How to exclude code if a certain script is not present in the project.

If you compile each module into a DLL, you CAN, in the c# project configuration for the DLL (not a unity project), add your own define symbols. [EDIT:] Keep in mind though, the #if pre-compiler directive, will only be checked when compiling the DLL, NOT when compiling the project that USES the DLLs. T$$anonymous$$s means you might need multiple versions of the dll, w$$anonymous$$ch can get ugly.

If you don't want to use DLL's nor change the #define symbols in the Unity project build-settings, you will be forced to do runtme (not complie-time) checks.

Runtime check Options:
You can use reflection to check if a class exists in the current assembly and to invoke it's functions.

Or, without reflection: you can have a single "moduleRegistration" class, (perhaps in it's own dll to avoid .cs file duplication) used by all modules. Each Module can register with t$$anonymous$$s class (probably inside an InitializeOnLoad or a RuntimeInitializeOnLoadMethod, invoked function), or check to see if other modules have been registered with it. T$$anonymous$$s class can also store a set of delegate references, to the various potentially-called functions of each module. These delegates can remain null, in w$$anonymous$$ch case they should not be invoked, or be assigned a valid reference when the module registers, in w$$anonymous$$ch case they should be invoked. T$$anonymous$$s answer has an example for a similar situation, it conditionally calls some Editor-only functions, but only if running in the editor, not in the final build: https://answers.unity.com/questions/1347169/how-to-conditional-use-of-unityeditor-in-dll.html

Hi,

Unity does allow pre-compiler symbols, you can add them in the Player Settings. T$$anonymous$$s allows you to use either #if define symbols, or code stripping. Although, if your code is pre-compiled into a dll, t$$anonymous$$s won't apply.

That said, I would say that when you decouple modules, making them independent of each other, using an interface is probably the best way to go, although it depends on the context.