Why do I get “left side cannot be assigned to” for TRect after upgrading Delphi?

Question

I am migrating the code from Delphi 7 to XE2 one of the Graphical module. we are using TRect variable , the old code is working in Delphi 7 without issue

Ex:

Var
  Beold : TRect
begin
  Beold.left := Beold.right;
end.

while porting the code to new XE2 we are facing the issue E0264 : Left side cannot be assigned to

Can you please explain what is the changes in XE2 TRect and D7, how we can assign the valuse

Please show code that exhibits the behaviour that you describe. Until we know what the problem is, it is hard to help. — David Heffernan
TRect is declared as an advanced record in XE2. But this will not explain what you are reporting. Please show real code. — LU RD
I removed the database and sqlserver-2008 tags, as there is no mention of either of those subjects in your question. Also, when you post questions, post real code. As I said below, the code you posted compiles fine on both XE and XE2, so it can't be the code you're actually using. Posting made-up code does exactly what this one did - it can hide the actual problem (or like this one, not even produce the problem at all, because it's not the real code). If you want help, ask a real question with real code that produces the problem you're trying to solve. — Ken White

Ken White Ken White · Accepted Answer · 2012-09-10T13:49:47

The code you posted compiles and runs fine in a quick Delphi test app, so it's not your real code.

I'd suspect what you've hit is a change in the with statement when it's related to using properties, however. There was a bug in previous versions of Delphi that existed for many years that was finally fixed recently. IIRC, it was first mentioned in a note in the README.HTML file for D2010. It's been added to the documentation in XE2 (not as a behavior change, but the new behavior is documented). The documentation is located here at the docwiki.

(Additional info: It must have been 2010 where it changed; Marco Cantù's Delphi 2010 Handbook mentions it on page 111 as "The With Statement Now Preserves Read-Only Properties" which describes this behavior and the solution I indicated below.)

Instead of accessing the property of a class directly using a with statement, you now need to declare a local variable, and read and write the whole thing directly (error handling omitted for clarity - yes, I know there should be a try..finally block to free the bitmap).

var
  R: TRect;
  Bmp: TBitmap;

begin
  Bmp := TBitmap.Create;
  Bmp.Width := 100;
  Bmp.Height := 100;
  R := Bmp.Canvas.ClipRect;
  { This block will not compile, with the `Left side cannot be assigned to` error
  with Bmp.Canvas.ClipRect do
  begin
    Left := 100;
    Right := 100;
  end;
  }
  // The next block compiles fine, because of the local variable being used instead
  R := Bmp.Canvas.ClipRect;
  with R do
  begin
    Left := 100;
    Right := 100;
  end;
  Bmp.Canvas.ClipRect := R;
  // Do other stuff with bitmap, and free it when you're done.
end.

Why do I get “left side cannot be assigned to” for TRect after upgrading Delphi?

2 Answers