我知道这是个意外，但是… 它们真的是不可变的吗? 考虑以下几点。

public static unsafe void MutableReplaceIndex(string s, char c, int i)
{
    fixed (char* ptr = s)
    {
        *((char*)(ptr + i)) = c;
    }
}

...

string s = "abc";
MutableReplaceIndex(s, '1', 0);
MutableReplaceIndex(s, '2', 1);
MutableReplaceIndex(s, '3', 2);
Console.WriteLine(s); // Prints 1 2 3

你甚至可以让它成为一个扩展方法。

public static class Extensions
{
    public static unsafe void MutableReplaceIndex(this string s, char c, int i)
    {
        fixed (char* ptr = s)
        {
            *((char*)(ptr + i)) = c;
        }
    }
}

是什么使下面的工作

s.MutableReplaceIndex('1', 0);
s.MutableReplaceIndex('2', 1);
s.MutableReplaceIndex('3', 2);

Conclusion: They're in an immutable state which is known by the compiler. Of couse the above only applies to .NET strings as Java doesn't have pointers. However a string can be entirely mutable using pointers in C#. It's not how pointers are intended to be used, has practical usage or is safely used; it's however possible, thus bending the whole "mutable" rule. You can normally not modify an index directly of a string and this is the only way. There is a way that this could be prevented by disallowing pointer instances of strings or making a copy when a string is pointed to, but neither is done, which makes strings in C# not entirely immutable.

至少有两个原因。

第一-安全http://www.javafaq.nu/java-article1060.html

The main reason why String made immutable was security. Look at this example: We have a file open method with login check. We pass a String to this method to process authentication which is necessary before the call will be passed to OS. If String was mutable it was possible somehow to modify its content after the authentication check before OS gets request from program then it is possible to request any file. So if you have a right to open text file in user directory but then on the fly when somehow you manage to change the file name you can request to open "passwd" file or any other. Then a file can be modified and it will be possible to login directly to OS.

第二-内存效率http://hikrish.blogspot.com/2006/07/why-string-class-is-immutable.html

JVM internally maintains the "String Pool". To achive the memory efficiency, JVM will refer the String object from pool. It will not create the new String objects. So, whenever you create a new string literal, JVM will check in the pool whether it already exists or not. If already present in the pool, just give the reference to the same object or create the new object in the pool. There will be many references point to the same String objects, if someone changes the value, it will affect all the references. So, sun decided to make it immutable.

我知道这是个意外，但是… 它们真的是不可变的吗? 考虑以下几点。

public static unsafe void MutableReplaceIndex(string s, char c, int i)
{
    fixed (char* ptr = s)
    {
        *((char*)(ptr + i)) = c;
    }
}

...

string s = "abc";
MutableReplaceIndex(s, '1', 0);
MutableReplaceIndex(s, '2', 1);
MutableReplaceIndex(s, '3', 2);
Console.WriteLine(s); // Prints 1 2 3

你甚至可以让它成为一个扩展方法。

public static class Extensions
{
    public static unsafe void MutableReplaceIndex(this string s, char c, int i)
    {
        fixed (char* ptr = s)
        {
            *((char*)(ptr + i)) = c;
        }
    }
}

是什么使下面的工作

s.MutableReplaceIndex('1', 0);
s.MutableReplaceIndex('2', 1);
s.MutableReplaceIndex('3', 2);

Conclusion: They're in an immutable state which is known by the compiler. Of couse the above only applies to .NET strings as Java doesn't have pointers. However a string can be entirely mutable using pointers in C#. It's not how pointers are intended to be used, has practical usage or is safely used; it's however possible, thus bending the whole "mutable" rule. You can normally not modify an index directly of a string and this is the only way. There is a way that this could be prevented by disallowing pointer instances of strings or making a copy when a string is pointed to, but neither is done, which makes strings in C# not entirely immutable.

这主要是出于安全考虑。如果您不能相信您的字符串是防篡改的，那么保护系统就困难得多。

Java中的字符串并不是真正不可变的，您可以使用反射和或类加载来更改它们的值。你不应该依赖这个属性来保证安全。 有关示例请参见:Java中的魔术

线程安全和性能。如果一个字符串不能被修改，那么在多个线程之间传递引用是安全且快速的。如果字符串是可变的，则总是必须将字符串的所有字节复制到新实例，或者提供同步。一个典型的应用程序在每次需要修改字符串时将读取字符串100次。参见维基百科关于不变性的内容。