How do code coverage tools work?

Question

How do code coverage tools like NCover know what parts of the code were executed and what parts were not?

Answer 1

From this source:

NCover uses the .NET Framework profiler API to monitor an application's execution. When a method is loaded by the CLR, NCover retrieves the IL and replaces it with instrumented IL code

So in short, it hooks itself into the just-in-time compilation.

Not all tools work the same way though. Other tools work by modifying the bytecode of your application after the code has been compiled.