using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string inputString = "zf3kabxcde224lkzf3mabxc51+crsdtzf3nab=";

        int patternLength = 3;

        try

        {

            Dictionary<string,int> patterns = FindMultipleOccurences(inputString, patternLength);

​

            foreach(var pattern in patterns)

            {

                Console.WriteLine(pattern.Key + " : " + pattern.Value.ToString());

            }

        }

        catch(Exception ex)

        {

            Console.WriteLine(ex.Message);

        }

    }

    public static Dictionary<string, int> FindMultipleOccurences(string inputString, int patternLength)

    {

        Dictionary<string, int> patterns = new Dictionary<string, int>();

        if(inputString.Length < patternLength)

        {

            throw new ArgumentOutOfRangeException("Pattern length must be less than or equal to input string length!");

        }

        for(int i = 0; i <= inputString.Length - patternLength; i++)

        {

            string patternFound = inputString.Substring(i, patternLength);

            if(patterns.ContainsKey(patternFound))

            {

                patterns[patternFound] += 1;

            }

            else

            {

                patterns.Add(patternFound, 1);

            }

        }

        return patterns.Where(p => p.Value > 1).ToDictionary(p => p.Key, p => p.Value);

    }

}