C# Linq 学习笔记

刚刚学习了 Siki老师 的C#教程Linq部分，以下是笔记

需要引用命名空间

using System.Linq;

然后我们需要准备数据

武林高手类

/// <summary>
/// 武林高手类
/// </summary>
class MartialArtsMaster
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public string MenPai { get; set; }
    public string Kongfu { get; set; }
    public int Level { get; set; }

    public override string ToString()
    {
        return string.Format("{0}--{1}--{2}--{3}--{4}--{5}", ID, Name, Age, MenPai, Kongfu, Level);
    }
}

武林绝学类

class Kongfu
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int Power { get; set; }

    public override string ToString()
    {
        return string.Format("{0}--{1}--{2}", ID, Name, Power);
    }
}

Main方法里的武林高手列表和武林绝学列表

List<MartialArtsMaster> masterList = new List<MartialArtsMaster>()
{
    new MartialArtsMaster() { ID = 1, Name = "黄蓉", Age = 18, MenPai = "丐帮", Kongfu = "打狗棒法", Level = 86 },
    new MartialArtsMaster() { ID = 2, Name = "洪七公", Age = 70, MenPai = "丐帮", Kongfu = "打狗棒法", Level = 10 },
    new MartialArtsMaster() { ID = 3, Name = "郭靖", Age = 22, MenPai = "丐帮", Kongfu = "降龙十八掌", Level = 10 },
    new MartialArtsMaster() { ID = 4, Name = "任我行", Age = 50, MenPai = "明教", Kongfu = "葵花宝典", Level = 10 },
    new MartialArtsMaster() { ID = 5, Name = "东方不败", Age = 35, MenPai = "明教", Kongfu = "葵花宝典", Level = 540 },
    new MartialArtsMaster() { ID = 6, Name = "林平之", Age = 23, MenPai = "华山", Kongfu = "葵花宝典", Level = 404 },
    new MartialArtsMaster() { ID = 7, Name = "岳不群", Age = 50, MenPai = "华山", Kongfu = "葵花宝典", Level = 40 },
    new MartialArtsMaster() { ID = 8, Name = "令狐冲", Age = 23, MenPai = "华山", Kongfu = "独孤九剑", Level = 4045 },
    new MartialArtsMaster() { ID = 9, Name = "梅超风", Age = 45, MenPai = "桃花岛", Kongfu = "九阴真经", Level = 078 },
    new MartialArtsMaster() { ID = 10, Name = "黄药师", Age = 28, MenPai = "梅花岛", Kongfu = "弹指神功", Level = 78 },
    new MartialArtsMaster() { ID = 11, Name = "风清扬", Age = 46, MenPai = "华山", Kongfu = "独孤九剑", Level =78 },
};
List<Kongfu> kongfuList = new List<Kongfu>()
{
    new Kongfu() {ID=1,Name="打狗棒法",Power=100 },
    new Kongfu() {ID=1,Name="降龙十八掌",Power=45 },
    new Kongfu() {ID=1,Name="葵花宝典",Power=75 },
    new Kongfu() {ID=1,Name="独孤九剑",Power=45 },
    new Kongfu() {ID=1,Name="九阴真经",Power=39 },
    new Kongfu() {ID=1,Name="弹指神功",Power=2 }
};

 

表达式基本写法

var res = from m in masterList
        where m.MenPai == "丐帮"//条件，布尔表达式
        select m.Name;//返回名字的集合

事实上，条件语句是可以用一个委托代替的，如下面的代码

delegate bool BoolDel(MartialArtsMaster mar);

var res = from m in masterList
        where boolDel(m)
        select m.Name;

也可以使用扩展方法的方式进行查询

var res = masterList.Where(new Func<MartialArtsMaster, bool>(boolDel));

接着我用下面的代码输出

Console.WriteLine("找到 " + res.Count() + " 条记录：");
foreach(var item in res)
{
    Console.WriteLine(item);
}

联合查询

使用多个from in 进行查询

var res = from m in masterList
        from k in kongfuList
        where m.Kongfu == k.Name && k.Power > 90
        select new { master = m, kongfu = k };

使用扩展方法进行联合查询

var res = masterList.SelectMany(m => kongfuList, (m, k) => new { master = m, kongfu = k })
    .Where(x => x.master.Kongfu == x.kongfu.Name && x.kongfu.Power > 90);

 

使用Join on 进行联合查询

var res = from m in masterList
        join k in kongfuList on m.Kongfu equals k.Name
        select new { master = m, kongfu = k };

 

对查询结果进行排序

var res = from m in masterList
        orderby m.Level, m.Age descending //级别升序，年龄降序
        select m;

扩展方法形式

var res = masterList.OrderBy(m => m.Level).ThenByDescending(m => m.Age);

 

分组查询

var res = from k in kongfuList
        join m in masterList on k.Name equals m.Kongfu
        into groups//根据 k 分组
        orderby groups.Count()
        select new { kongfu = k, count = groups.Count() };

 

按照自身字段分组

var res = from m in masterList
        group m by m.MenPai into g
        select new { MenPai = g.Key, Count = g.Count() };

 

量词操作

其实就是查询这个集合里是否包含我要查的东西

Console.WriteLine(masterList.Any(m => m.MenPai == "少林寺"));//任一满足，则返回true
Console.WriteLine(masterList.All(m => m.MenPai == "丐帮"));//所有满足，则返回true

第一行查询武林高手列表里有没有少林寺的，第二行查询列表里的是不是都是丐帮的