Find error "Namespace or type specified in the Imports" when refer to another DLL

Dear all,

I'm developing on VB.NET .Net framework 4.0

And I have build a class library and call successfully from some test project

But when I create a Command Line Console Application and ref to that dll, I find the following error when I compile the console application

warning BC40056: Namespace or type specified in the Imports 'MyTestDll.MySession' doesn't contain any public member or cannot be found. Make sure the namespace or the type is defined and contains at least one public member. Make sure the imported element name doesn't use any aliases.

And seem it cannot reference to the dll successfully

Do you know any possible cause of such error?