using System;

using System.Linq;

using System.Collections.Generic;

static class Example

{

​

   public static IEnumerable<IEnumerable<int>> GroupConsecutive(this IEnumerable<int> list)

   {

    var group = new List<int>();

    foreach (var i in list)

    {

​

        if (group.Count == 0 || i - group[group.Count - 1] == 1)

        {

             group.Add(i);

        }

        else

        {

             if (group.Count >= 2) yield return group;

             group = new List<int> { i };

        }

      }

    }

}

​

public class Program

{

  public static void Main(string[] args)

  {

    var groups = new[] {2, 4, 7, 8, 9, 5, 6}.GroupConsecutive();

​

    foreach (var g in groups)

    {

         Console.WriteLine("group: {0}", string.Join(",", g.Select(x => 

                                       x.ToString()).ToList()));

         Console.ReadLine();

   }

  }

}