Can I have global preprocessor definitions in C# library?

Question

In C# you can have conditional compilation by using macros similar to the C/C++ syntax. This would enable the following to happen:

#define MYMACRO
....
#if MYMACRO
//some C# code logic 
#else
//some other C# code logic

I need to define some macros in a dedicated file in a C# library project, and I need these macros to be visible inside the entire library, once defined. The problem is that the above code works only for a single file.

Another way I know to work around this, is to add the macros to the build command. This would take care of defining the macros for the entire .dll and I will have the #if - #else checks working wherever I want inside the library. The issues with this approach is that I want to be able to maintain the macros easily. Having them in a file inside the project will be perfect. I'd like to have some comments inside too, so that I will know what each macro is doing. This will not be applicable if I have to pass the macros as build parameters. Another reason is being able to turn a macro on/off by simply commenting it and examining the behavior.

Is there a decent way to achieve my requirement? I'd prefer not to deal with any build automation tools like MSBuild, NAnt or anything like this, still if no other way is possible I'd appreciate an advice which one you consider a better choice.

Answer 1

You #define them for an entire project with Project + Properties, Build tab, "Conditional compilation symbols" setting. This sets the <DefineConstants> element in the project file. You override this property with msbuild by giving it the /property:DefineConstants="MYMACRO" command line option.

Answer 2

I'd also advise putting the macros in the project settings (csproj file) as @Hans Passant suggests.

If you need the defines documented, you could add a documentation file to the solution explaining what the settings mean.

If there aren't too many variants, you could define a new project configuration for each one. That will allow you to pre-configure the necessary list of #defines for each variant, and then simply switch between them from the configuration combo box in the toolbar. If you want to temporarily disable one option, you could duplicate the current configuration and remove the #define, then delete the config later when you've tested it.

The next option I can suggest to make it "easier" (by combining the settings and docs into a single file as you've suggested) would be to use a simple text file (settings + comments) to configure the project, and spend 15 minutes writing a quick c# app to read this file and write the settings it contains into the .csproj file - it's just XML so should be a trivial app to write. You'd be able to easily tweak this file and run your updater app to chnage the project settings. If it's something you will do often, spend 30 minutes on it and add a UI with checkboxes to choose the settings more easily.

The concept you're describing sounds rather odd, though. The point of a library is usually that you have one standardised lump of code that can be shared by many clients, so changing these sort of defines to reconfigure the whole library a lot is not something that I'd expect to need to do very often. Perhaps you have good reasons, but it may be worth reviewing why you need to solve this #define problem.

(e.g. If you have lots of customers who need different variants of the "library", the best approach will be to use configurations (described above) to allow you to build all needed variants in a batch build. If you are just trying out lots of different algorithms/techniques then can you redesign chunks of the library so that you can restrict the impact of most #defines to just to a single .cs file so they no longer need to be global? Perhaps the library shouldn't be in a single dll, or a plug-in architecture is needed to allow you to pick and choose the "modules" that are included within the library)