使用带有 this 关键字的 ref 参数？

发布于 2024-08-28 15:51:03 字数 380 浏览 8 评论 0原文

有没有办法强制 this 关键字充当 ref 参数？我想传递一个修改对象上多个属性的访问者，但这只想充当值参数。

对象中的代码：

public void Accept(Visitor<MyObject> visitor)
{
    visitor.Visit(this);
}

访问者中的代码：

public void Visit(ref Visitor<MyObject> receiver)
{
    receiver.Property = new PropertyValue();
    receiver.Property2 = new PropertyValue();
}

原文

Is there a way to force the this keyword to act as a ref argument? I would like to pass in a visitor that modifies multiple properties on the object, but this only wants to act like a value parameter.

Code in Object:

public void Accept(Visitor<MyObject> visitor)
{
    visitor.Visit(this);
}

Code in Visitor:

public void Visit(ref Visitor<MyObject> receiver)
{
    receiver.Property = new PropertyValue();
    receiver.Property2 = new PropertyValue();
}

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就像说晚安 2024-09-04 15:51:03

由于您实际上并未更改 receiver 所指的内容，因此不需要 ref 关键字。但是，如果是这种情况，您将无法使 this 引用另一个实例。

为了将 this 作为 ref 参数传递，您需要这样编写：

Visit(ref this);

该代码将无法编译："Cannot pass '; '作为 ref 或 out 参数，因为它是只读的”

Since you are not actually changing what receiver refers to there is no need for the ref keyword. However, if that would have been the case, you would not be able to make this refer to another instance.

In order to pass this as a ref parameter, you would need to write it like this:

Visit(ref this);

That code will not compile: "Cannot pass '<this>' as a ref or out argument because it is read-only"

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天生の放荡 2024-09-04 15:51:03

如果您的 Visitor 是一个类，那么您不需要 ref 关键字。

如果在方法中更改，类中的所有信息都会自动更改

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只是在用心讲痛 2024-09-04 15:51:03

听起来您已经得到了更大问题的答案。但无论如何，让我们考虑一下这个问题。

有没有办法强制 this 关键字充当 ref 参数？

是的。逻辑地完成它。

ref 参数为变量创建别名。
“this”不是引用类型中的变量
“this”是值类型中的变量，
因此传递“ref this”的唯一方法是从值类型的实例方法。

事实上，这就是“this”在幕后以值类型实现的方式。当你有一个（最糟糕的做法！实际上不要这样做）可变值类型时：

struct S
{
    private int x;
    public void M()
    {
        this.x = 123;
    }
}
...
S s = new S();
s.M();

既然 S 的实例是按值传递的，那么 M 是如何改变 s 的呢？ s 必须通过引用传递！事实上，我们生成的代码就像您编写的代码一样：

struct S
{
    private int x;
    public static void M(ref S THIS)
    {
        THIS.x = 123;
    }
}
...
S s = new S();
S.M(ref s);

简而言之，结构中的“this”已经作为 ref 参数传递，因此再次传递它是没有问题的。这样做几乎总是一个糟糕的想法，但它是合法的。

It sounds like you got an answer to your larger question already. But let's consider this one anyway.

Is there a way to force the this keyword to act as a ref argument?

Yes. Work it through logically.

A ref argument makes an alias for a variable.
"this" is not a variable in a reference type
"this" is a variable in a value type
therefore the only way to pass a "ref this" is from an instance method of a value type.

And in fact that is how "this" is implemented in a value type behind the scenes. When you have a (worst practice! do not actually do this) mutable value type:

struct S
{
    private int x;
    public void M()
    {
        this.x = 123;
    }
}
...
S s = new S();
s.M();

Since instances of S are passed by value how is it that M mutates s? s must be passed by reference! In fact the code we generate is just as if you'd written something like:

struct S
{
    private int x;
    public static void M(ref S THIS)
    {
        THIS.x = 123;
    }
}
...
S s = new S();
S.M(ref s);

In short, a "this" in a struct already is passed as a ref parameter, so there is no problem passing it along again. Doing so is almost always a terrible idea but it is legal.

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