Q:

Why would Visual Studio 2010 not allow debug mode? 

I have a website that was MVC 1.0 in VS 2008.   When in MVC 1.0/VS2008, I had no issues and could debug.   I upgraded and now it is MVC 2.0 in VS2010.   The site builds but will not allow me to debug.   It only shows me the little green arrow (start debugging) when in release mode.  When I put it in debug mode, the green arrow is  grayed out.   What could cause this?   Web.config?  Upgrade bug?   Please let me know if you have any ideas. 

A:

The problem was setting a consistent platform target for all configurations.   I had the release configuration set to Any CPU and the debug to x86.   After changing the debug configuration to Any CPU, it allowed me to debug.