Then I launched AutoCAD 2012, loaded my arx solution in VS2010 and from the Tools menu, Attach to Process… I tried to select Attach to Managed v3.5 but it does not exist! All I see is Managed (v2.0, v1.1, v1.0) code or Managed (v4.0) code.
Am I the only one having this much fun with debugging? After sifting the internet I found no similar issues.
"Attach to process" does not work correctly when it comes to debugging AutoCAD plug-ins. This is to do with the use the fibers in AutoCAD.
Can you please try setting AutoCAD exe path as the external exe to be launched in the project properties and then use F5 to start AutoCAD ? The .arx can then be apploaded and the breakpoint should be hit when your custom command is executed.
I have attached a screenshot of the project setting.