Question

i need to select duplicates from a list using linq and return list of arrays where each of the array contains the duplicates. for example if my list is

{2,2,3,13,4,4,15,5,7,8,9,12,13}

then i need returned

{2,2},{4,4}

the following code only returns a single value for each duplicate

int[] listOfItems = new[] {2,2,3,13,4,4,15,5,7,8,9,12,13};
var duplicates = listOfItems
    .GroupBy(i => i)
    .Where(g => g.Count() > 1)
    .Select(g => g.Key);
foreach (var d in duplicates)
    Console.WriteLine(d);

This produces the following:

4
2

Answer 1

Well this is the problem:

    .Select(g => g.Key)

You’re explicitly just selecting the key.

Remove that call, and you’ll select all the groups instead – you’ll get an IEnumerable<IGrouping<int, int>>. You’ll need to change your output as well, of course. For example:

foreach (var group in duplicates)
{
    Console.WriteLine("{{{0}}}", group.Join(","));
}

EDIT: For a List<int[]> you just need:

var duplicates = listOfItems
    .GroupBy(i => i)
    .Where(g => g.Count() > 1)
    .Select(g => g.ToArray())
    .ToList();