using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        string string1 = "ahappycode";

        string string2 = "happymeal";

​

        string intersection = string1.GetLongestConsecutiveIntersection(string2);

        Console.Write(intersection);

    }

}

​

​

​

public static class Extensions

{

    public static string GetLongestConsecutiveIntersection(this string string1, string string2, StringComparison comparison = StringComparison.Ordinal)

    {

        string first = string1.Length >= string2.Length ? string1 : string2;

        string second = string1.Length >= string2.Length ? string2 : string1;

        var allSubstrings = first.GetAllSubstring().OrderByDescending(s => s.Length);

        foreach(string substring in allSubstrings)

        {

            if(second.IndexOf(substring, comparison) >= 0)

            {

                return substring;

            }

        }

        return null;

    }

    public static List<string> GetAllSubstring(this string input)

    {

        var stringList = new List<string>();

        for (int i = 0; i < input.Length; i++)

            for (int j = i; j < input.Length; j++)

                stringList.Add(input.Substring(i, j - i + 1));

        return stringList;

    }

}

​