为什么可以通过ref参数修改只读字段? [英] Why can readonly fields be modified through ref parameters?

问题描述

考虑:

class Foo
{
    private readonly string _value;

    public Foo()
    {
        Bar(ref _value);
    }

    private void Bar(ref string value)
    {
        value = "hello world";
    }

    public string Value
    {
        get { return _value; }
    }
}

// ...

var foo = new Foo();
Console.WriteLine(foo.Value); // "hello world"

这甚至如何编译，但仍然有效?我不应该为构造函数之外的 _value 字段分配不同的值，因为它用 readonly 标记.但是，通过ref传递给一个方法，确实是可以操作的.

How does this even compile, nonetheless work? I should not be able to assign a the different value to _value field outside of the constructor, as it's marked with readonly. However, pass it by ref to a method, and it can indeed be manipulated.

这很危险吗?为什么?我感觉不对，但我无法完全理解它.

Is this dangerous? Why? It feels wrong to me, but I can't quite put my finger on it.

推荐答案

它可以编译，因为您只是在构造函数中将值作为 ref 参数传递(其中 readonly 允许设置字段).如果您将对 Bar() 的调用移动到其他方法中，它将失败.

It compiles because you're only passing the value as a ref parameter in the constructor (where readonly fields are allowed to be set). If you moved your call to Bar() into some other method, it would fail.

class Foo
{
    private readonly string _value;

    public Foo()
    {
        Bar(ref _value);
    }

    public void Baz()
    {
        Bar(ref _value);
    }

    private void Bar(ref string value)
    {
        value = "hello world";
    }

    public string Value
    {
        get { return _value; }
    }
}

上面的代码提供了一个非常明显的编译器错误:

The above code provides a very telling compiler error:

只读字段不能传递 ref 或 out(构造函数中除外)

A readonly field cannot be passed ref or out (except in a constructor)

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