Best practices with Unit testing on Third Party software API's (AutoCAD)

The first step is to triage your code for parts which need AutoCAD and parts which are really independent. Create unit tests for the independent parts as you usually would.

For the other parts, you need mockups which behave like AutoCAD. Make them as simple as possible (for example, just return the correct answers in the methods without doing any calculations). Now, you need several sets of classes:

1. A set of interfaces which your code uses to achieve something (for example, load a drawing).

2. A set of implementations for said set of interfaces which call the AutoCAD dlls.

3. A set of classes which try the implementations within the context of AutoCAD. Just create a small UI with a couple of buttons where you can run this code. It is used to reassure yourself that your mockups do the right thing. Log method parameters and results to some file so you can try how AutoCAD responds. If a mockup breaks, you can use this code to verify what AutoCAD is doing and you can use it as a reference when developing the mockups.

4. When you know how AutoCAD responds, create the mockups. In your tests, create them with the desired results (and errors, so you can test error handling, too). So when you have boolean loadDrawing(File filename), create a mockup which returns true for the filename exists.dxf and false for anything else.

5. Use a factory or DI to tell your application code which implementation to use. I tend to have a big global config class with a lot of public fields where I simply store the objects to use. I can set this up in the beginning, it's fast, it's easy to understand. If you need to create objects at runtime, then put factories in the config class which generate the objects for you, so you can swap them out.