using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

          var myArray = new[] { 1, 1, 1, 2, 2, 1, 1,  3, 9, 9, 4, 5, 5, 9, 1, 1 };

            IEnumerable<int> myList = myArray.ToList();

​

            IEnumerable<int> result = new List<int>();

            var query = myList.GroupBy(x => x)

              .Select((y, index) => new { Element = y.Key, Counter = y.Count() });

​

            var firstCounter = query.First().Counter == 1 ? 0 : query.First().Counter;

            var lastCounter = query.Last().Counter == 1 ? 0 : query.Last().Counter;

​

​

            if (firstCounter > 0)

                myList = myList.SkipWhile(l => l == query.First().Element);

            if(lastCounter > 0)

                myList = myList.SkipLastWhile(i => i == query.Last().Element);

        Console.WriteLine(string.Join(", ", myList));

    }

}

​

public static class Helper

{

    public static IEnumerable<TSource> SkipLastWhile<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

    {

        var buffer = new List<TSource>();

​

        foreach (var item in source)

        {

            if (predicate(item))

            {

                buffer.Add(item);

            }

            else

            {

                if (buffer.Count > 0)

                {

                    foreach (var bufferedItem in buffer)

                    {

                        yield return bufferedItem;

                    }

​

                    buffer.Clear();

                }

​

                yield return item;

            }

        }

    }

}