Demonstrates compile-time string constants are interned in C#, and side-effects when using direct memory access to those strings.

TestInternedStrings.cs

using System;

namespace ConsoleApplication1
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var s1 = "Immutable String";
            var s2 = "Immutable String";
            var s3 = s1;
            var s4 = s2;

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

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

            Console.ReadKey();
        }
    }
}