C# - why is it not possible to access 'this' in a field initializer?

Question

Why does this produce a compiler error:

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

But this does not:

class Foo
{
    public Bar Baz;

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

Conceptually the two are equivalent, are they not?

Answer 1

It is not logical to access the class that is not initialized yet. If you would be able to access this it would mean that Bar is already initialized that is not true till the constructor is not finished.

Answer 2

Because at that point (in you first example) the object has not been constructed yet. In the constructor it is already.

In the constructor body you have an option to do extra initialization but only if you wish - change field values, call other methods, whatever. But this is optional. So you may count on a technically constructed object here, while it may not be really constructed according to your application logic. But then it is you who has to watch out, not compiler any more.

Answer 3

Because the class you're referencing hasn't been initialised yet.

Answer 4

No, they're not quite equivalent... the variable initializer executes before any base class constructors are run. The body of the constructor executes after base class constructors are run. (This is different from Java, where variable initializers execute after base class constructors but before the constructor body.)

Therefore it's safer to access this within the constructor body: you can be sure that the object is initialized at least in respect to its base class (and upwards).

I believe that's the reasoning, anyway...

Answer 5

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

"this" represents the object of a "Foo" class. And you don't have the class object initiated at this point.

Suppose, "Bar" constructor don't need the "Foo" instance to initiate (or call its constructor) like Bar("hello!") then the above statement could be compiled as:

class Foo
{
    public static Bar Baz = new Bar("Hello!");
}

Which can be accessed through:

Foo.Baz.something();

A class can initiate an instance which should be marked with "static" keyword and don't need the class instance to initiate.