Linq

1. 数据的初始化和LINQ的第一个基础使用

Kongfu Class

class KungFu
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int Power { get; set; }

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

MartialArtsMaster Class

/// <summary>
/// 武林高手
/// </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("Id:{0},Name:{1},Age:{2},Menpai:{3},Kongfu:{4},Level:{5}", Id, Name, Age, Menpai, Kongfu, Level);
    }
}

Foreach查询

    internal class Program
        {
            private static void Main(string[] args)
            {
                //初始化武林高手
                List<MartialArtsMaster> masterList = new List<MartialArtsMaster>()
                {
                    new MartialArtsMaster() {Id = 1,Name = "黄蓉",Age = 18,Menpai = "丐帮",Kongfu = "打狗棒法",Level = 9},
                    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 = 1},
                    new MartialArtsMaster() {Id = 5,Name = "东方不败",Age = 35,Menpai = "明教",Kongfu = "葵花宝典",Level = 10},
                    new MartialArtsMaster() {Id = 6,Name = "林平之",Age = 23,Menpai = "华山",Kongfu = "葵花宝典",Level = 7},
                    new MartialArtsMaster() {Id = 7,Name = "岳不群",Age = 50,Menpai = "华山",Kongfu = "葵花宝典",Level = 8},
                    new MartialArtsMaster() {Id = 8,Name = "令狐冲",Age = 23,Menpai = "华山",Kongfu = "独孤九剑",Level = 10},
                    new MartialArtsMaster() {Id = 9,Name = "梅超风",Age = 23,Menpai = "梅花岛",Kongfu = "九阴真经",Level = 8},
                    new MartialArtsMaster() {Id = 10,Name = "黄药师",Age = 23,Menpai = "梅花岛",Kongfu = "弹指神通",Level = 10},
                    new MartialArtsMaster() {Id = 11,Name = "风清扬",Age = 23,Menpai = "华山",Kongfu = "独孤九剑",Level = 10}
                };
                //初始化武学
                var kongfuList = new List<KungFu>()
                {
                    new KungFu() {Id = 1,Name = "打狗棒法",Power = 90},
                    new KungFu() {Id = 2,Name = "降龙十八掌",Power = 95},
                    new KungFu() {Id = 3,Name = "葵花宝典",Power = 100},
                    new KungFu() {Id = 4,Name = "独孤九剑",Power = 100},
                    new KungFu() {Id = 5,Name = "九阴真经",Power = 100},
                    new KungFu() {Id = 6,Name = "弹指神通",Power = 100}
                };
                //查询所有武学大于八的武林高手,生成在一个表里
                var res = new List<MartialArtsMaster>();
                foreach (var item in masterList)
                {
                    if(item.Level>8)
                        res.Add(item);
                }
                foreach (var item in res)
                {
                    Console.WriteLine(item);
                }

            }
        }
//output:
//Id : 1,Name : "黄蓉",Age : 18,Menpai : "丐帮",Kongfu : "打狗棒法",Level : 9
//Id : 2,Name : "洪七公",Age : 70,Menpai : "丐帮",Kongfu : "打狗棒法",Level : 10
//Id : 3,Name : "郭靖",Age : 22,Menpai : "丐帮",Kongfu : "降龙十八掌",Level : 10
//Id : 5,Name : "东方不败",Age : 35,Menpai : "明教",Kongfu : "葵花宝典",Level : 10
//Id : 8,Name : "令狐冲",Age : 23,Menpai : "华山",Kongfu : "独孤九剑",Level : 10
//Id : 10,Name : "黄药师",Age : 23,Menpai : "梅花岛",Kongfu : "弹指神通",Level : 10
//Id : 11,Name : "风清扬",Age : 23,Menpai : "华山",Kongfu : "独孤九剑",Level : 10

2. linq查询：Linq to Object

     //使用LInq查询
                var res = from m in masterList //from后面设置查询的集合
                          where m.Level > 8//where后面跟上查询的条件
                          select m.Name;//表示m的结果结合返回
                foreach (var item in res)
                {
                    Console.WriteLine(item);
                }
//output:
//黄蓉
//洪七公
//郭靖
//东方不败
//令狐冲
//黄药师
//风清扬

3. Linq拓展方法查询

    //使用Linq拓展方法查询
                var res2 = masterList
                    .Where(ml => ml.Level > 8 && ml.Menpai == "丐帮")
                    .Select(ml => ml.Name);
                foreach (var item in res2)
                {
                    Console.WriteLine(item);
                }
//output
//黄蓉
//洪七公
//郭靖

4. LINQ集合联合查询

//使用Linq联合查询
            var res = from m in masterList
                      from k in kongfuList
                      where k.Name == m.Kongfu
                      select new { master = m, kongfu = k };//使用master和kongfu作为标志生成新集合
            foreach (var item in res)
            {
                Console.WriteLine(item);
            }

取得功夫杀伤力大于90的武林高手

//取得武学杀伤力大于90的高手
            var res = from m in masterList
                      from k in kongfuList
                      where k.Name == m.Kongfu && k.Power>90
                      select m;
            foreach (var item in res)
            {
                Console.WriteLine(item);
            }

5. LINQ使用扩展方法集合联合查询

//使用linq拓展方法联合查询
            var res = masterList
                .SelectMany(kl => kongfuList, (m, k) => new { master = m, KungFu = k })
                .Where(x => x.master.Kongfu == x.KungFu.Name && x.KungFu.Power > 90);
            foreach (var item in res)
            {
                Console.WriteLine(item);
            }

SelectMany拓展方法的第一个参数是Func<MartialArtsMaster,IEnumable<KungFu>>形参名为collectionSelector，第二个参数是Func<MartialArtsMaster,KungFu,'a>形参名为resultSelector，返回的集合相当于上面集合联合查询的第一个结果，在声明里面这个集合用'a表示
Where拓展方法的参数是Func<'a,bool>形参名为predicate返回的集合相当于上面集合联合查询的第二个结果

6. 对结果进行排序 order by descending

    //使用linq对结果进行排序
                var res = from m in masterList
                          where m.Level > 8 && m.Menpai == "丐帮"
                          //orderby m.Age 默认正序 descending时倒序
                          orderby m.Age descending
                          select m;
                foreach (var item in res)
                {
                    Console.WriteLine(item);
                }
//output:
//Id: 2, Name: 洪七公, Age: 70, Menpai: 丐帮, Kongfu: 打狗棒法,Level: 10
//Id: 3, Name: 郭靖, Age: 22, Menpai: 丐帮, Kongfu: 降龙十八掌,Level: 10
//Id: 1, Name: 黄蓉, Age: 18, Menpai: 丐帮, Kongfu: 打狗棒法,Level: 9

多条件排序

                var res = from m in masterList
                          where m.Level > 8 && m.Menpai == "丐帮"
                          orderby m.Level descending , m.Age descending//先按第一个条件排序，第一个相同时按第二个排序
                          select m;
                foreach (var item in res)
                {
                    Console.WriteLine(item);
                }
//output:
//Id: 2, Name: 洪七公, Age: 70, Menpai: 丐帮, Kongfu: 打狗棒法,Level: 10
//Id: 3, Name: 郭靖, Age: 22, Menpai: 丐帮, Kongfu: 降龙十八掌,Level: 10
//Id: 1, Name: 黄蓉, Age: 18, Menpai: 丐帮, Kongfu: 打狗棒法,Level: 9

拓展方法排序

//使用linq拓展方法对结果进行排序
           var res = masterList
               .Where(m => m.Level > 8)
               .OrderByDescending(m => m.Age)
               .ThenBy(m => m.Level);
           foreach (var item in res)
           {
               Console.WriteLine(item);
           }

7. Join on集合联合查询

//使用join on联合查询
            var res = from m in masterList
                      join k in kongfuList on m.Kongfu equals k.Name
                      where k.Power>90
                      select new { master = m, kongfu = k };
            foreach (var item in res)
            {
                Console.WriteLine(item);
            }

8. 对结果进行分组操作into

//使用into 计数组
            var res = from k in kongfuList
                      join m in masterList on k.Name equals m.Kongfu
                      into groups
                      orderby groups.Count()
                      select new { kongfu = k, count = groups.Count() };
            foreach (var item in res)
            {
                Console.WriteLine(item);
            }

9. 对结果进行分组操作group by

    //使用 group by自身分组
                var res = from m in masterList
                          group m by m.Kongfu into g
                          orderby g.Count()
                          select new { kongfu = g.Key, count = g.Count() };
                foreach (var item in res)
                {
                    Console.WriteLine(item);
                }
//output：
//{ count = 1, key = 降龙十八掌 }
//{ count = 1, key = 九阴真经 }
//{ count = 1, key = 弹指神通 }
//{ count = 2, key = 打狗棒法 }
//{ count = 2, key = 独孤九剑 }
//{ count = 4, key = 葵花宝典 }

10. 量词操作符 any all

//使用All Any返回判断
            bool res1 = masterList.Any(m => m.Kongfu == "葵花宝典");
            bool res2 = masterList.All(m => m.Kongfu == "葵花宝典");
            Console.WriteLine(res1);//true
            Console.WriteLine(res2);//false