Linq group by and select particular element

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Hi, I have a list which looks like following:
PartNumber ID
---------- --
ABC 1
ABC 2
ABC 3
XYZ 4
XYZ 5

I want to select all the IDs group by the PartNumber and join the ID into a string for further processing later in my system.
I have code before group by which looks like this:

string stringIDs = string.Join(",", myList.Where(x => x.ID != 0).Select(p => p.ID).ToList());

Therefore my "stringIDs" would look like "1,2,3,4,5" but it is not group by the PartNumber.

How should I modify my codes to make the final output to become something like:
PartNumber ID stringIDs
---------- -- ---------
ABC 1 1,2,3
ABC 2 1,2,3
ABC 3 1,2,3
XYZ 4 4,5
XYZ 5 4,5

Thank you.

What I have tried:

1. Tried changing the codes to become:

string stringIDs = string.Join(",", myList.Where(x => x.ID != 0).GroupBy(y => y.PartNumber).Select(p => p.ID).ToList());

but return error.

Posted 3-Aug-21 22:21pm

Jamie888

Updated 5-Aug-21 1:34am

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Richard MacCutchan 4-Aug-21 4:40am

"but return error."
What error?

Jamie888 4-Aug-21 4:49am

The error is "IGrouping<string> does not contain a definition for "ID" and no accessible extension method "ID" accepting a first argument of type...."

Richard MacCutchan 4-Aug-21 7:22am

What is the structure and content of myList?

Jamie888 4-Aug-21 4:50am

The error happened at the .Select()

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George Swan · Answer 1 · 2021-08-04T00:02:00

Your final output example seems to be just duplicating the information in a collection grouped by PartNumber. Here is an example of how to group your list.Each item in the list is split into a 2 element array from which a collection of value tuples of type (string id, string part) is constructed. That collection is grouped by id. The nested foreach loops just iterate over each group.

C#

  List<string> parts = new List<string> { "ABC 1", "ABC 2", "ABC 3", "XYZ 4", "XYZ 5" };
var grps=  parts.Select(s => s.Split(' ')).Select(a => (id:a[0],part: a[1])).GroupBy(v=>v.id);
foreach (var group in grps)
  {
      Console.WriteLine("Group {0}", group.Key);
      foreach (var p in group)
      {
          Console.WriteLine("  {0}", p.part);
      }
  }

Richard Deeming · Answer 2 · 2021-08-05T01:34:00

Seems simple enough:

C#

var stringIDs = myList
    .Where(x => x.ID != 0)
    .GroupBy(x => x.PartNumber)
    .Select(g => new { PartNumber = g.Key, stringIDs = string.Join(", ", g.Select(x => x.ID)) })
    .ToList();

Or:

C#

var stringIDs = myList
    .Where(x => x.ID != 0)
    .GroupBy(x => x.PartNumber, (PartNumber, items) => new { PartNumber, stringIds = string.Join(", ", items.Select(x => x.ID)) })
    .ToList();

Linq group by and select particular element

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