C#中如何证明字符串的不变性？

Question

在我上次c#面试中，
我被要求证明C#字符串的不可变性，我知道c#字符串的不可变性是什么意思，但是是否可以通过代码证明c#字符串的不可变性？请给我一个示例代码片段。提前致谢

Answer 1

是的，可以使用 ObjectIDGenerator Class.

证明 c# 字符串的不变性

Following answer is taken from dotmob article on String Vs Stringbuilder in C#

实际上 ObjectIDGenerator 将为我们在 programs.With 中创建的实例 return 一个唯一的整数值借助此 class 我们可以检查是否为各种创建了新实例string 和 stringbuilder 上的操作。考虑以下程序

using System;
using System.Text;
using System.Runtime.Serialization;

class Program
{
  static void Main(string[] args)
  {
    ObjectIDGenerator idGenerator = new ObjectIDGenerator();
    bool blStatus = new bool();
    //just ignore this blStatus Now.
    String str = "My first string was ";
    Console.WriteLine("str = {0}", str);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(str, out blStatus));
    //here blStatus get True for new instace otherwise it will be false
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    str += "Hello World";
    Console.WriteLine("str = {0}", str);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(str, out blStatus));
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    //Now str="My first string was Hello World"
    StringBuilder sbr = new StringBuilder("My Favourate Programming Font is ");
    Console.WriteLine("sbr = {0}", sbr);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(sbr, out blStatus));
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    sbr.Append("Inconsolata");
    Console.WriteLine("sbr = {0}", sbr);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(sbr, out blStatus));
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    //Now sbr="My Favourate Programming Font is Inconsolata"
    Console.ReadKey();
  }
}

输出将如下所示

当 str 与“Hello World”连接时，字符串的实例 ID 从 1 变为 2。而 sbr 的实例 ID 在追加操作后也保持为 3。这说明了有关可变性和不变性的所有信息。 blStatus 变量表示实例是否是新的。

您可以从以下位置找到有关该主题的完整文章：http://dotnetmob.com/csharp-article/difference-string-stringbuilder-c/

Answer 2

我可以证明 string 是而非不可变的。我需要做的就是展示一些改变 string 的代码，像这样：

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;
    }
}

现在这是否应该被视为 C# 中的错误是另一回事。 :) （答案可能是 FieldOffset 应该被认为是 unsafe - 上面的代码据称是 safe 因此 string 不应该是可变的。）

此外，我认为您可以合理地争辩说 string 在精神上是不可变的，即使在所谓的安全代码中存在违反其不可变性的愚蠢边缘情况。

C#中如何证明字符串的不变性？

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