Task2.cs

using System;
using System.Linq;

namespace _4Phil
{
    class Program
    {
        static void Main(string[] args)
        {
            // Tests
            int maxLength = LengthOfLongestString(
                new string[3] { "ezy", "jnx", "btp" });
            Console.WriteLine(maxLength == 9);

            maxLength = LengthOfLongestString(
                new string[3] { "co", "dil", "ity" });
            Console.WriteLine(maxLength  == 5);
            
            maxLength = LengthOfLongestString(
                new string[3] { "banana", "racecar", "potato" });
            Console.WriteLine(maxLength);
        }

        static private int LengthOfLongestString(string[] aStringArray)
        {
            // Combine the elements into a single string
            string s = String.Join(String.Empty, aStringArray);
            
            // Fail fast. We need a minimum of one char to create a string
            // with no repeating chars.
            if(String.Empty == s) return -1;

            int stopIndex = s.Length-1;
            int currentMaxLength = 0;

            for(int startIndex = 0; startIndex <= stopIndex; startIndex++ ){
                for(int take = 1; take <= s.Length-startIndex; take++) {
                    string current = s.Substring(startIndex, take);
                    if(current.Distinct().Count() != current.Length) break;
                    if(current.Length > currentMaxLength)
                        currentMaxLength = current.Length;                    
                }
            }
            
            return currentMaxLength;
        }
    }
}