Cannot assign value to object with array of records

Question

I'm writing a simple object that contain array of records. Just like invoice.(ID,date,customer name and array of records about the items).

type
  Trows = record
  private
    Fcode: string;
    Qty: Double;
    cena: Currency;
    procedure Setcode(const value: string);
  public
    property code: string read Fcode write SetCode;
  end;

  Tcart = class(TObject)
  private
    Frow: array of Trows;
    function  Getrow(Index: Integer): Trows;
    procedure Setrow(Index: Integer; const Value: Trows);
  public
    ID: integer;
    CustName: string;
    Suma: currency;
    Payed: boolean;
    constructor Create(const Num: Integer);
    destructor Destroy;
    function carttostr: string;
    procedure setcode(Index: integer;val: string);
    property Row[Index: Integer]: Trows read Getrow write setrow;
  end;

Everything seems fine since I'm trying to change value of one record. I have found 3 ways to do it. First, and second worked ok, but I'd like to simplify the code modifying the value of this record like this:

cart.row[0].code:='333';

but it doesn't work.

What am I'm missing?

Here is the code:

procedure TForm1.Button1Click(Sender: TObject);
var
  Arows: Trows;
begin
  Cart:=Tcart.Create(0);
  cart.custName:='Customer 1';
  cart.Suma:=5.55;
  cart.Payed:=false;
  Arows.code:='123';
  cart.setrow(0,Arows);  // this way working
  cart.setcode(0,'333');   // this way also working
  cart.row[0].code:='555';      //this way doesn''t change value. How to make it work?
  memo1.Lines.Text:=cart.carttostr;
end;

Answer 1

It doesn't work because your Row[] property returns a TRows record by value, which means the caller receives a copy of the original record. Any modifications you make to the copy are not reflected in the original.

You need to assign the copy back to the property to apply changes:

procedure TForm1.Button1Click(Sender: TObject);
var
  Arows: Trows;
begin
  ...
  Arows := cart.row[0];
  Arows.code:='555';
  cart.row[0] := Arows; // <-- equivalent to 'cart.setrow(0,Arows);'
  ...
end;