Pass VB6 object to .NET object via interop?

Question

I have a VB6 app that shows a .NET DLL form via interop.

I would like an event in the .NET DLL to cause a form in the VB6 app to be shown.

My idea is to have the VB6 app pass a reference to a form to the .NET DLL. Eg:

[VB6]
Dim objNetDllObject As New NetDllObject
objNetDllObject.PassVb6Form(MyForm)
objNetDllObject.ShowForm

[C#]
object Vb6Form; 
private void PassVb6Form(object form) { Vb6Form = form; }
private void button1_Click(object sender, EventArgs e) { Vb6Form.Show(); }

Will this work?

I've read elsewhere that sending objects across a 'process boundary' can cause problems. Is this correct?

Answer 1

One route would be to define a COM Interface in .NET:

<System.Runtime.InteropServices.GuidAttribute("0896D946-8A8B-4E7D-9D0D-BB29A52B5D08"), _
InterfaceType(ComInterfaceType.InterfaceIsIDispatch)> _
Public Interface IEventHandler
    Sub OnEvent(ByRef sender As Object, ByRef e As Object)
End Interface

Implement this interface in VB6

Implements MyInterop.IEventHandler

Private Sub IEventHandler_OnEvent(ByRef sender As Variant, ByRef e As Variant)
    Dim id
    id = e.Entity.Id
    ' As long as e is COM Visible (not necessarily COM registered, this will work)
End Sub

and then have a Registrar in .NET with a static collection of IEventHandlers:

<ComClass(ComRegistrar.ClassId, ComRegistrar.InterfaceId, ComRegistrar.EventsId>
Public Class ComRegistrar

   Private Shared ReadOnly _eventHandlers As New Dictionary(Of String, List(Of IEventHandler))


   ' This is called by .NET code to fire events to VB6
   Public Shared Sub FireEvent(ByVal eventName As String, ByVal sender As Object, ByVal e As Object)
        For Each eventHandler In _eventHandlers(eventName)
                eventHandler.OnEvent(sender, e)
        Next
   End Sub

   Public Sub RegisterHandler(ByVal eventName As String, ByVal handler As IEventHandler)
        Dim handlers as List(Of IEventHandler)
        If Not _eventHandlers.TryGetValue(eventName, handlers)
             handlers = New List(Of IEventHandler)
             _eventHandlers(eventName) = handlers
        End If
        handlers.Add(handler)
   End Sub

End Class

Your .NET code would call FireEvent and if VB6 had previously called RegisterHandler, your VB6 IEventHandler would get called.