如何在C#中证明字符串的不变性？

Question

在我上次的c#访谈中,
我被要求证明C#字符串的不变性,我知道c#字符串的不变性意味着什么,但是有可能通过代码证明c#字符串的不变性吗？请问我有一个示例代码片段吗？提前致谢

Answer 1

我能证明string是不是一成不变的.我需要做的就是显示一些变异的代码string,如下所示:

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;
    }
}

现在是否应该将其视为C#中的错误是另一回事.:)(答案可能FieldOffset应该被认为是unsafe- 上面的代码据称是safe,因此不string应该是可变的.)

此外,我认为你可以合理地认为这string在精神上是不可改变的,即使有一些愚蠢的边缘案例违反了它在所谓的安全代码中的不变性.