Why one project (exe) does not see the namespace of another project (dll) in the same solution?

Question

Why one project (exe) does not see the namespace of another project (dll) in the same solution?

Answer 1

You need to add a reference from the using project to the DLL first.

Select Project|Add Reference, Projects Tab.

Answer 2

A "solution" in Visual Studio is a collection of projects. Each project is independent of all the others. The solution is just a convenient way of organizing projects and opening them all together.

If one project is going to use the public objects defined in another project, then it must be compiled with a reference to the other project. This is true whether the projects are part of a single solution or not.

To signal to Visual Studio that the EXE must be compiled with a reference to the DLL, you must add the DLL to the EXE's list of references in the Solution Explorer.