Cake: Support for #define

[jnm2]

The Cake script

```c#

define SOME_SYMBOL

```

Causes:

error CS1032: Cannot define/undefine preprocessor symbols after first token in file

(Version 0.21.1+Branch.main.Sha.9ff85bb53ea1890fabeb709d38ba79d3bf5cb16f)

Is there a technical reason this can't work or am I the first to try?

[bjorkstromm]

@jnm2 there are technical reasons why it does't currently work. This is due to code generation done for aliases behind the scenes. Have a look at this gist showing the difference between plain .cake snd what's passed to Roslyn.

That said, there's no technical reason why we couldn't get it to work. But first, what is your use case?

[jnm2]

Cool! My use case is that I prefer #define SOME_SYMBOL to use with #if SYMBOL rather than const bool someSymbol = true; with if (someSymbol). Purely style.

Shouldn't be too hard to hoist #defines to the top then?

[jnm2]

I have a real use case now: avoiding warning CS0162: Unreachable code detected when I use a const bool instead of preprocessor.

Would you take a PR for this?

[devlead]

If you like to submit a PR we'll review it.

Tried this in csi.exe

#define DEBUG
#if (DEBUG)
Console.WriteLine("hello");
#endif

it works there, so should work in Cake too, but it needs to come first in the file so these defines would need to be moved to top of script like using statements much similar to what I propose in #1854 .

[jnm2]

much similar to what I propose

If it's fresh in your mind, would you do it? That would be perhaps more efficient.

[bjorkstromm]

To spin this even further I’d say we should support defines via cmdline/config/environment (similar to DefineConstants in MSBuild). Also we should take #ifdef into account when preprocessing. This would open up possibility to conditionally load another file or conditionally install tool/addin.

[devlead]

If it's fresh in your mind, would you do it? That would be perhaps more efficient.
@jnm2 even if that were true, it still takes some effort to do it and time is limited 😎 But if no one takes it on I might take a stab at it when I can, have a few outstanding PRs awaiting @cake-build/cake-team review and could potentially look at this post that. But other things might also take precedence.

@mholo65
So tested plain scripting and csi.exe

#if (FOO)
#r ".\Newtonsoft.Json.dll"
#endif


Console.WriteLine(
    Newtonsoft.Json.JsonConvert.SerializeObject(
            new {
                Hello = "World"
            }
        )
    );

and it works regardless if i define FOO or not, so would seem as #r is processed at same scope as #if/#define
and gives error CS0103: The name 'Newtonsoft' does not exist in the current context if i remove the line.

I propose we first align with what Roslyn / csi.exe does as that's likely a fairly simple fix and later on look if we can / should improve it in any way.

I do like the DefineConstants as cmdline/config/environment idea though - essentially just a cake config for constants, makes a lot of sense.

[devlead]

Submitted PR #1858 as a proposed fixed for this issue.

[jnm2]

Thank you @devlead!