Implement strStr()

using System;
using System.Collections.Generic;

namespace test
{
    class Program
    {
        public static int StrStr(string haystack, string needle)
        {
            bool match = false;
            int i = 0;
            if (haystack == "" && needle == "")
                return 0;
            if (haystack != "" && needle == "")
                return 0;
            if (haystack.Length < needle.Length)
                return -1;
            for (i = 0; i < haystack.Length; i++)
            {
                if (match == false)
                {
                    if (haystack[i] != needle[0])
                        continue;
                    if (i + needle.Length > haystack.Length)
                        break;
                    int j = 0;
                    while (haystack[i + j] == needle[j])
                    {
                        j++;
                        if (j != needle.Length) continue;
                        match = true;
                        break;
                    }
                }
                else
                    break;
            }

            if (match == true)
            {
                return i - 1;
            }
            return -1;
        }
        public static void Main(string[] args)
        {
            string a = "mississippi";
            string b = "issipi";
            Console.WriteLine(StrStr(a, b));
        }

    }
}