Enquiry about Unit Test Concept in .NET Custom Environment

I have an environment that would consume my custom DLL.

So, what I do are:

1. Create C# class library project in Visual Studio.


2. Add references of Environment DLLs to my class library project.


3. I call certain function or API from the Environment DLLs in my class library project.


4. I build my class library project and deploy it into the environment.


5. Setup the environment to call my DLL.


6. If I am required to debug my custom DLL, I need to attach process to my environment process.

All are good in development and debugging. But, I am not sure about the unit testing.

How to do the unit testing of my custom DLL since my custom DLL is really dependable with the environment DLL?

Is there any Unit Testing Library that I could use to perform Unit test similar like NUnit?

Thanks for your attention.