DuplicateValues

Returns all distinct values in a collection.

• Use IEnumerable.GroupBy() to create groups for each distinct value in the enumerable.
• Use IEnumerable.Where() to create select only the groups with a count greater than 1.
• Use IEnumerable.Select() to return the Key property of each group.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static IEnumerable<T> DuplicateValues<T>(IEnumerable<T> items)
  {
    return items
      .GroupBy(c => c)
      .Where(g => g.Count() > 1)
      .Select(i => i.Key);
  }
}

int[] arr = {1, 2, 1, 3, 2, 4};

_30s.DuplicateValues(arr); // {1, 2}