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

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

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

  TForm1 = class(TForm)
  private
    FRec : TRec;
  public
    procedure DoSomething(ARec: TRec);
    property Rec : TRec read FRec write FRec;
  end;

If I try to assign a value to any of the members of Rec property, I'll get "Left side cannot be assigned to" error:

procedure TForm1.DoSomething(ARec: TRec);
begin
  Rec.A := ARec.A;
end;

while doing the same with the underlying field is allowed:

procedure TForm1.DoSomething(ARec: TRec);
begin
  FRec.A := ARec.A;
end;

Is there any explanation for that behavior?

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.

Yes this is a problem. But the problem can be solved using record properties:

type
  TRec = record
  private
    FA : integer;
    FB : string;
    procedure SetA(const Value: Integer);
    procedure SetB(const Value: string);
  public
    property A: Integer read FA write SetA;
    property B: string read FB write SetB;
  end;

procedure TRec.SetA(const Value: Integer);
begin
  FA := Value;
end;

procedure TRec.SetB(const Value: string);
begin
  FB := Value;
end;

TForm1 = class(TForm)
  Button1: TButton;
  procedure Button1Click(Sender: TObject);
private
  FRec : TRec;
public
  property Rec : TRec read FRec write FRec;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Rec.A := 21;
  Rec.B := 'Hi';
end;

This compiles and workes without problem.