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

public class Test
{
     public static IEnumerable<string> getAllSubstrings(string word)
        {
            return from charIndex1 in Enumerable.Range(0, word.Length)
                   from charIndex2 in Enumerable.Range(0, word.Length - charIndex1 + 1)
                   where charIndex2 >= 2
                   select word.Substring(charIndex1, charIndex2);
        }
  
	public static void Main()
	{
             var list = new List<string>() { 
                "ToCommondays","MonCommonday_","TuesCommonda","WednesCommon_day"
            };

            string shortest = list.OrderBy(s => s.Length).First();
            IEnumerable<string> shortestSubstrings = getAllSubstrings(shortest)
                .OrderByDescending(s => s.Length);
            var other = list.Where(s => s != shortest).ToArray();
            string longestCommonIntersection = string.Empty;
            foreach (string subStr in shortestSubstrings)
            {
                bool allContains = other.All(s => s.Contains(subStr));
                if (allContains)
                {
                    longestCommonIntersection = subStr;
                    break;
                }
            }
            Console.Write("longestCommonIntersection=" + longestCommonIntersection);
	}
}