When you want to debug an assembly that has been deployed to the GAC (e.g. an assembly that is part of a Sharepoint solution), the easiest way is to change one of the settings in Visual Studio (2005, 2008 or 2010):
- Open the Visual Studio Options dialog (Tools -> Options)
- Select Debugging in the left pane
- In the right pane, locate the Enable Just My Code (Managed only) node
- Uncheck all the childnodes under the Enable Just My Code (Managed only) node
- Uncheck the Enable Just My Code (Managed only) node
- Select OK
