using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var str = "testawannawao";

        var result = GetResult(str);

        Console.WriteLine(result);

    }

    public static string GetResult(string str)

    {

        var list = new List<string>();

        for(var i = 0; i < str.Length; i++)

        {

            for(var y = str.Length-i; y > 0; y--)

            {

                var term = str.Substring(i, y);

                string first = term.Substring(0, term.Length / 2);

                char[] arr   = term.ToCharArray();

​

                Array.Reverse(arr);

​

                string temp   = new string(arr);

                string second = temp.Substring(0, temp.Length / 2);

                if(first.Equals(second))

                {

                    list.Add(term);

                }

            }

        }

        var max = list.Max(s => s.Length);

        return list.FirstOrDefault(s => s.Length == max);

    }

}