c#-LINQ与lambda表达式学习笔记

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 arr1from b in arr2select a + b;var query3 = from n in arr2 select n * 10;var query4 = from n in arr2select new{Id = n,name = n.ToString()};var query11 = from n in arr1where n < 3select 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 studentsgroup s by s.Class into gselect 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 arrwhere 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 arrlet n = num % 2where n==0select num;foreach (var n in query)
{Console.WriteLine(n);}

orderby

var query = from n in arrwhere n > 1 && n < 6orderby n descendingselect n;

group by

int[] arr = new int[] {1,2,3,4,5,6,7,8 };
var query = from n in arrwhere n > 1 && n < 6group 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 arrwhere n > 1 && n < 6group n by n % 2 into gfrom sn in gselect 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 arr1where a < 7join b in arr2 on a equals bselect 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 arr1where score > 2select score;var scoreQuery2 = from score in arr1where score > 2select score;

key

string[] langusges = { "java", "c#", "c++", "python", "vb" };
var query = from item in langusgesgroup item by item.Length into lengthgroupsorderby lengthgroups.Keyselect 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 masterwhere 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 }};//test1var 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);}//test2var 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 kongfuwhere k.Lethality > 90orderby k.Lethality descendingselect new { KongfuId = k.kuongfuId, kongfuName = k.kongfuName, Lethality = k.Lethality, KongfuTop = (ii++) };//var masterLethalityTop = from m in masterTopjoin k in KongfuTop on m.masterKongfu equals k.kongfuNameorderby m.Leverl * k.Lethality descendingselect 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 mastergroup m by m.Menpai into gorderby g.Keyselect 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 masterjoin k in kongfu on m.Kungfu equals k.kongfuNameorderby m.Level * k.Lethality descendingselect 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 masterwhere 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 +" ");