“Unable to attach to process. A debugger is already attached.” (VS 2010, Classic ASP)
A fellow developer was using Visual Studio 2010 SP1 to debug a classic ASP application. He attempted to attach to the IIS process, w3wp.exe and received the error message: “Unable to attach to process. A debugger is already attached.”
He is running Windows 7, 32 bit. (But I reproduced the error on Win7 64 bit.)
He is using IIS 7.5.
Yesterday he installed IE 10 32 bit. (Yesterday I installed IE 10 64 bit.)
We looked in Process Explorer but no debugger was running that we could see.
We tried changing the “Attach to” type in the “Attach to Process” dialog, but it had no effect.
We tried all sorts of things but nothing made any difference.
I found this forum article: Visual Studio 2010 debugger already attached classic ASP
I tried debugging the same ASP site by attaching to the IIS process, using VS 2012 and it worked perfectly.
Use VS 2012 or revert from IE 10 for Win7 to IE 9.
My fellow developer is installing VS 2012. (I don’t know why he didn’t do it already – heck, it’s worth it just for that cool Dark Theme.)
I hope that helps anyone hitting this issue.