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c#-LINQ与lambda表达式学习笔记

news 来源：原创 2025/5/25 11:22:20

https://blog.csdn.net/m0_56259289/article/details/144134122

 static void Main(string[] args) //程序入口
 {
     int[] arr1 = new int[] { 1, 2, 3, 4, 5, 6, 7 };
     int[] arr2 = new int[] { 1, 2, 3, 4, 5, 6, 7 };
     var query1 = from n in arr1 select n;
     var query2 = from a in arr1
                  from b in arr2
                  select a + b;
     var query3 = from n in arr2 select n * 10;
     var query4 = from n in arr2
                  select new
                  {
                      Id = n,
                      name = n.ToString()
                  };
     var query11 = from n in arr1
                   where n < 3
                   select n;


 }

class Student
{
    public string Name { get; set; }
    public int Age { get; set; }
    public string Class { get; set; }
}
 
List<Student> students = new List<Student>
{
    new Student { Name = "Alice", Age = 20, Class = "A" },
    new Student { Name = "Bob", Age = 22, Class = "B" },
    new Student { Name = "Charlie", Age = 21, Class = "A" },
    new Student { Name = "David", Age = 23, Class = "B" },
    new Student { Name = "Eve", Age = 20, Class = "C" }
};
 
var studentGroups = from s in students
                    group s by s.Class into g
                    select new { Class = g.Key, Count = g.Count() };
 
foreach (var group in studentGroups)
{
    Console.WriteLine($"Class: {group.Class}, Number of Students: {group.Count}");
}

方法调用where

public static bool IsEven(int n)
{
    return n % 2 == 0 ? true : false;
}

static void Main(string[] args) //程序入口
{
    int[] arr = new int[] {1,2,3,4,5,6,7,8 };
    var query = from n in arr
                where IsEven(n)
                select n;

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

let

int[] arr = new int[] {1,2,3,4,5,6,7,8 };
var query = from num in arr
            let n = num % 2
            where n==0
            select num;

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

orderby

var query = from n in arr
            where n > 1 && n < 6
            orderby n descending
            select n;

group by

int[] arr = new int[] {1,2,3,4,5,6,7,8 };
var query = from n in arr
            where n > 1 && n < 6
            group n by n % 2;
            

foreach (var n in query)
{
    foreach (var j in n)
    {
        Console.WriteLine(j);
        }
 }
int[] arr = new int[] {1,2,3,4,5,6,7,8 };
var query = from n in arr
            where n > 1 && n < 6
            group n by n % 2 into g
            from sn in g
            select sn;
    
foreach (var n in query)
{
        Console.WriteLine(n);  
 }

join

nt[] arr1 = new int[] {1,2,3,4,5,6,7,8 };
            int[] arr2 = new int[] { 1,4, 5, 6,8 };

            var query = from a in arr1
                        where a < 7
                        join b in arr2 on a equals b
                        select a;
         //将arr1和arr2数组连接，同时满足a,b相等的条件，其中b是arr2中的元素

LINQ构架

IEnumerable  var 二者相等

IEnumerable<int> scoreQuery =
                           from score in arr1
                           where score > 2
                           select score;

var scoreQuery2 = from score in arr1
                  where score > 2
                  select score;

key

string[] langusges = { "java", "c#", "c++", "python", "vb" };
var query = from item in langusges
            group item by item.Length into lengthgroups
            orderby lengthgroups.Key
            select lengthgroups;
foreach (var n in query)
{
        Console.WriteLine(n.Key);  
 }

 

武学案例

 

 var master = new List<MartialArtsMaster>()
 {
     new MartialArtsMaster() { Id = 1, Name = "黄蓉", Age = 18, Menpai = "丐帮", Kungfu = "打狗棒法", Level = 9 },
      new MartialArtsMaster() { Id = 7, Name = "黄蓉2", Age = 18, Menpai = "丐帮", Kungfu = "打狗棒法", Level = 93 },
     new MartialArtsMaster() { Id = 2, Name = "洪七公", Age = 34, Menpai = "丐帮", Kungfu = "没有", Level = 4 },

     new MartialArtsMaster() { Id = 3, Name = "郭靖", Age = 54, Menpai = "黑帮", Kungfu = "细心打法", Level = 10 },
     new MartialArtsMaster() { Id = 4, Name = "任我行", Age = 76, Menpai = "明教", Kungfu = "葵花宝典", Level = 14 },
     new MartialArtsMaster() { Id = 5, Name = "东方不败", Age = 95, Menpai = "华山", Kungfu = "葵花宝典", Level = 20 },
     new MartialArtsMaster() { Id = 6, Name = "岳不群", Age = 26, Menpai = "华山", Kungfu = "葵花宝典", Level = 20 }
 };

 var kongfu = new List<kungfu>()
 {
     new kungfu() { kuongfuId = 1, kongfuName = "打狗棒法", Lethality = 90 },
     new kungfu() { kuongfuId = 2, kongfuName = "没有", Lethality = 95 },
     new kungfu() { kuongfuId = 3, kongfuName = "细心打法", Lethality = 50 },
     new kungfu() { kuongfuId = 4, kongfuName = "葵花宝典", Lethality = 99 }

 };


 var GaiBangMaster = from m in master
                     where m.Level > 8 && m.Menpai == "丐帮"
                     select m;
 var GaiBangMethod = master.Where(m => m.Level > 8 && m.Menpai == "丐帮");
 string matserLevel = "查询丐帮中武力值大于8：";
 foreach(var num in GaiBangMaster)
 {
     matserLevel = num.Id + " "+num.Name + " "+ num.Age + " " +num.Menpai +" "+ num.Kungfu +" "+ num.Level +"\n" ;
     Console.WriteLine(matserLevel);
 }

 foreach (var num in GaiBangMethod)
 {
     matserLevel = num.Id + " " + num.Name + " " + num.Age + " " + num.Menpai + " " + num.Kungfu + " " + num.Level + "\n";
     Console.WriteLine(matserLevel);
 }
 var masterkongfumethod = master.SelectMany(k => kongfu, (m, k) => new { mt = m, kf = k })
     .Where(x => x.kf.Lethality > 90 && x.mt.Kungfu == x.kf.kongfuName)
     .OrderBy(m => m.mt.Level)
     .Select(m => m.mt + " " + m.mt.Name + " " + m.mt.Age + " " + m.mt.Menpai + "\n");

var topmaster = from m in master
from k in kongfu
where k.kongfuName == m.kungfu
orderby m.leverl * k.lethality + m.age
select m.id + " " + m.Name + " " + m.age + " " + m.kunggu + " " + m.level;


var topmaster = master
    .Join(kongfu, m => m.kungfu, k => k.kongfuName, (m, k) => new { m, k })
    .OrderBy(x => x.m.leverl * x.k.lethality + x.m.age)
    .Select(x => $"{x.m.id} {x.m.Name} {x.m.age} {x.m.kunggu} {x.m.level}");

 .Select(x => new { id = x.Id, Name = x.Name, masterJongfu = x.Kungfu, Leverl = x.Level, Top = (++i) });这里的意思是并不想直接返回master对象，而是创建一个匿名对象，返回master的部分属性，和自己添加的属性

int i = 1;
var masterTop = master.Where(x => x.Level > 8)
    .OrderByDescending(x => x.Level)
    .Select(x => new { id = x.Id, Name = x.Name, masterJongfu = x.Kungfu, Leverl = x.Level, Top = (++i) });

案例

using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Web;
namespace studytest  //命名空间
{
    class Program //类
    {


        static void Main(string[] args) //程序入口
        {

            var master = new List<MartialArtsMaster>()
            {
                new MartialArtsMaster() { Id = 1, Name = "黄蓉", Age = 18, Menpai = "丐帮", Kungfu = "打狗棒法", Level = 9 },
                new MartialArtsMaster() { Id = 7, Name = "黄蓉2", Age = 18, Menpai = "丐帮", Kungfu = "打狗棒法", Level = 93 },
                new MartialArtsMaster() { Id = 2, Name = "洪七公", Age = 34, Menpai = "丐帮", Kungfu = "没有", Level = 4 },

                new MartialArtsMaster() { Id = 3, Name = "郭靖", Age = 54, Menpai = "黑帮", Kungfu = "细心打法", Level = 10 },
                new MartialArtsMaster() { Id = 4, Name = "任我行", Age = 76, Menpai = "明教", Kungfu = "葵花宝典", Level = 14 },
                new MartialArtsMaster() { Id = 5, Name = "东方不败", Age = 95, Menpai = "华山", Kungfu = "葵花宝典", Level = 20 },
                new MartialArtsMaster() { Id = 6, Name = "岳不群", Age = 26, Menpai = "华山", Kungfu = "葵花宝典", Level = 20 }
            };
            var kongfu = new List<kungfu>()
            {
                new kungfu() { kuongfuId = 1, kongfuName = "打狗棒法", Lethality = 90 },
                new kungfu() { kuongfuId = 2, kongfuName = "没有", Lethality = 95 },
                new kungfu() { kuongfuId = 3, kongfuName = "细心打法", Lethality = 50 },
                new kungfu() { kuongfuId = 4, kongfuName = "葵花宝典", Lethality = 99 },
                new kungfu() { kuongfuId = 5, kongfuName = "九阳真经", Lethality = 95 }

            };
            //test1
            var menpaiNum = master.Where(x => x.Menpai == "丐帮")
                .OrderByDescending(x => x.Level)
                .Select(x => new { Name = x.Name, Menpai = x.Menpai, kungfu = x.Kungfu, Level = x.Level, gg = "111" });
            foreach (var m in menpaiNum)
            {
                Console.WriteLine(m.Name + " " + m.Menpai + " " + m.kungfu + " " + m.Level + " " + m.gg);
            }
            //test2
            var eqkungfuNum = master.Join(kongfu, m => m.Kungfu, k => k.kongfuName, (m, k) =>
                new { m.Name, m.Kungfu, k.kongfuName,k.Lethality });
            foreach(var m in eqkungfuNum)
            {
                Console.WriteLine($"master: {m.Name},kungfu:{m.Kungfu}m.Lethality");
            }
            //lambda表达式
            int i = 1;
            var masterTop = master.Where(x => x.Level > 8)
                .OrderByDescending(x => x.Level)
                .Select(x => new { id = x.Id, Name = x.Name, masterKongfu = x.Kungfu, Leverl = x.Level, Top = (++i) });
            //LINQ经典
            int ii = 1;
            var KongfuTop = from k in kongfu
                            where k.Lethality > 90
                            orderby k.Lethality descending
                            select new { KongfuId = k.kuongfuId, kongfuName = k.kongfuName, Lethality = k.Lethality, KongfuTop = (ii++) };

            //
            var masterLethalityTop = from m in masterTop
                                     join k in KongfuTop on m.masterKongfu equals k.kongfuName
                                     orderby m.Leverl * k.Lethality descending
                                     select new { Id = m.id, Name = m.Name, kongfu = m.masterKongfu, Lever = m.Leverl, Kill = m.Leverl * k.Lethality };
            //
            var masterLethalityTopMethod =
                masterTop.Join(KongfuTop, m => m.masterKongfu, k => k.kongfuName, (m, k) =>
                new { id = m.id, Name = m.Name, kongfu = m.masterKongfu, Level = m.Leverl, Kill = m.Leverl * k.Lethality })
                .OrderByDescending(m => m.Kill);

            foreach (var ma in masterLethalityTop)
            {
                Console.WriteLine(ma.Id + " " + ma.Name + " " + ma.kongfu + " " + ma.Lever + " " + ma.Kill + "\n");
            }

            foreach (var ma in masterLethalityTopMethod)
            {
                Console.WriteLine(ma.id + " " + ma.Name + " " + ma.kongfu + " " + ma.Level + " " + ma.Kill + "\n");
            }


        }

    }
}

问题：$

  • $符号在字符串前面告诉 C# 编译器：这个字符串是一个带插值的字符串，即字符串内部可以包含变量或表达式。

问题 lambda

总结：

  • 为什么要用 .Select(x => new {...}) 而不是 Select(x)：

  • 选择性返回字段：通过 Select(x => new {...}) 你可以选择性地返回 MartialArtsMaster 中的某些字段。例如，如果你只对 Id, Name, Kungfu, Level 等信息感兴趣，可以只选择这些字段，而不返回整个对象。
  • 添加额外的属性：通过匿名类型，你还可以在查询中添加额外的属性，比如这里的 Top，它是基于其他字段计算出来的一个额外字段。
  • 避免暴露不需要的字段：如果你只关心某些字段，使用 Select 可以避免暴露整个对象的所有字段，减少不必要的数据传输。
  • Select(x => x) 会返回原始的 MartialArtsMaster 对象，包含所有属性。
  • Select(x => new { ... }) 通过创建一个匿名类型，可以返回特定字段，还可以进行加工，比如计算排名等。

列表add

master.Add(new MartialArtsMaster() { Id = 1, Name = "黄蓉", Age = 18, Menpai = "丐帮", Kungfu = "打狗棒法", Level = 9 });

 分组

 groupjoin

var mastergroup = kongfu.GroupJoin(master, k => k.kongfuName, m => m.Kungfu, (k, m) =>
    new { k.kuongfuId, k.kongfuName, k.Lethality, Count = m.Count() })
    .OrderByDescending(k => k.Count);
foreach (var kk in mastergroup)
{
    Console.WriteLine(kk.kuongfuId+kk.kongfuName+kk.Lethality+kk.Count);
}

groupby

 问题

1. from m in master

这部分指定了查询的源是 master 列表中的元素。每个元素 m 表示 MartialArtsMaster 类的一个实例。

2. group m by m.Menpai into g

这行代码是关键的分组操作：

  • group m by m.Menpai：表示按照每个 MartialArtsMaster 实例的 Menpai 属性进行分组。
  • into g：这部分将分组结果命名为 g，它代表一个分组后的集合。每个分组都会包含一个 Menpai 值及其对应的所有成员。

3. orderby g.Key

这部分是排序操作，作用是按 Menpai（即分组的依据）对分组后的结果进行排序：

  • g.Key：每个分组 g 都有一个 Key，它就是该分组依据的值。在本例中，g.Key 对应的就是 m.Menpai（即门派名称）。所以，orderby g.Key 就是按门派名称（Menpai）对分组结果进行升序排序。

    为什么要加上 orderby 呢？

    • orderby g.Key 让查询的结果按照 Menpai 字段排序。这样，可以确保结果按门派名称的字母顺序排列，易于查看。
    • 如果不加 orderby，则返回的分组顺序将是不确定的，通常是根据原始数据中分组的顺序来排序的，但这种顺序可能会导致数据不易理解或处理。

groupby问题 

var groupitems = from m in master
                 group m by m.Menpai into g
                 orderby g.Key
                 select new
                 {
                     menpai = g.Key,
                     Count = g.Count()
                 };
           
var groupmethod = master.GroupBy(m => m.Menpai, (k, m) => new { menpai = k, count = m.Count() });

 

(k, m) => new { menpai = k, count = m.Count() }

  • 这是 GroupBy 的第二个参数，它是一个用于生成结果的投影（projection）函数。它的作用是为每个分组生成一个新的对象。
    • k：代表每个分组的键（即 m.Menpai 的值），这就是分组的依据。在这个例子中，k 是每个 Menpai。
    • m：代表分组中的元素集合。它是一个集合，包含了所有具有相同 Menpai 的 master 元素。

3. menpai = k

  • 在生成新的匿名对象时，menpai = k 将分组的键 k 赋值给 menpai 属性。因为 k 是我们用来分组的依据（即门派），所以 menpai 就代表该分组的门派名称。

为什么 menpai = k 而不能使用 menpai = m.Menpai？

  • 原因：m 在这里代表的是分组后的元素集合（即一组有相同 Menpai 值的元素）。所以，m 是一个集合，不能直接使用 m.Menpai，因为 m 是一个包含多个元素的集合，而不是单个元素。

  • k 是 GroupBy 方法的分组键，它代表了当前分组的标识。在这个例子中，k 对应的是每个分组的 Menpai 值，而这个值是每个分组的共同属性。因此，menpai = k 赋值给 menpai 属性是正确的，它表示该分组的 Menpai。

如果你使用 menpai = m.Menpai，会发生编译错误，因为 m 是一个集合（IEnumerable 类型），它本身并没有 Menpai 属性。你需要使用分组的键 k 来获取每个分组的 Menpai 属性。

量词

 var any = master.Any(m => m.Menpai == "丐帮");
 Console.WriteLine(any + "\n");

 var all = master.All(m => m.Menpai == "丐帮");
 Console.WriteLine(all + "\n");

 var ouyangfeng = new MartialArtsMaster { Id = 1, Name = "黄蓉", Age = 18, Menpai = "丐帮", Kungfu = "打狗棒法", Level = 9 };
 var huangrong = master[0];
 var isouyangfeng = master.Contains(ouyangfeng);
 var ishuangrong = master.Contains(huangrong as MartialArtsMaster);
 Console.WriteLine(isouyangfeng +"\n");
 Console.WriteLine(ishuangrong);

分页 

    int pageSize = 2;
    int pageNum =(int) Math.Ceiling(master.Count() /(double) pageSize);

    Console.WriteLine(master.Count + " " + pageNum + " " + pageSize + "\n");
    for( i = 0; i < pageNum; i++)
    {
        var pageMaster = (from m in master
                          join k in kongfu on m.Kungfu equals k.kongfuName
                          orderby m.Level * k.Lethality descending
                          select new
                          {
                              m.Name,
                              m.Menpai,
                              m.Kungfu,
                              m.Level,
                              k.Lethality,
                              kill = m.Level * k.Lethality
                          }).Skip(i * pageSize).Take(pageSize);
        
        Console.WriteLine("姓名 门派武功 杀伤力 修炼等级 总武力");

        foreach(var m in pageMaster)
        {
            Console.WriteLine(m.Name + " " + m.Menpai + " " + m.Kungfu + " " + m.Lethality + " " + m.kill+"\n");
        }
    }


}


 for (i = 0; i < pageNum; i++)
 {
     var pageMaster = master
         .Join(kongfu, m => m.Kungfu, k => k.kongfuName,(m,k) => new
         {
             m.Name,
             m.Menpai,
             m.Kungfu,
             m.Level,
             k.Lethality,
             kill = m.Level * k.Lethality
         }).OrderByDescending(x => x.Level * x.Lethality)
         .Skip(i * pageSize)
         .Take(pageSize);

  1. .Skip(i * pageSize)：

    • 这个方法跳过查询结果中的前 i * pageSize 条记录。
    • i 表示当前页的索引，从 0 开始。
    • pageSize 表示每页显示的记录数。
    • 所以，.Skip(i * pageSize) 会跳过前面 i * pageSize 条数据，以确保你从第 i + 1 页开始加载数据。

  2. .Take(pageSize)：

    • 这个方法会从跳过的记录之后，取出接下来的 pageSize 条记录。
    • 这样，每一页就会获取到 pageSize 条数据。

集合 

 var itemintersect = (from m in master
                      where m.Menpai == "明教" || m.Menpai == "丐帮"
                      select m).Intersect(from m in master where m.Kungfu == "葵花宝典" select m);

 var itemmethod = master.Where(m => m.Menpai == "明教" || m.Menpai == "丐帮")
     .Intersect(master.Where(m => m.Kungfu == "葵花宝典"));
 foreach( var m in itemmethod)
 {
     Console.WriteLine(m.Name + " " + m.Menpai + " "+ m.Kungfu);
 }

合计

 int[] arr = new int[] { 1, 2, 3, 4 };
 int xx = arr.Aggregate((prod, n) => prod + n);
 int y = arr.Aggregate(0, (prod, n) => prod + n);
 int z = arr.Aggregate(0, (prod, n) => prod + n, r => r * 2);
 Console.WriteLine(xx +" " + y+" " +z +" ");

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