"Immutable" String test

ImmutableStringTest.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestingStringImutability
{
    class Program
    {
        static void Main(string[] args)
        {

            var s1 = "Immutable String";
            var s2 = "Immutable String";
            var s3 = s2;
            var sb = new StringBuilder(s1);
            var s4 = sb.ToString();
            var s5 = sb.ToString();

            unsafe
            {
                fixed (char* ptr = s1)
                {
                    *(ptr + 1) = '_';
                    *(ptr + 2) = '_';
                }
                fixed (char* ptr = s4)
                {
                    *(ptr + 1) = '_';
                    *(ptr + 2) = '_';
                }
            }

            Console.WriteLine("S1 = " + s1); // Prints I__utable String
            Console.WriteLine("S2 = " + s2); // Prints I__utable String
            Console.WriteLine("S3 = " + s3); // Prints I__utable String
            Console.WriteLine("S4 = " + s4); // Prints Immutable String
            Console.WriteLine("S5 = " + s5); // Prints Immutable String

            Console.ReadKey();
        }
    }
}