英文:
What should be the C# equivalent of Rust's Box<> type?
问题
Rust中有一种叫做 Box<> 的类型，据说它是C++中智能指针的等价物。
C#中应该使用什么类型来等效于Rust的 Box<> 类型？
英文:
I am translating Rust code into C#.
Rust has some type called Box<> which is said to be C++'s equivalent of smart pointers.
What should be the C# equivalent of Rust's Box<> type?
答案1
得分: 4
答案很简短，没有一个完全等同的类型。
Box 用于将值存储在堆上。有许多原因可以使用它，但这就是它的全部作用。
在 C# 中，引用类型始终指向堆分配。因此，在引用类型上进行"装箱"是没有意义的，因为它们已经被有效地装箱了。
相反，C# 值类型直接分配，可以分配在堆栈上，或者如果作为非静态类字段使用，则直接分配在对象的堆分配内部。您可以通过将其强制转换为兼容的引用类型（object 或值类型实现的接口类型）来装箱 C# 值类型。
但是，C# 中的引用类型不仅被装箱，还具有共享所有权以及共享可变性，并且因此 C# 引用类型更接近于实现这些行为的 Rust 类型，例如 Rc<RefCell<T>> 或 Arc<Mutex<T>>，尽管这些类型和 C# 引用类型之间仍然存在非常重要的语义差异（Rc<_> 不能发送到其他线程，Arc<Mutex<_>> 必须锁定才能访问内部值，Rc 和 Arc 都可以创建引用循环，可能导致内存泄漏等）。特别是共享可变性需要某种同步/原子性才能通过 Rust 编译器，而在 C# 中，您可以轻松创建数据竞争而没有问题。
换句话说，您需要看看值为什么被装箱。
- 是否是为了在一组异构值上启用多态性（
Box<dyn _>）？只需使用 C# 接口。 - 是否是为了启用递归结构？只需使用 C# 类，它们可以自我递归而没有问题。
英文:
The short answer is that there isn't an exactly-equivalent type.
Box is used to put a value on the heap. There are several reasons why you would want to do that, but that's all it does.
In C#, reference types always point to a heap allocation. "Boxing" is therefore pointless on reference types, as they are already effectively boxed.
C# value types are instead allocated directly, either on the stack or directly inside an object's heap allocation if used as a non-static class field. You can box a C# value type by casting it to a compatible reference type: either object or an interface type implemented by the value type.
However, reference types in C# are not just boxed, they also have shared ownership as well as shared mutability and so C# reference types are closer to the Rust types that implement those behaviors, such as Rc<RefCell<T>> or Arc<Mutex<T>>, though there are still very relevant semantic differences between those types and C# reference types (Rc<_> can't be sent to other threads, Arc<Mutex<_>> has to be locked to access the inner value, both Rc and Arc can create reference cycles that could cause a memory leak, etc.). Shared mutability in particular requires some kind of synchronization/atomicity to even pass the Rust compiler, where C# has no problem letting you create data races.
In other words, you need to look at why the value is boxed.
- Is it to enable polymorphism on a set of heterogeneous values (
Box<dyn _>)? Just use C# interfaces. - Is it to enable a recursive structure? Just use C# classes, which can self-recurse without issue.
答案2
得分: -1
public class Box<T>(T val) {
public T _val = val;
}
英文:
public class Box<T>(T val) {
public T _val = val;
}
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