学完了Linq的核心原理的实与性能的分析，我们可以更加合理的使用Linq，其帮助我们完成功能。 那么真正实现功能的时候，往往使用的是查询表达式的语法规则，这样更加便于读写。那么现在就写几个例子来看看真正的实现方式，与实际应用方式：

简单得查询不多做实例了，主要练习一下新的实例

1、let子句

这let更像一个范围变量，只在表达式中有效，例子：

//这里就提供了一个范围变量n，来实现判断的用法（虽然多此一举）
int[] arr = new[] {0, 1, 2, 3, 4, 5, 6, 7};
var query = from a in arr
    let n = a % 2
    where n == 0
    select a;

2、OrderBy排序 （关键字descending 降序排列， ascending升序排列。）

int[] arr = new[] {0, 1, 2, 3, 4, 5, 6, 7};
var query = from a in arr
    orderby a descending 
    select a;

3、GroupBy分组

int[] arr = new[] {0, 1, 2, 3, 4, 5, 6, 7};
var query =
    from a in arr
    group a by a % 2 == 0;
//返回值为两个分组的序列

4、Into临时标识符

Into用于创建一个临时的标识符，指针去指向一个序列或集合。也相当与合并了两个序列。

int[] arr = new[] {0, 1, 2, 3, 4, 5, 6, 7};
var query =
    from a in arr
    group a by a % 2 == 0 into g
    from sn in g
    select sn;

5、join联合查询

由条件关联两个表。

int[] arr = new[] {0, 1, 2, 3, 4, 5, 6, 7};
int[] arr1 = new[] {0,  2 , 4};
var query1 = from a in arr
    join b in arr1 on a equals b
    select a;

Linq的综合案例

查询一系列的语言，并按照长度进行分组，然后进行升序排列

string[] languane = { "java", "C#", "C++", "python", "golang" };
var query = from a in languane
    group a by a.Length into g
    orderby g.Key
    select g;
foreach (var items in query)
{
    Console.WriteLine(items.Key);
    foreach (var item in items)
    {
        Console.WriteLine(item);
    }
}

建立例子

定义类

//大侠
public 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; }
}
//功夫
class Kongfu
{
    public int KongfuId { get; set; }
    public string KongfuName { get; set; }
    public int Lethality { get; set; }
}

实例化

//初始化大侠
var master = new List<MartialArtsMaster>()
{
    new MartialArtsMaster(){Id = 1,Name = "黄蓉",Age = 8,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 = 9 }
};
//初始化武学
var Kongfu = new List<Kongfu>()
{
    new Kongfu(){KongfuId = 1, KongfuName = "打狗棒法" , Lethality = 90},
    new Kongfu(){KongfuId = 2, KongfuName = "降龙十八掌" , Lethality = 95},
    new Kongfu(){KongfuId = 3, KongfuName = "葵花宝典" , Lethality = 100}
};

1、在武林大侠中选出Level大于8级且门派为丐帮的大侠

var sequence = from a in master
    where a.Level > 8 && a.Menpai == "丐帮"
    select a;
var sequenceMethod = master.Where(x => x.Level > 8 && x.Menpai == "丐帮");

2、过滤所学武功杀伤力大于90的大侠，且按等级进行升序排序。

var sequence1 = (
    from a in master
    from b in Kongfu
    where b.Lethality > 90 && a.Kongfu == b.KongfuName
    orderby a.Level
    select new
           {
               a.Id,
               a.Name,
               a.Kongfu,
               a.Level,
               a.Menpai,
           });
var sequence1Method = master.SelectMany(k => Kongfu, (m, k) => new { mt = m, kf = k })
                .Where(x => x.kf.Lethality > 90 && x.kf.KongfuName == x.mt.Kongfu)
                .OrderBy(m => m.mt.Level)
                .Select(m => new
                {
                    m.mt.Id,
                    m.mt.Name,
                    m.mt.Kongfu,
                    m.mt.Level,
                    m.mt.Menpai,
                });

3、武林高手排名

int i = 0;
var sequence2 = from m in master
                from k in Kongfu
                where m.Kongfu == k.KongfuName
                orderby m.Level * k.Lethality descending, m.Age descending, m.Name
                select new
                {
                    m.Name,
                    m.Id,
                    m.Kongfu,
                    weili = m.Level * k.Lethality,
                    paiming = ++i
                };
var sequence2Method = master.SelectMany(m => Kongfu, (m, k) => new { mt = m, kf = k }).Where(m => m.mt.Kongfu == m.kf.KongfuName).OrderByDescending(x => x.kf.Lethality * x.mt.Level).ThenByDescending(x => x.mt.Age).Select(
                x => new { 
                    x.mt.Id,
                    x.mt.Name,
                    V = x.mt.Level *x.kf.Lethality,
                    x.mt.Kongfu,
                    paimign = ++i
                });

4、武林高手排名（多次分类排名）

var sequenceM = from m in master
                where m.Level > 8
                orderby m.Level descending
                select m;
var sequenceK = from k in Kongfu
                where k.Lethality > 90
               orderby k.Lethality descending
               select k;

var sequence3 = from m in sequenceM
                from k in sequenceK 
                where m.Kongfu == k.KongfuName
                //join k in Kongfu on m.Kongfu equals k.KongfuName //这种join为左连接方式
                orderby m.Level * k.Lethality descending
               select new
                    {
                        paiming = ++i,
                        m.Id,
                        m.Name,
                        m.Age,
                        V = m.Level * k.Lethality
                    };

5、对功夫进行分组

var sequence4 = from k in Kongfu
                join m in master on k.KongfuName equals m.Kongfu 
                into groups
                orderby groups.Count() descending
                select new
                {
                    V = ++i,
                    k.KongfuId,
                    k.KongfuName,
                   V1 = groups.Count()
                };

6、华山与明教都使用葵花宝典的大侠

//Intersect:相等比较器，生成两个序列的交集
            var sequence5 = (from m in master
                             where m.Menpai == "明教" || m.Menpai == "华山"
                             select m).Intersect(from m in master where m.Kongfu == "葵花宝典"                                 select m);