How to Prove Immutabiltiy of String in C#?

Question

In my last c# interview,
I was asked to prove immutability of C# string,I know what is meant by immutability of c# string,But is it possible to prove immutability of c# string through code ? can i have a sample code snippet please. Thanks in advance

Answer 1

I can prove that a string is not immutable. All I need to do is to show some code which mutates a string, like so:

using System;
using System.Runtime.InteropServices;

namespace Demo
{
    class Program
    {
        static void Main(string[] args)
        {
            const string test = "ABCDEF"; // Strings are immutable, right?
            char[] chars = new StringToChar {str = test}.chr;
            chars[0] = 'X';

            // On an x32 release or debug build or on an x64 debug build, 
            // the following prints "XBCDEF".
            // On an x64 release build, it prints "ABXDEF".
            // In both cases, we have changed the contents of 'test' without using
            // any 'unsafe' code...

            Console.WriteLine(test);

            // The following line is even more disturbing, since the constant
            // string "ABCDEF" has been mutated too (because the interned 'constant' string was mutated).

            Console.WriteLine("ABCDEF");
        }
    }

    [StructLayout(LayoutKind.Explicit)]
    public struct StringToChar
    {
        [FieldOffset(0)] public string str;
        [FieldOffset(0)] public char[] chr;
    }
}

Now whether this should be considered a bug in C# is a different matter. :) (The answer is probably that FieldOffset should be considered to be unsafe - the code above is purportedly safe and therefore the string should not be mutatable.)

Also, I think you could legitimately argue that string is immutable in spirit, even if there are silly edge cases which violate its immutability in supposedly safe code.