VS 2015 Update 1 - Claiming I am debugging a release build
I noticed the answers here are incomplete, I was having the same issue and it was solved by opening the project properties and under the build tab and debug configuration unchecking "optimize code". You also should check the configuration manager as mentioned above to make sure that is also sound. The answer came from this post and they should get the credit: VS2015 Project no longer runs in debug mode
Thanks,
As mentioned by @romanoza, Microsoft updated the (now missing) Microsoft Connect bug report, (previously located here, in case you are able to find an archive somewhere) with the following information:
Uncheck the setting Debug -> Options -> Suppress JIT optimization on module load (Managed only)
This is the workaround. They go on to say later:
We recommend folks leaving it unchecked as having it unchecked will improve both performance and the behavior of just my code in specific scenarios.
Lastly, the acknowledgement:
It is a bug that it doesn't work with that setting enabled and we're working on a fix for that situation in case some customers still want to debug with that setting turned on.
Update: Based on the comments, it appears that the box is now unchecked by default for some developers, and that checking it can fix the exact same problem in some cases. Very strange.
I solved the problem setting the configuration to Debug in the Configuration Manger window as suggested in this answer.
The word from Microsoft is that this is a known issue (it originally went to the Debugger team, but was determined it was a build issue, and is now in the Project system team's hands. There are other bugs open on this issue, and it's rated Priority 1, so should be on track for the next update. Though as would be expected, no promises can be made as to when it will be released (or what is actually in the update).
So. It's known and is being worked on. At least turning off the “Enable Just My Code” in the Debugging General Options seems to be a work around for now.