How do I use LINQ to group join two different object lists that share a common key value in C#?

Question

I want to join these two lists together with common key being the date but whatever I tried doesn’t work, any suggestions?

Classes:

  public class item_mod 
   {
       [JsonProperty(PropertyName = "item.price")]
       public string price { get; set; }

       [JsonProperty(PropertyName = "item.name")]
       public string name { get; set; }

       [JsonProperty(PropertyName = "@modify_stamp")]
       public string stamp { get; set; }

       [JsonProperty(PropertyName = "$trans")]
       public string trans { get; set; }

       [JsonProperty(PropertyName = "trans.date")]
       public string date { get; set; }
   }


public class trans_mod 
   {
   [DataMember]
   public string refer { get; set; }

   [DataMember]
   public string date { get; set; }

   [DataMember]
   public string time { get; set; }

   [DataMember]
   public string location { get; set; }

   [DataMember]
   public int points { get; set; }

   [DataMember]
   public string _total { get; set; }

   [DataMember]
   public string _discount { get; set; }

   [JsonProperty(PropertyName = "$$position")]
   public string position { get; set; }

   [JsonProperty(PropertyName = "@modify_stamp")]
   public string stamp { get; set; }

   [JsonProperty(PropertyName = "$trans")]
   public string trans { get; set; }
   }

Lists: (I grouped them with Date being the key)

var q1 = it.GroupBy(x => x.date)
                          .Select(g => new
                          {
                              Date = g.Key,
                              Details = g.Select(x => new
                              {
                                  Name = x.name,
                                  Price = x.price,
                                  Trans = x.trans,
                              })
                            .ToList()
                          })
                  .ToList();
var q2 = t.GroupBy(x => x.date)
                          .Select(g => new
                          {
                              Date = g.Key,
                              Details = g.Select(x => new
                              {
                                  Location = x.location,
                                  Points = x.points,
                                  Pos = x.position,
                                  Time = x.time,
                                  Discount = x._discount,
                                  Totals = x._total,
                                  Trans = x.trans,
                                  Ref = x.refer
                              })
                            .ToList()
                          });

I want to merge these two together in a group join - any suggestions how to?

What I've tried so far:

var query = from trans in q2
                                 join item in q1 on q2 equals q1.Date into gj
                                 select new { Date = q2.Date, Item = gj };

=> doesn't work returns blanks

var mergedList = q1.Union(q2).ToList();

=> also doesn't work.

Any help is appreciated, I'm also noting that I am very new to object oriented programming and I'm just trying to figure out how to do things!

Can you show an example of input data and your desired output? — palindrom, Sep 01 '21 at 10:44
You can first do the join and then the grouping if that's what you want. — palindrom, Sep 01 '21 at 10:46
Yes - input example: `public List GetTrans() { return new List() { new trans_mod { refer = "123", date = "12/08/2021" ... }}` — Stalo Hadjitofi, Sep 01 '21 at 14:23
`public List GetTrans() { return new List() { new item_mod { name = "socks", date = "12/08/2021" ... }}` — Stalo Hadjitofi, Sep 01 '21 at 14:27
And output: Date : 12/08/2021, Details : "name: socks" "refer: 123" etc. — Stalo Hadjitofi, Sep 01 '21 at 14:27

score 0 · Answer 1 · answered Sep 01 '21 at 11:09

you can merge by SelectMany

var query = q2.SelectMany(x =>
              q1.Where(z => z.Date == x.Date)
                .Select(z => new
                 {
                     Date=z.Date,
                     q1Details = z.Details,
                     q2Details= x.Details
                 })
        ).ToList();

score 0 · Answer 2 · answered Sep 01 '21 at 14:54

var myvar = from a in it
            join b in t on a.date equals b.date
            select new { a.date, a.price, b.name, b.refer };

or:

var myvar = from a in it
            join b in t on a.date equals b.date
            select new { a.date, details= new {a.price, b.name, b.refer} };

or if you have multiple entries for same dates and want to group by date:

var myvar = from a in it
            join b in t on a.date equals b.date
            group new {a, b} by new {a.date} into g
            select new { a.date, deatils = g.ToLİst() };

How do I use LINQ to group join two different object lists that share a common key value in C#?

2 Answers2