Relax 'this' keyword restriction in field initializers

[TahirAhmadov]

Is it possible to allow accessing other fields within field initializers, using this?

class Foo
{
  int _x = 1;
  int _y = this._x + 1;
}

Calling methods (and properties) would still be prohibited, only field access would be allowed. I think it should work because similar access is allowed in static context:

class Bar
{
  static int _x = 1;
  static int _y = _x + 1;
}

[HaloFour]

IIRC it's the runtime that forbids accessing this before the base constructor is called (which is when initializers are executed).

[TahirAhmadov]

Is accessing across files allowed in static fields?

[CyrusNajmabadi]

yes. i would not want to make that mistake again.

[TahirAhmadov]

What do you think about topological sort by dependency? That way placement doesn't matter and across files is allowed.

[CyrusNajmabadi]

i will never ever support this. There's nothing in the language that makes me think data dependency should have any influence on this sort of thing.

[TahirAhmadov]

I think about it in a similar way to C# vs C++ WRT "includes". Whereas C++ needs the definition to be "included" to be available, in C# the compiler is able to link all the identifiers without it; basically all identifiers are available everywhere (with the right access). Similarly, instead of relying on lexical order, it would be cool if C# could basically put all the fields into a bucket and make them available to within the class.

But for me personally, it's not a deal breaker.

[CyrusNajmabadi]

Definitely an interesting possibility.

[TahirAhmadov]

Some other discussions I found which are related:
#5813
#5845

Along with allowing to access other fields in field initializers, I think it should also be allowed to access other fields when calling other constructors, like this: : this(this._field) or : base(this._field).

[bbartels]

I come across this issue relatively often with primary constructors, as there is no way to store a temporary value as part of a computation from a constructor parameter. Wanting to set two fields to the same value as a result of an operation in the constructor. For instance:

public class SomeService(TimeProvider timeProvider)
{
    private readonly DateTimeOffset StartTime = timeProvider.GetUtcNow();
    private DateTimeOffset TimeEventLastSeen = StartTime;
}

Lexical order, not across files sounds like a very intuitive way of relaxing this restriction.

[AndersKehlet]

This is how F# works. With primary constructors we effectively treat the body of the type as the scope of the constructor. It makes a ton of sense then to also allow simple assignments in lexical order.

[RikkiGibson]

#6591 suggests allowing a "final initializer" that runs after the constructor returns, but before the instance is used at the creation site. That might address this scenario.

[AndersKehlet]

In C# we'd probably need something like init for more complex scenarios with temp vars, but I'd still want this feature. Roslyn emits a constructor for these assignments anyway.