The 'using' directive imports namespaces into the current file - in other words, it enables you to uses classes which are defined in different namespaces (either in the current assembly or a different assembly) without the need to qualify them with the namespace name.

To use classes in a different assembly (CILib1) from the current assembly (ClassLibraryApp1) you first need to add a reference to CILib1.dll to the current project which you say you've done.

The .cs files which make up CILib1.dll are immaterial once the library has been compiled. It just becomes one big collection of namespaces and classes.

So if, for example, CILib1.dll contains a class called bye2009 which is in a namespace called CILib1, then to use that from ClassLibraryApp1 you have two choices:

1. Add this using directive:

using CILib1;

You can then refer to bye2009 without fully qualifying it:

bye2009 bye = new bye2009();


2. Don't bother with the using directive but fully qualify the class name instead:

CILib1.bye2009 bye = new CILib1.bye2009();


It's also possible to provide an alias for a class with the 'using' directive. For example:

using Bye = CILib1.bye2009;

You can then do:

Bye bye = new Bye();