Linq查询中的链式连接(?)

本文关键字：链式连接 查询 Linq | 更新日期: 2023-09-27 18:04:01

我很难相信我一直在使用Linq这么长时间，不能弄清楚这个…我以前一定做过这件事，但是我的脑子出问题了。

我想做的要点是

from a in A
join b in B on a.aId equals b.aId into Bees
join c in C on b.bId equals c.bId into Cees
select new { a.id, Bees, Cees }

这可以做到吗?如果可以，如何做到?我得到"无法解析符号b"如果我在c连接条件下将b更改为Bees，当然这是一个集合，所以我不能连接id。

Linq查询中的链式连接(?)

from a in A
join b in B
on a.aId equals b.aId into Bees
from bs in Bees
join c in C 
on bs.bId equals c.bId into Cees
select new { a.Id, Bees, Cees };

类定义

class AA
{
    public int Id { get; set; }
    public int aId { get; set; }
}
class BB
{
    public int Id { get; set; }
    public int aId { get; set; }
    public int bId { get; set; }
}
class CC
{
    public int Id { get; set; }
    public int bId { get; set; }
}

主程序

class Program
{
    static void Main(string[] args)
    {
        var A = new List<AA>();
        var B = new List<BB>();
        var C = new List<CC>();
        // make up some data
        var bid = 1;
        var cid = 1;
        for (var a = 1; a < 4; a++)
        {
            A.Add(new AA { Id = a, aId = a });
            for (var b = 1; b < 4; b++)
            {
                B.Add(new BB { Id = bid++, aId = a, bId = b });
                for (var c = 1; c < 4; c++)
                    C.Add(new CC { Id = cid++, bId = b });
            }
        }
        // display the item count for each list
        Console.WriteLine("A: {0}", A.Count());
        Console.WriteLine("B: {0}", B.Count());
        Console.WriteLine("C: {0}", C.Count());
        // define the query
        var abc = from a in A
                  join b in B
                  on a.aId equals b.aId into Bees
                  from bs in Bees
                  join c in C 
                  on bs.bId equals c.bId into Cees
                  select new { a.Id, Bees, Cees };
        // display the results of the query in the console
        foreach (var x in abc)
        {
            var bees = x.Bees.Count();
            var cees = x.Cees.Count();
            var str = String.Format("Id: {0}, Bees: {1}, Cees: {2} ", x.Id, bees, cees);
            Console.WriteLine(str);
        }
     }
}

输出
A: 3
B: 9
C: 27
Id: 1, Bees: 3, Cees: 9
Id: 1, Bees: 3, Cees: 9
Id: 1, Bees: 3, Cees: 9
Id: 2, Bees: 3, Cees: 9
Id: 2, Bees: 3, Cees: 9
Id: 2, Bees: 3, Cees: 9
Id: 3, Bees: 3, Cees: 9
Id: 3, Bees: 3, Cees: 9
Id: 3, Bees: 3, Cees: 9

请记住，当您到达join c行时，b不存在，因为它在Bees集合中。因为除了a中的ID之外，您还加入了其他ID，所以不止一个ID可以加入。对于每个Bees集合，您需要将b与c 连接起来。

from a in A
join b in B on a.aId equals  b.aId into Bees
select new { a.id, Bees, 
    Cees = from b in Bees 
    join c in C on b.bId equals c.bId
    select v}

如果这不是在做一个组连接，而是在做一个常规的连接，这会更有意义:

from a in A
join b in B on a.aId equals b.aId
join c in C on b.bId equals c.bId
select new { a.id, b, c}

现在不知道你的问题的背景，很难说这是否有助于解决你真正的问题。

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