Fórum Borders, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

#define BONUS

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Solution{
        public int MaxBorderLength(string s){
            if(s==default(string)||s.Length<3)throw new ArgumentException("*** Hey!");
            #if BONUS
                Console.WriteLine("\tstring={0}",s);
            #endif
            IEnumerable<string> allBorders=_findAllBorders(s).Where(x=>x!=default(string));
            if(allBorders.Count()==0)return 0;
            int maxLength=allBorders.Max(x=>x.Length);
            IEnumerable<string> theBorder=allBorders.Where(x=>x.Length==maxLength);
            if(theBorder.Count()!=1){throw new Exception("Oops! (impossible by logic)");}
            string maxBorder=(theBorder.ToArray())[0];
            #if BONUS
                Console.WriteLine("\tborder={0}\n\tlength={1}",maxBorder,maxBorder.Length);
            #endif
            return maxBorder.Length;
        }
        
        private IEnumerable<string> _findAllBorders(string s){
            int lenMax=s.Length/3;
            for(int len=1;len<=lenMax;++len){
                string start=s.Substring(0,len);
                if(s.EndsWith(start)==false){yield return default(string);continue;}
                int nextBorder=s.IndexOf(start,len),lastBorder=s.LastIndexOf(start);
                if(nextBorder==lastBorder)yield return default(string);
                else yield return start;
            }
            yield break;
        }
    }
    public class Program
    {
        public static void Main(string[] args)
        {
            string[] testStrings={
                "codility",
                "abc",
                "kkk",
                "barbararhubarb",
                "barbarabarbrhubarb",
                "xyzaxyzbxyz",
                "abab",
                "ababab",
                "baaab"
            };
            
            Solution sol=new Solution();
            foreach(string ts in testStrings){
                Console.WriteLine(sol.MaxBorderLength(ts));
            }
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,11 sec, cpu time: 0,11 sec, average memory usage: 13 Mb, average nr of threads: 3