Property & indexer access not fully updated for ref get accessors

[Nigel-Ecma]

In §12.2.2 we have:

• The value of a property access expression is obtained by invoking the get accessor of the property. If the property has no get accessor, a compile-time error occurs. Otherwise, a function member invocation (§12.6.6) is performed, and the result of the invocation becomes the value of the property access expression.
• The value of an indexer access expression is obtained by invoking the get accessor of the indexer. If the indexer has no get accessor, a compile-time error occurs. Otherwise, a function member invocation (§12.6.6) is performed with the argument list associated with the indexer access expression, and the result of the invocation becomes the value of the indexer access expression.

Both of these do not allow for a property or indexer with a ref get accessor – in such a case a compile-time error does not occur, a value is the result, etc.

Are ref get accessors classified as something other than property/indexer accesses somewhere?

Property access comes under member_reference, indexer under element_access, do these handle ref get and correctly classify the expression (§12.2), indeed does classification handle ref valued expressions correctly?

Then there is setting a property or indexer element (§12.21.2), is ref get handled correctly in these cases?

Above questions are not claimed to be exhaustive.

The above observations come from adding support for Index and Range (#1369, draft at time of writing) which has added text such as (§12.8.12.3 Indexer access in #1369):

• Using the best indexer determined at binding-time:
  • If the indexer access is the target of an assignment, the set accessor or ref get accessor is invoked to assign a new value (§12.21.2).
  • In all other cases, the get accessor or ref get accesor is invoked to obtain the current value (§12.2.2).

An ancient (2018 ;-)) issue was raised in csharplang regarding ref get properties, transferred here as #295 years ago (2021 ;-)), and has no milestone…

This surely needs to be sorted for v8.

[BillWagner]

I do think we should add this to the C# 8 milestone, or a new pre-C#9 milestone.

[jnm2]

Both of these do not allow for a property or indexer with a ref get accessor

For my benefit, why do they not allow for a ref get accessor? Is the spec missing a mention of when a dereference happens somewhere along the way?

To me, this seems to be to be an example of "a function member invocation is performed, and the result of the invocation becomes the value of the property access expression":

ref var x = ref SomeProp;

In this scenario, the value is a reference.

[Nigel-Ecma]

@jnm2 – You wrote:

Both of these do not allow for a property or indexer with a ref get accessor

For my benefit, why do they not allow for a ref get accessor? Is the spec missing a mention of when a dereference happens somewhere along the way?

Just look at the quoted text from §12.2.2 “The value of a property access expression is obtained by invoking the get accessor of the property. If the property has no get accessor, a compile-time error occurs.” This is no longer true, if there is a ref get accessor it is invoked and the resultant variable reference is used to obtain the value. There is no mention of a ref get accessor at all.

There is a similar situation when assigning to a property, a ref get accessor can be used.

There is a similar issue for indexers, as a by-product #1369 includes some text to cover it – but it is a by-product of the purpose of #1369 and there are other places needing updating, e.g. the indexer access section of §12.2.2.

Further there is a related binding time issue as well. For indexers V7 selects the best indexer at binding time but does not state that the best indexer must also have a suitable accessor. C# requires that the accessor is checked for at binding time. Text for this is included for indexers only in #1369’s §12.8.12.4 – but again this is a by-product and there may be other places that need changing for indexers and for the #1369 work properties were not even looked at.

[jnm2]

I believe a ref get accessor is a special case of a get accessor- in particular, a ref get accessor is a get accessor whose return type is a by-ref type, and which is contained within a property whose type is a by-ref type.

For assignments to a ref-returning property, something does need to specify that you first call the get accessor to obtain a reference (classified a variable I think), and secondly to assign to that variable.

[Nigel-Ecma]

@jnm2 – A get accessor and a ref get accessor are distinct in the current descriptions, a property/indexer has either a set and or get accessor OR it has a single ref get accessor which has different syntax and semantic descriptions – albeit of course related to those of get.

Even if you propose to unify the two for getting, for setting the use of a getter which returns a ref (rather than the current distinct ref get) still needs to be covered wherever it currently states the setter should be used. I expect in unifying the two you’ll find you’ve just moved sideways, but I’m happy to be convinced otherwise!

[KalleOlaviNiemitalo]

Is the term "ref get accessor" defined?

In standard-v7, subclause 15.7.3 talks about a "get accessor for a ref-valued property":

csharpstandard/standard/classes.md

Line 3272 in 02d1a90

A get accessor for a ref-valued property corresponds to a parameterless method with a return value of a *variable_reference* to a variable of the property type. When such a property is referenced in an expression its get accessor is invoked to compute the *variable_reference* value of the property. That *variable reference*, like any other, is then used to read or, for non-readonly *variable_reference*s, write the referenced variable as required by the context.

There is a grammar rule for ref_get_accessor_declaration but I'm not convinced this implies a definition for "ref get accessor":

csharpstandard/standard/classes.md

Lines 3231 to 3233 in 02d1a90

	ref_get_accessor_declaration
	: attributes? accessor_modifier? 'get' ref_accessor_body
	;