“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

Because you have implicit getter and setter functions and you cannot modify the Result of a function as it is a const parameter.

(Note: In case you transform the record in an object, the result would actually be a pointer, thus equivalent to a var parameter).

If you want to stay with a Record, you have to use an intermediate variable (or the Field variable) or use a WITH statement.

See the different behaviors in the following code with the explicit getter and setter functions:

type
  TRec = record
    A: Integer;
    B: string;
  end;

  TForm2 = class(TForm)
  private
    FRec : TRec;
    FRec2: TRec;
    procedure SetRec2(const Value: TRec);
    function GetRec2: TRec;
  public
    procedure DoSomething(ARec: TRec);
    property Rec: TRec read FRec write FRec;
    property Rec2: TRec  read GetRec2 write SetRec2;
  end;

var
  Form2: TForm2;

implementation

{$R *.dfm}

{ TForm2 }

procedure TForm2.DoSomething(ARec: TRec);
var
  LocalRec: TRec;
begin
  // copy in a local variable
  LocalRec := Rec2;
  LocalRec.A := Arec.A; // works

  // try to modify the Result of a function (a const) => NOT ALLOWED
  Rec2.A := Arec.A; // compiler refused!

  with Rec do
    A := ARec.A; // works with original property and with!
end;

function TForm2.GetRec2: TRec;
begin
  Result:=FRec2;
end;

procedure TForm2.SetRec2(const Value: TRec);
begin
  FRec2 := Value;
end;