Console.WriteLine("姓名: " + item.Name + "  年龄:" + item.Age);
}

运行结果：