linq中let关键字学习

2022-09-17 19:40:15

linq中let关键字就是对子查询的一个别名，let子句用于在查询中添加一个新的局部变量，使其在后面的查询中可见。

linq中let关键字实例

1、传统下的子查询与LET关键字的区别

C# 代码复制


static void Main(string[] args)
{
    int[] numbers = new[] { 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);
    }

    Console.Read();

}

2、把每个单词开头包含a或者e的找出来

C# 代码复制


using System;
using System.Linq;

public class Test
{
    static void Main(string[] args)
    {
        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);
        }
        Console.ReadLine();
    }
}

3、linq实例3

C# 代码复制


var query = from p in persons
            let friendlyName = p.Gender == "男" ? "Mr" : "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);
}

4、linq实例4

C# 代码复制


public class Singer
    {
        public string Name { set; get; }
        public int Age { set; get; }
    }

 List<Singer> list = new List<Singer>(){
              new Singer{Name="zhangs" ,Age=21},
              new Singer{Name="zhangs",Age=25},
              new Singer{Name="margy",Age=21}
            };


            var query = from a in list
                        let b = a.Name
                        let c=a.Age
                        where b =="zhangs" & c>21
                        select a;
            foreach (var item in query)
            {
                Response.Write("姓名: "+item.Name+"  年龄:"+item.Age);
            }

//结果           姓名: zhangs 年龄:25
//使用let 建立了个范围变量，这个范围变量在后续的where子句中使用，如果不使用let子句，where子句的表达式将写成这样：
//where a.Name=="zhangs" & a.Age>21</span>

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