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)   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?

Answer 1

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.