How to Prove Immutabiltiy of String in C#?

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灰色年华
灰色年华 2021-02-08 11:06

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# str

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  •  花落未央
    2021-02-08 11:15

    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.

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