I guess you are working with .Net on a x64 OS.
If you are running a x64 OS you should take care of your compiler settings.
If you compile your project with the for 'Any CPU' settings - you would need the 64-bit versions of the native bass.dll !
Or set your build to target the 'x86' platform!
See the docs for detailed info - or here:http://www.bass.radio42.com/help/html/b8b8a713-7af4-465e-a612-1acd769d4639.htm#32vs64
In addition you should add the 'Bass.Net.dll' in your project references and not bass.dll - as bass.dll is an unmanaged lib.
You guessed correctly that I'm working with .NET on a x64 OS.
However, not only is the .NET version referenced but the project is set to build for x86 and the exception is thrown.