Subst

using System;
using System.Text;

namespace LiveDemo
{
    class SubstringDemo
    {
        public static void Main(string[] args)
        {
            string text = "ABCDEFGHIJKL";
            Console.WriteLine(Substring(text,5));
            Console.WriteLine(text.Substring(5));
            Console.WriteLine("=============================");
            Console.WriteLine(Substring(text,5,1));
            Console.WriteLine(text.Substring(5,1));
        }

        static String Substring(string text, int startIndex, int length)
        {
            if (startIndex < 0 || startIndex > text.Length || length < 0 || startIndex > text.Length - length)
            {
                throw new ArgumentOutOfRangeException();
            }

            if (length == 0)
            {
                return String.Empty;
            }

            if (startIndex == 0 && length == text.Length)
            {
                return text;
            }
            StringBuilder subString = new StringBuilder();
            for (int i = 0; i < length; i++)
            {
                subString.Append(text[startIndex++]);
            }
            return subString.ToString();
        }

        static string Substring(string text, int startIndex)
        {
            return Substring(text, startIndex, text.Length - startIndex);
        }
    }
}