Quote needed: Preprocessor usage is bad OO practice

Henry Spencer wrote a paper called #ifdef Considered Harmful.

Also, Bjarne Stroustrup himself, in the chapter 18 of his book The Design and Evolution of C++, frowns on the use of preprocessor and wishes to eliminate it completely. However, Stroustrup also recognizes the necessity for #ifdef directive and the conditional compilation and goes on to illustrate that there is no good alternative for it in C++.

Finally, Pete Goodliffe, in chapter 13 of his book Code Craft: The Practice of Writing Excellent Code, gives an example how, even when used for its original purpose, #ifdef can make a mess out of your code.

Hope this helps. However, if your co-workers won't listen to reasonable arguments in the first place, I doubt book quotes will help convince them ;)

Preprocessor directives in C# have very clearly defined and practical uses cases. The ones you're specifically talking about, called conditional directives, are used to control which parts of the code are compiled and which aren't.

There is a very important difference between not compiling parts of code and controlling how your object graph is wired via IoC. Let's look at a real-world example: XNA. When you're developing XNA games that you plan to deploy on both Windows and XBox 360, your solution will typically have at least two platforms that you can switch between, in your IDE. There will be several differences between them, but one of those differences will be that the XBox 360 platform will define a conditional symbol XBOX360 which you can use in your source code with a following idiom:

#if (XBOX360)
// some XBOX360-specific code here
#else
// some Windows-specific code here
#endif

You could, of course, factor out these differences using a Strategy design pattern and control via IoC which one gets instantiated, but the conditional compilation offers at least three major advantages:

1. You don't ship code you don't need.
2. You can see the differences between platform-specific code for both platforms in the rightful context of that code.
3. There's no indirection overhead. The appropriate code is compiled, the other isn't and that's it.