C# Linq 學習筆記

剛剛學習了 Siki老師 的C#教程Linq部分，以下是筆記

需要引用命名空間

1 using System.Linq;

然後我們需要準備數據

武林高手類

/// <summary>
/// 武林高手類
/// </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);
    }
}

武林絕學類

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絕學列表

List<MartialArtsMaster> masterList = new List<MartialArtsMaster>()
{
    new MartialArtsMaster() { ID = 1, Name = "黃蓉", Age = 18, MenPai = "丐幫", Kongfu = "打狗棒法", Level = 86 },
    new MartialArtsMaster() { ID = 2, Name = "洪七公", Age = 70, MenPai = "丐幫", Kongfu = "打狗棒法", Level = 10 },
    new MartialArtsMaster() { ID = 3, Name = "郭靖", Age = 22, MenPai = "丐幫", Kongfu = "降龍十八掌", Level = 10 },
    new MartialArtsMaster() { ID = 4, Name = "任我行", Age = 50, MenPai = "明教", Kongfu = "葵花寶典", Level = 10 },
    new MartialArtsMaster() { ID = 5, Name = "東方不敗", Age = 35, MenPai = "明教", Kongfu = "葵花寶典", Level = 540 },
    new MartialArtsMaster() { ID = 6, Name = "林平之", Age = 23, MenPai = "華山", Kongfu = "葵花寶典", Level = 404 },
    new MartialArtsMaster() { ID = 7, Name = "岳不群", Age = 50, MenPai = "華山", Kongfu = "葵花寶典", Level = 40 },
    new MartialArtsMaster() { ID = 8, Name = "令狐沖", Age = 23, MenPai = "華山", Kongfu = "獨孤九劍", Level = 4045 },
    new MartialArtsMaster() { ID = 9, Name = "梅超風", Age = 45, MenPai = "桃花島", Kongfu = "九陰真經", Level = 078 },
    new MartialArtsMaster() { ID = 10, Name = "黃藥師", Age = 28, MenPai = "梅花島", Kongfu = "彈指神功", Level = 78 },
    new MartialArtsMaster() { ID = 11, Name = "風清揚", Age = 46, MenPai = "華山", Kongfu = "獨孤九劍", Level =78 },
};
List<Kongfu> kongfuList = new List<Kongfu>()
{
    new Kongfu() {ID=1,Name="打狗棒法",Power=100 },
    new Kongfu() {ID=1,Name="降龍十八掌",Power=45 },
    new Kongfu() {ID=1,Name="葵花寶典",Power=75 },
    new Kongfu() {ID=1,Name="獨孤九劍",Power=45 },
    new Kongfu() {ID=1,Name="九陰真經",Power=39 },
    new Kongfu() {ID=1,Name="彈指神功",Power=2 }
};

表達式基本寫法

var res = from m in masterList
        where m.MenPai == "丐幫"//條件，布爾表達式
        select m.Name;//返回名字的集合

事實上，條件語句是可以用一個委托代替的，如下麵的代碼

delegate bool BoolDel(MartialArtsMaster mar);

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

也可以使用擴展方法的方式進行查詢

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

接著我用下面的代碼輸出

1 Console.WriteLine("找到 " + res.Count() + " 條記錄：");
2 foreach(var item in res)
3 {
4     Console.WriteLine(item);
5 }

聯合查詢

使用多個from in 進行查詢

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

使用擴展方法進行聯合查詢

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 進行聯合查詢

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

對查詢結果進行排序

var res = from m in masterList
        orderby m.Level, m.Age descending //級別升序，年齡降序
        select m;

擴展方法形式

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

分組查詢

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

按照自身欄位分組

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

量詞操作

其實就是查詢這個集合里是否包含我要查的東西

Console.WriteLine(masterList.Any(m => m.MenPai == "少林寺"));//任一滿足，則返回true
Console.WriteLine(masterList.All(m => m.MenPai == "丐幫"));//所有滿足，則返回true

第一行查詢武林高手列表裡有沒有少林寺的，第二行查詢列表裡的是不是都是丐幫的