using System;
using System.Linq;

class Foo
{
  public int A { get; set; }
  public int B { get; set; }
}

public class Test
{
	public static void Main()
	{
           var list = new System.Collections.Generic.List<Foo>(){
                new Foo(){ A = 1, B = 1 },
                new Foo(){ A = 1, B = 2 },
                new Foo(){ A = 2, B = 3 },
                new Foo(){ A = 2, B = 4 },
                new Foo(){ A = 1, B = 5 },
                new Foo(){ A = 3, B = 6 }
            };

            var groups = list
    		.Select((f, i) => new
    		{
        		Obj = f,
        		Next = list.ElementAtOrDefault(i + 1),
        		Prev = list.ElementAtOrDefault(i - 1)
    		})
    		.Select(x => new
    		{
        		A = x.Obj.A,
        		x.Obj,
        		Consecutive = (x.Next != null && x.Next.A == x.Obj.A)
                   		   || (x.Prev != null && x.Prev.A == x.Obj.A)
    		})
    		.GroupBy(x => new { x.Consecutive, x.A });

            foreach (var abGroup in groups)
            {
                int aKey = abGroup.Key.A;
                var bList = string.Join(",", abGroup.Select(x => x.Obj.B.ToString()).ToArray());
                Console.WriteLine("A = {0}, Bs = [ {1} ] ", aKey, bList);
            }
	}
}