如何证明C#中字符串的不可变性?
在我上一次的c#访谈中,
我被要求证明C#字符串的不变性,我知道c#字符串的不变性是什么意思,但是有可能通过代码证明c#字符串的不变性吗? 我可以有一个示例代码片段吗? 提前致谢
我可以证明一个string
不是不可变的。 我需要做的就是显示一些代码,这些代码会改变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,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。)
另外,我认为你可以合理地认为, string
在精神上是不变的,即使在假定的安全代码中存在违反其不可变性的愚蠢的边缘案例。
是的,可以使用ObjectIDGenerator
Class来证明c#字符串的不变性。
下面的答案取自Stringm在String#
实际上,ObjectIDGenerator将为我们在程序中创建的实例返回一个唯一的整数值。借助这个类,我们可以检查是否为string和stringbuilder上的各种操作创建了新实例。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();
}
}
输出将看起来像这样
当str与“Hello World”串联时,字符串的实例id从1变为2,同时追加操作后sbr的实例ID与3保持一致。 这告诉所有关于易变性和不变性。 blStatus变量指示实例是否是新的。
您可以从以下网址找到有关该主题的完整文章:http://dotnetmob.com/csharp-article/difference-string-stringbuilder-c/
一个简单的例子:
string str = "test";string str2 = str;Console.WriteLine(str2);
//输出:测试
str = "DDD";Console.WriteLine(str2);
//输出:测试
试试这个代码,如果字符串是可变的,“str2”应该是“DDD”。
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