How to access Unity's preprocessor directives from a custom C# class library project?

Pre processor directives as the name suggests are interpreted by the preprocessor of the compiler. Therefore once the code is compiled they do not exist anymore. The pre processor literally removes things that are in a conditional code block from the source files before they are compiled. Since you compiled your DLL already there is no way to change anything in the code. You would need to compile seperate assemblies for the different versions you want to support. You have to set the defines manually in your Visual Studio project to control which version you are compiling.

You can use the Conditional attribute. https://msdn.microsoft.com/en-us/library/system.diagnostics.conditionalattribute(v=vs.110).aspx
e.g.

 [Conditional("CONDITION1")]
 public static void Method1(int x){..condition1 specific stuff..}

If the pragma, CONDITION1, is defined, the function will be called, if not, it will be skipped. (Careful, even it's parameters will NOT be evaluated)

It looks like, this even works in a class library DLL! The attribute's pragma is "applied/evaluated" when compiling the CLIENT executable, NOT your class library dll. https://stackoverflow.com/questions/26634744/conditional-compilation-in-referenced-assemblies

So, you can write a function for each version, applying to each, a conditional attribute with its own unique "Version" pragma. Then, in your public/primary function, call them all: one will actually execute (assuming you have (only) one of the unique pragmas defined in the client project).