former worked code does not work - delphi

Question

There are code below and worked well before
unit Unit1;
{$DEFINE _Full}
// {$DEFINE _Trial}
interface
uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms;
{$ifdef _Trial}
_programname='abc';
{$endif}
{$ifdef _Full}
_programname='abc';
{$endif}
but today I run Delphi and try to compile, it reported
Identifier redeclared: '_programname'
it looks like
{$DEFINE _Full}
does not work
your comment welcome

Answer 1

Both conditionals are defined. That can be inferred from the compiler error.
Either single line // comments do not comment out defines. Or you are defining _Trial at the project level. To the very best of my knowledge, a single line // comment will comment out a directive. So I presume that _Trial is defined at the project level.
The idiomatic way to comment out defines is like this:
{.$DEFINE _Trial}
For an either or condition it might be simpler with a single conditional:
{$IFDEF _Trial}
.... stuff for trial version
{$ELSE}
.... stuff for full version
{$END}
All that said, perhaps your actual problem is different because the code you show does not match the error message you reported. I'd expect an error saying that a keyword was expected, but identifier _programname found. The code in the question appears to omit the const keyword before the declaration of _programname.

Answer 2

I am submitting this under the assumption that _programname is supposed to be a constant, as that is most consistent with t
he code you are showing. Your constant declarations need to be declared in a CONST section block. Try this:
unit Unit1;
{$DEFINE _Full}
// {$DEFINE _Trial}
interface
uses Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms;
CONST //<< Changed Code
{$ifdef _Trial}
_programname='abc';
{$endif}
{$ifdef _Full}
_programname='abc';
{$endif}