“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.

Answer 2

Since "Rec" is a property, the compiler treats it a little differently because it has to first evaluate the "read" of the property decl. Consider this, which is semantically equivalent to your example:

...
property Rec: TRec read GetRec write FRec;
...

If you look at it like this, you can see that the first reference to "Rec" (before the dot '.'), has to call GetRec, which will create a temporary local copy of Rec. These temporaries are by design "read-only." This is what you're running into.

Another thing you can do here is to break out the individual fields of the record as properties on the containing class:

...
property RecField: Integer read FRec.A write FRec.A;
...

This will allow you to directly assign through the property to the field of that embedded record in the class instance.