c# - 使用双分组后如何展平 Linq 查询？

Question

假设我有一个代表订单行的类，例如

public class Line
{
    public string Code ;
    public string No ; // Invoice Number
    public DateTime Date ;
    public string Product ;
    public decimal Quantity ;
}

和行列表，例如

List<Line> myList = new List<Line>();
myList.Add(new Line() { Code = "ABC001", No = "1001" ,Date = new DateTime(2012,4,1) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC001", No = "1001" ,Date = new DateTime(2012,4,1) ,  Product = "Y", Quantity= 1m});

myList.Add(new Line() { Code = "ABC002", No = "1002" ,Date = new DateTime(2012,4,2) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC002", No = "1002" ,Date = new DateTime(2012,4,2) ,  Product = "Y", Quantity= 1m});
myList.Add(new Line() { Code = "ABC002", No = "1003" ,Date = new DateTime(2012,4,3) ,  Product = "Z", Quantity= 1m});
myList.Add(new Line() { Code = "ABC002", No = "1004" ,Date = new DateTime(2012,4,4) ,  Product = "X", Quantity= 1m});

myList.Add(new Line() { Code = "ABC003", No = "1005" ,Date = new DateTime(2012,4,4) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC003", No = "1006" ,Date = new DateTime(2012,4,4) ,  Product = "X", Quantity= 1m});
myList.Add(new Line() { Code = "ABC003", No = "1006" ,Date = new DateTime(2012,4,4) ,  Product = "Y", Quantity= 1m});

我希望检索客户代码有多个发票的所有行。为此，我首先按代码、编号和日期分组，然后按客户代码分组，对于有两条或更多记录的任何客户，我选择除第一条记录以外的所有记录。

像这样:

var query1 = 
    (from r in myList
        group r by new { r.Code ,  r.No , r.Date } into results
        group results by new { results.Key.Code } into results2 
        where results2.Count() > 1
        select new 
        {   
            results2.Key.Code , 
            Count = results2.Count(), 
            Results = results2.OrderBy(i=>i.Key.Date).Skip(1).ToList() 
          // Skip the first invoice
        } 
     ).ToList();

query1 现在包含正确的记录，但包含在 IGrouping 中，我无法将结果作为 List<Line> 输出

我试过了 query1.SelectMany(r=>r.Results).ToList();

但这仍然给我留下了 IGrouping，这就是我被困的地方。

我可以求助于嵌套的 for 循环

List<Line> output = new List<Line>();
foreach (var r1 in query1)
{
    foreach(var r2 in r1.Results)
        foreach(var r3 in r2)
            output.Add(r3);     
}

但是有更好的/Linq 方式吗？

实际输出应该是四行

// Code    No    Date              Product Quantity 
// ABC002 1003 03/04/2012 00:00:00 Z 1 
// ABC002 1004 04/04/2012 00:00:00 X 1 
// ABC003 1006 04/04/2012 00:00:00 X 1 
// ABC003 1006 04/04/2012 00:00:00 Y 1 

Answer 1

你会踢自己:

query1.SelectMany(q => q);

ABC002 1003 3/04/2012 12:00:00 AM Z 1 
ABC002 1004 4/04/2012 12:00:00 AM X 1 
ABC003 1006 4/04/2012 12:00:00 AM X 1 
ABC003 1006 4/04/2012 12:00:00 AM Y 1 

query1 的返回值是 IGrouping 的可枚举值(我删除了你的列表)，而 IGrouping 本身也是可枚举值，因此我们可以直接压平。

参见此处:http://mtaulty.com/CommunityServer/blogs/mike_taultys_blog/archive/2007/09/28/9836.aspx

编辑:记得我还简化了您的代码:

var query1 = 
(from r in myList
    group r by new { r.Code ,  r.No , r.Date } into results
    group results by new { results.Key.Code } into results2 
    where results2.Count() > 1
    from result in results2.OrderBy(i=>i.Key.Date).Skip(1)
    select result
 );