为什么在C＃6.0自动属性初始化中没有`this`？

Question

I have the following code class:

public class Foo
{
    public Nested Bar { get; } = new Nested(this);

    public class Nested
    {
        public Nested(Foo foo)
        {
            foo.DoSomething();
        }
    }

    private void DoSomething()
    {

    }
}

However, I get this compile error:

Keyword 'this' is not available in the current context

I can fix it by simply not using Auto-Property Initializer, and explicitly move it into a constructor instead:

public Nested Bar { get; }

public Foo()
{
    this.Bar = new Nested(this);
}

Why is it so? Isn't Auto-Property Initializer actually translated into constructor code in IL?

Answer 1

Simply: you can't use this in initializers. The idea is to prevent an incomplete object from escaping - Nested(this) could do anything to your object, leading to very confusing and hard to understand bugs. Keep in mind that initializers execute before any constructor that you add. The same thing fails for field initializers too, in exactly the same way:

private Nested _field = new Nested(this);

Essentially, initializers are intended to perform simple initializations - fixing the 98% problem. Anything involving this is more complex, and you'll need to write your own constructor - and take the blame for any timing issues :)

Answer 2

Why is it so? Isn't Auto-Property Initializer actually translated into constructor code in IL?

The rules for automatically implemented property initializers are the same as those for field initializers, for the same reason. Note that property initializers are executed before base class bodies, just like field initializers - so you're still in the context of a "somewhat uninitialized" object; more so than during a constructor body.

So you should imagine that the property is being converted into this:

private readonly Nested bar = new Nested(this); // Invalid

public Nested Bar
{
    get { return bar; }
}

In short, this restriction is to stop you from getting yourself into trouble. If you need to refer to this when initializing a property, just do it manually in a constructor, as per your second example. (It's relatively rare in my experience.)