in VB.NET how do i get a dll instance to access a method from the main form class?

Hello!

I have two projects.  One is a standard vb.net windows application and the second is a DLL with one function that returns a simple string.
Public Class DLLClass
     public function TestDLL () as string
        return "Dude, I'm here"
     end function
End Class

In my windows app, i late bind this DLL and invoke its method:
        Dim assAssembly As Reflection.Assembly = Nothing
        Dim typeClassType As Type = Nothing
        Dim strReturn As String = ""

         assAssembly = Reflection.Assembly.LoadFrom(strApplicationPath & strDLLName)
         typeClassType = assAssembly.GetType("DLLClass.DLLClass")
         objClassInstance = Activator.CreateInstance(typeClassType)

         MessageBox.Show(objClassInstance.TestDLL)

This works fine.  But what if I want the DLL to access a method from the main application?  Can someone please show me an example of how that is done?  Again, keep in mind that the dll is late bound.
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