"The breakpoint will not currently be hit. The source code is different from the original version." What does this mean?

As it says, the "source code is different from the original version".

Right click on the project folder inside the solution explorer and choose to Clean. Build a new version of the project and the breakpoint will work again!

If you have unchecked the DLL project in the Debug build configuration, your new code will never be built!

Go to Build --> Configuration Manager ... (in VS2010) and check if the project with the code you're trying to debug is checked for the current build configuration.

For me it was while working on a WebSite project. After cleaning up these temp folders I got the proper compiler errors back:

• C:\Documents and Settings\%username%\AppData\Local\Temp\Temporary ASP.NET Files
• C:\windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files

I finally resolved the problem when I discovered that a class file I had intentionally moved into a subfolder, somehow reappeared in the root folder. VS was using that one while I was editing the other.