using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        string a  = "hi"; 

        a.Replace("hi","Yi"); //Imuutable 

        StringBuilder sb = new StringBuilder("hi"); 

        sb.Replace(sb.ToString(),"Yi"); // Mutables

        Console.WriteLine(a);

        Console.WriteLine(sb.ToString());

​

    }

​

}