Question

I have a VB6 project that currently Compiles even though I am accessing a Property that does not exist.

The code looks a bit like this:

Public vizSvrEmp As VizualServer.Employees
Set vizSvrEmp = New VizualServer.Employees

Fn = FreeFile
Open vizInfo.Root & "FILE.DAT" For Random As #Fn Len = RecLen
Do While Not EOF(Fn)
    Get #Fn, , ClkRecord
    With vizSvrEmp
        Index = .Add(ClkRecord.No)
        .NotAvailable(Index) = ClkRecord.NotAvailable
        .Bananas(Index) = ClkRecord.Start
        'Plus lots more properties
    End With
Loop

The Bananas property does not exist in the object yet it still compiles.
My vizSvrEmp Object is a .NET COM Interop DLL and is early bound and if I type the dot in I get the Intellisense correctly (which does not show Bananas)

I tried removing the With but that behaves the same

How can I make sure these errors are picked up by the compiler?

Answer 1

I know you’ve got this sorted out with the help of Hans but just for completeness, the alternative to using ClassInterface(ClassInterfaceType.AutoDual) is to use ClassInterface(ClassInterfaceType.None) and then Implement an explicit interface that is decorated with InterfaceType(ComInterfaceType.InterfaceIsDual)>.

It is more work, but gives you complete control over the interface GUIDs. The AutoDual will auto generate unique GUIDs for the interfaces when you compile, which is time saving but you don’t have control over them.

In use, this would look something like this:

<ComVisible(True), _
Guid(Guids.IEmployeeGuid), _
InterfaceType(ComInterfaceType.InterfaceIsDual)> _
Public Interface IEmployee 

   <DispIdAttribute(1)> _
   ReadOnly Property FirstName() As String

   <DispIdAttribute(2)> _
   ReadOnly Property LastName() As String

   <DispIdAttribute(3)> _
   Function EtcEtc(ByVal arg As String) As Boolean

End Interface


<ComVisible(True), _
Guid(Guids.EmployeeGuid), _
ClassInterface(ClassInterfaceType.None)> _
Public NotInheritable Class Employee
   Implements IEmployee 

   Public ReadOnly Property FirstName() As String Implements IEmployee.FirstName
      Get
         Return "Santa"
      End Get
   End Function

   'etc, etc

End Class

Note how the GUIDs are declared. I find creating a helper class to consolidate the GUIDs and provide Intellisense works out well:

Friend Class Guids
   Public Const AssemblyGuid As String = "BEFFC920-75D2-4e59-BE49-531EEAE35534"   
   Public Const IEmployeeGuid As String = "EF0FF26B-29EB-4d0a-A7E1-687370C58F3C"
   Public Const EmployeeGuid As String = "DE01FFF0-F9CB-42a9-8EC3-4967B451DE40"
End Class

Finally, I use these at the assembly level:

'The following GUID is for the ID of the typelib if this project is exposed to COM
<Assembly: Guid(Guids.AssemblyGuid)> 

'NOTE:  The following attribute explicitly hides the classes, methods, etc in 
'        this assembly from being exported to a TypeLib.  We can then explicitly 
'        expose just the ones we need to on a case-by-case basis.
<Assembly: ComVisible(False)> 
<Assembly: ClassInterface(ClassInterfaceType.None)>