using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string str = "this is john, who are you?";

string strrev = "";

char[] punctuation = new char[] { ' ', ',', '.', ':', '\t', '?', '!', '@', '#' };   // note the space!

// convert the input to char[]

char[] inputAsChars = str.ToCharArray();

// step through the char[] looking for punctuation and spaces

string temp = "";

foreach (char c in inputAsChars)

{

    if (punctuation.Contains(c))

    {

        //found a word separator so reverse what we have so far

        char[] tx = temp.ToCharArray();

        Array.Reverse(tx);

        temp = new string(tx);

        strrev += temp + c;    // make sure we add on the separator

        temp = "";

    }

    else

        temp += c; //otherwise add the char to the end of our temporary word

}

System.Diagnostics.Debug.Print(strrev);

    }

}