While a plug-in is loaded in Softimage, the plug-in DLL is in use and you cannot recompile it (LINK will report a "cannot open file" error). If you need to recompile the plug-in, you'll have to do one of the following:
Unload a plug-in the plug-in, and then load it again after you recompile.
Disable the Cached option for the plug-in. When you disable caching, Softimage only holds on to the DLL during the invocation of a callback. Disabling caching may impact performance. Also, you cannot disable caching of some types of plug-ins, such as graphics sequencer plug-ins.
Use the Debug
Apply Code Changes command in Visual Studio.
To use Apply Code Changes, you need the /INCREMENTAL linker option (Configuration Properties > Linker > General > Enable Increment Linking).