[SOLVED] Generics.Collections: Argument cannot be assigned to

[Eugene Loza]

Hi all!
Again I'm having a stupid question
Generics.Collections offers a TList for records with TList.Items to access the values. However, I can't assign fields of the record this way:

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. uses SysUtils,Generics.Collections;
4.  
5. type TRec = record
6.   a: integer;
7. end;
8. type TRecList = specialize TList<TRec>;
9.  
10. var List: TRecList;
11.     r: TRec;
12.  
13. begin
14.   List := TRecList.Create;
15.   r.a := 1;
16.   List.Add(r);
17.   List.Items[0].a := -1; //<------ project1.lpr(17,16) Error: Argument cannot be assigned to
18.   FreeAndNil(List);
19. end.
20.  

TObjectList works perfectly in such situation.
Moreover List.Items[0].a += -1; works but doesn't change the value. That really makes me only to guess what's going on here
Yes, the "items" definition is
property Items[Index: SizeInt]: T read GetItem write SetItem; default;
So, it's relatively clear why it's read-only. The direct access to the items by
  protected
    FItems: array of T; 
is actually protected... Is there anything I can do here except for moving FItems to Public (i.e. editing the Generics.Collections)?

------

or is there a way to raise FItems up to public without modifying external Generics.Collections unit?

[michalis]

Castle Game Engine has TStructList (descendant of TList from Generics.Collections) that has methods "List" and "L" to help with this: https://github.com/castle-engine/castle-engine/blob/master/src/base/castleutils_struct_list.inc .

So you can write

Code: [Select]

MyList.List^[I].MyField := 123;

or (only in FPC ObjFpc mode)

Code: [Select]

MyList.L[I].MyField := 123;

See also https://github.com/castle-engine/castle-engine/issues/63 and https://github.com/eugeneloza/decoherence/issues/291

[Eugene Loza]

Thanks a lot, Michalis!
Yes, I've missed the part with TGenericStructList -> TStructList. I've fixed it and it's working perfectly!