If a class has a public reader property like this:

         TLexemeList= class(TObject)
         protected
           fValue: TPrimaevalValue;
         public
           property Value: TPrimaevalValue read fValue;
         ...

is there a way of allowing a class helper to augment this with a protected writer property? My attempt as below messed up the reader:

         TLLEvaluator = class helper for TLexemeList
         private
           procedure SetValue(v: TPrimaevalValue);
         protected
           property Value: TPrimaevalValue write SetValue;
         ...

In your exact example fValue is protected so the following should work:

property Value: TPrimaevalValue read fValue write SetValue;

Another possibility (for non protected properties):

property Value: TPrimaevalValue read GetValue write SetValue;

...

function TLLEvaluator.GetValue: TPrimaevalValue;
begin
   Result := inherited Value;
end;

Other possibilities don't exist, as a helper's declaration will hide the declaration of the extended class (exceptions are methods with "overload" defined).

Regards,
Sven

Note that below is in a different unit.

That didn't work since once we're in the helper it can't see fValue since it's only visible in the original unit.

Obviously code in the helper can simply call SetValue(), but I thought it worth raising in case I was missing something obvious.

You are right that it does not work, but you're not right about the reason. Helpers are able to access published, public and protected members of its extended class (e.g. you're able to write "fValue := aValue" in your setter inside the helper), but you can't make use of fields and methods of the extended class in the helper's declaration.

Obviously code in the helper can simply call SetValue(), but I thought it worth raising in case I was missing something obvious.

What about my second suggestion?

Regards,
Sven

Sorry, it was getting late and I think I missed the GetValue() detail.

property Value: TPrimaevalValue read GetValue write SetValue;

..

function TLLEvaluator.GetValue: TPrimaevalValue;
begin
   Result := inherited Value;
end;

Thanks, I think that works but I need to do more testing.