compiler error with preprocessing directives

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cody - 29 Mar 2006 13:36 GMT

the following leads to an error (note that TEST is not defined):

#if TEST
string s = @"
#"; // <-- the error is "preprocessing directive expected"
#endif

also, here we get the same error:

#if TEST
/*
# */
#endif

iam not sure wheather this is a problem in the compiler or the spec, but in
my opinion even in such directives the compiler should parse comments and
verbatim string to ensure we can use #if statements everywhere and do not
have to change to code to make if work.

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Jon Skeet [C# MVP] - 29 Mar 2006 14:39 GMT

> the following leads to an error (note that TEST is not defined):
>
[quoted text clipped - 14 lines]
> verbatim string to ensure we can use #if statements everywhere and do not
> have to change to code to make if work.

The spec disagrees:

<quote>
The remaining conditional-sections, if any, are processed as skipped-
sections: except for pre-processing directives, the source code in the
section need not adhere to the lexical grammar; no tokens are generated
from the source code in the section; and pre-processing directives in
the section must be lexically correct but are not otherwise processed.
Within a conditional-section that is being processed as a skipped-
section, any nested conditional-sections (contained in nested #if...
#endif and #region...#endregion constructs) are also processed as
skipped-sections.
</quote>

Basically, #"; isn't lexically correct as a preprocessing directive.
The comments business is interesting - I assume that's where the "no
tokens are generated" bit comes in.

Can you give an example where it's actually *useful* to have lexically
invalid preprocessor directives in your code?

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cody - 30 Mar 2006 19:09 GMT

> <quote>
> The remaining conditional-sections, if any, are processed as skipped-
[quoted text clipped - 14 lines]
> Can you give an example where it's actually *useful* to have lexically
> invalid preprocessor directives in your code?

Useful or not but it happened. I copied sql script from a file into my code
and used @ to make a verbatim string so that I do not have to change
something in the string and do not need to make "stuff"+ for every single
line.
The sql code contained sql comments (#) and they were located at line
beginnings.
Because this code wasn't ready for release I decided to put #if..#endif
around it.
So that was my story :-)

Iam sure there are more situations where similar things might happen. imho
the best thing would be if the the compiler would ignore all # inside a #if
block unless it is the #endif directive which would really minimize the
chance that such things happen.

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Jon Skeet [C# MVP] - 30 Mar 2006 20:36 GMT

> > Basically, #"; isn't lexically correct as a preprocessing directive.
> > The comments business is interesting - I assume that's where the "no
[quoted text clipped - 17 lines]
> block unless it is the #endif directive which would really minimize the
> chance that such things happen.

Except it also has to look for #if so that it can match the right
number of levels of #endif.

#if isn't meant to be a way of commenting out code - do that with
multi-line comments, or get the IDE to add // at the start of each line
(I think you can just highlight the relevant section and then press a
shortcut - I don't know the key off hand.)

So, to me it sounds like changing your behaviour in this case would be
a better answer than changing the spec to suit this particular case.

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cody - 31 Mar 2006 10:58 GMT

>> Iam sure there are more situations where similar things might happen.
>> imho
[quoted text clipped - 5 lines]
> Except it also has to look for #if so that it can match the right
> number of levels of #endif.

Which it also has to do at the moment, otherwise the following wouldn't
compile:

#if false
#if false
#endif
#endif

> #if isn't meant to be a way of commenting out code - do that with
> multi-line comments, or get the IDE to add // at the start of each line
[quoted text clipped - 3 lines]
> So, to me it sounds like changing your behaviour in this case would be
> a better answer than changing the spec to suit this particular case.

I was conditionally compiling my code to that I have unfinished code only in
DEBUG mode.

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Jon Skeet [C# MVP] - 31 Mar 2006 17:32 GMT

> >> Iam sure there are more situations where similar things might happen.
> >> imho
[quoted text clipped - 13 lines]
> #endif
> #endif

That's my point - it has to validate *some* preprocessor directives, so
I don't see that it's a big problem to have to validate all
preprocessor directives.

> > #if isn't meant to be a way of commenting out code - do that with
> > multi-line comments, or get the IDE to add // at the start of each line
[quoted text clipped - 6 lines]
> I was conditionally compiling my code to that I have unfinished code only in
> DEBUG mode.

But the code you "conditioned" out was invalid - if it had been valid
code, it would have been okay. The only exception to that is the
comment with the preprocessor directive in it - it would be reasonable
to expect that to work.

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compiler error with preprocessing directives

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