Q47510: Watch Value and Watchpoint Ignored During Event or ERROR Trap

Article: Q47510
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 13-DEC-1989

When using ON ERROR GOTO or any of the ON event GOSUB statements
(where event includes KEY, TIMER, COM, PEN, PLAY, and STRIG),
watchpoints or watch values will not function while you are in the ON
ERROR GOTO or ON <event> GOSUB subroutine. Watch values will show the
default value for the type of variable (either 0 or a null string),
and watchpoints will not halt the program.

This behavior occurs in the QB.EXE editor for QuickBASIC Versions
4.00, 4.00b, 4.50 and the QB.EXE editor for Microsoft BASIC Compiler
Versions 6.00 and 6.00b. This behavior is a known design limitation of
these versions of QuickBASIC and is not an error.

The QBX.EXE editor for Microsoft BASIC PDS 7.00 has been enhanced
so that you may now use watchpoints and watch values in ON ERROR GOTO
and ON <event> GOSUB subroutines.

The following example demonstrates the limitation:

ON TIMER (5) GOSUB 1000
TIMER ON
WHILE INKEY$ = "" : WEND
END

1000 PRINT "In routine"
     FOR x = 1 TO 10
       PRINT x
     NEXT x
     RETURN

Execute the program with a watchpoint or watch value for the variable
"x". The watchpoint will not halt the program in the FOR NEXT loop as
you would expect. If you set a breakpoint inside the FOR NEXT loop,
you will also see that a watch value on "x" will always report a value
of 0.

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