When to use preprocessor directives in .net?

Question

I think this is a simple question so I assume I'm missing something obvious. I don't really ever use preprocessor directives but I was looking at someone's code which did and thought it was something I should be familiar with.

So I looked at the msdn example here it has the code:

#define DEBUG // ... #if DEBUG     Console.WriteLine("Debug version"); #endif 

My two questions are:

• in the example above why do they define DEBUG? I was under the impression that was set if you compile in debug v. release mode?
• looking at the other example which has #define MYTEST and then writes to the console dependent on if it 'defined', but how does this differ from just using a variable? What am I missing here?

Answer 1

I would actually recommend using the Conditional Attribute instead of inline #if statements.

[Conditional("DEBUG")] private void DeleteTempProcessFiles() { } 

Not only is this cleaner and easier to read since you don't end up having #if, #else within your code. This style is less prone to errors either during normal code edits and well as logic flow errors.

Answer 2

Generally, the optional/conditional compilation symbols will be provided by the build script. It is pretty rare to see #define, except for very debug code (if you see what I mean).

Re using a variable; I often use such conditions to handle code that must run on different runtimes (mono, cf, silverlight, etc). A variable cannot suffice because the code cannot be compiled against the wrong platform (missing types/methods etc).

In the example presented I would probably just have used Debug.WriteLine; since this is decorated with [Conditional("DEBUG")], all calls to it are automatically removed if DEBUG is not defined at build.