“Left side cannot be assigned to” for record type properties in Delphi

Question

I\'m curious to know why Delphi treats record type properties as read only:

  TRec = record
    A : integer;
    B : string;
  end;

  TForm1 = class(TForm)
         


        

Answer 1

Like others have said - the read property will return a copy of the record, so the assignment of fields isn't acting on the copy owned by TForm1.

Another option is something like:

  TRec = record
    A : integer;
    B : string;
  end;
  PRec = ^TRec;

  TForm1 = class(TForm)
  private
    FRec : PRec;
  public
    constructor Create;
    destructor Destroy; override;

    procedure DoSomething(ARec: TRec);
    property Rec : PRec read FRec; 
  end;

constructor TForm1.Create;
begin
  inherited;
  FRec := AllocMem(sizeof(TRec));
end;

destructor TForm1.Destroy;
begin
  FreeMem(FRec);

  inherited;
end;

Delphi will dereference the PRec pointer for you, so things like this will still work:

Form1.Rec.A := 1234; 

There's no need for a write part of the property, unless you want to swap the PRec buffer that FRec points at. I really wouldn't suggest to do such swapping via a property anyway.