Another good read and valuable reference on HOW to test. Thanks!

How do you decide WHAT to test? Personally, I've been a fan for years of Pragmatic Unit Testing (happens to be in Java using JUnit, but pretty easily relatable to C#).

I especially appreciate the FIRST, Right-BICEP, and CORRECT acronyms.

Fast

Isolated

Repeatable

Self-validating

Timely

Are the Results Right?

Check Boundary Conditions

Check Inverse Relationships

Cross-Check Using Other Means

Force Error Conditions

Check Performance Characteristics

Conformance

Ordering

Range

Reference

Existence

Cardinality

Time

https://pragprog.com/titles/utj2/pragmatic-unit-testing-in-java-8-with-junit/