Breakpoints not being hit in JetBrains Rider?

For .Net Framework projects:

This can happen if Rider attached the debugger to a different process than the process that IIS is using.

• Open Run on Rider menu and click on "Attach to process" or ctrl+alt+F5
• Type w3wp, this will find all the processes IIS is running on. If you found multiple processes you can hover on them to see which AppPool is related to that process.
• Click on the process to attach debugger to it.
• The breakpoint should be hit now.

And make sure you are using the URL that Rider provides you when you run your project (http://localhost:XXXX), because the URL that Rider uses might be different than the Host Name you set for your IIS.

One more caveat: I started using Rider again after an absence, I was pressing the "play" (triangle) button rather than the button with a picture of a bug.

(D'oh!)

I've found what was causing it.

As of Rider 2018.1.3, the solution you're running must be on the same disk as the copy of Rider you're using.

This issue is referenced on JetBrains support here.

You can temporarily solve this by either moving your solution to the same drive, or by reinstalling Rider.