using System;

​

public class Program

{

    static void Main()

    {

        Console.WriteLine(substring("hello world", 2));

    }

    static bool check(string s) {

        return (s == null || s == String.Empty) ? true : false;

    }

    static string substring(string value, int startIndex) {

        if (check(value)) {

            throw new CustomException("value must not be null or empty");

        }

        var str = string.Empty;

        for (int i = startIndex; i < value.Length; i++) {

            str+= value[i];

        }

        return str;

    }

}

​

public class CustomException : Exception {

    public CustomException(string message) : base (message) {    

    }

}