Rafael,
The short answer is you cannot.
The long answer is use a Separated Interface Pattern:
http://www.martinfowler.com/eaaCatalog/separatedInterface.html
The first project defines a class and an interface that the class expects a
different class to implement. In the second project define a class that
implements the interface.
' Project 1
Public Interface INeedThis
Sub DoSomeWork()
End Interface
Public Class Class1
Public Sub DoSomething(ByVal needThis As INeedThis)
needThis.DoSomeWork()
End Sub
End Class
' Project 2
' references project 1
Public Class NeedThis
Implements INeedThis
Public Sub DoSomeWork() Implements INeedThis.DoSomeWork
End Sub
End Class
Notice that Project 2 references Project 1, but Project 1 does not reference
Project 2. If you want you can put the interface in its own project if you
have a number of interfaces.
Project 1 will need to be a class library. Project 2 can be a class library
or an Application.
Hope this helps
Jay