using System;

using System.Text;

​

namespace SubstringExtension

{

    public static class Helper

    {

        public static StringBuilder Substring(this StringBuilder stringBuilder, int startIndex, int length)

        {

            StringBuilder resultSb = new StringBuilder();

            string convertedString = stringBuilder.ToString();

            string subString = convertedString.Substring(startIndex, length);

            resultSb.Append(subString);

            return resultSb;

        }

        public static string MakeUpper(string entryString)

        {

            string resultString = entryString.ToUpper();

            return resultString;

        }

    }

    public static class Program

    {

        static void Main(string[] args)

        {

            StringBuilder resultStringBuilder = new StringBuilder();

​

            string helloWorldString = "Hello, world!";

            helloWorldString = MakeUpper(helloWorldString);

            StringBuilder helloWorldSb = new StringBuilder();

            helloWorldSb.Append(helloWorldString);

​

            resultStringBuilder = helloWorldSb.Substring(0, 5);

            Console.WriteLine(resultStringBuilder);

        }

    }

}

​

​