custom enumerator, com interop, and vb6 client

custom enumerator, com interop, and vb6 client

• Wayner7

  Hot Dawg, Peter! This was what I needed to get over the hump. It turns out that I was darn close on my own, with your earlier posts. The problem that I had was in the custom enumerator class, I was returning a DAO object, and not one of my abstract objects. Looking at your code gave me the clue, and it's now working!

  Thanks ever so much!

  Sean

  
  

• Aldrenn

  Well, after posting a while ago, I discovered that the syntax in question is intended to decorate the return type of the GetEnumerator property in VB, so it ought to look something like:

  Public Function GetEnumerator() As <System.Runtime.InteropServices.MarshalAsAttribute(UnmanagedType.CustomMarshaler, MarshalType = "System.Runtime.Interop-Services.CustomMarshalers.EnumeratorToEnumVariantMarshaler,CustomMarshalers, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")> System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator

  However, it's now complaining that the

  Name 'MarshalType' is not declared.

  I'm guessing that C# (which I admittedly know little about) handles named parameters while VB does not. I thought that I rememberd VB6 doing this, but I may be thinking of something else.

  I still don't understand all of the implications of the MarshalAsAttribute decoration - I see that it has lots of parameters, but so far, I've not found a construction that compiles.

  I must be getting closer, but I still feel far, far away.

  Sean

  
  

• mjoc69

  Hi ,

  Based on my understanding, you wanted to expose the .NET class to COM. So that the COM client(VB6 application) can access to the class via foreach.
  If I misunderstood, please feel free let me know.

  I think you may try to decorate the GetEnumerator with the Marshal attribute.
  e.g.
  [DispId(-4)]
  [return:
  MarshalAs(UnmanagedType.CustomMarshaler,MarshalType="System.Runtime.Interop-S
  ervices.CustomMarshalers.EnumeratorToEnumVariantMarshaler,CustomMarshalers,
  Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
  public IDictionaryEnumerator GetEnumerator()

  Also as you said, we need to expose the type we hope the current to return too.
  What do you mean by "ABXDB.Index"
  Do you mean the type DAO.Index in VB6
  If so, I think you may try add reference to the DAO COM Library, so that you will get a .NET wrapper for the type. Or you can define one yourself.

  Here is a link for your reference.
  http://groups.google.com/group/microsoft.public.dotnet.framework.interop/browse_frm/thread/7ca14c08352adb69/94ca86ddae382ead lnk=st&q=%22v-phuang%22+IEnumerable+for+each+vbscript&rnum=1&hl=en#94ca86ddae382ead

  
  If you still have any concern, please feel free to post here.

  Best regards,
  Peter Huang

  
  
  

• LiLo

  Digging deeper, I discovered that named parameters in attributes use :=, so my declaration needed to look like

  <System.Runtime.InteropServices.DispId(-4)> _

  Public Function GetEnumerator() As <System.Runtime.InteropServices.MarshalAsAttribute(UnmanagedType.CustomMarshaler, MarshalType:="System.Runtime.Interop­Services.CustomMarshalers.EnumeratorToEnumVariantMarshaler,CustomMarshalers, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")> System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator

  Return New CTableDefsEnumerator(moInternal_DAO_TableDefs)

  End Function

  By the way, I discovered this by reading through the O'Reilly VB.Net 2005 in a Nutshell. I LOVE the O'Reilly Nutshell books - I've got a half dozen of them for various topics. If you don't use them, you should!

  So, it now compiles cleanly, but the GetEnumerator routine is never called. I've set a breakpoint on the on the return statement and on the appropriate routines in the CTableDefsEnumerator class and they are never hit. Something is still quite wrong.

      Sean

   

  
  

• windiswebwerkstatt

  Hi,

  I am sorry for that.
  I am considered the issue as a ComInterop issue and I did not notice this is a VB.NET forum.
  Anyway, here goes the code in VB.NET version for your reference.
  '[.NET class] which will expose a customer class TestObj.
  Imports System.Runtime.InteropServices
  '<ClassInterface(ClassInterfaceType.AutoDual)> _
  Public Class TestObj
  Dim _name As String
  Public Sub New(ByVal index As Integer)
  _name = "Object " & index.ToString() & ": " & Rnd().ToString()
  End Sub
  Public Property Name() As String
  Get
  Return _name
  End Get
  Set(ByVal value As String)
  _name = value
  End Set
  End Property
  End Class
  '<ClassInterface(ClassInterfaceType.AutoDual)> _
  Public Class TestObjCol
  Implements ICollection, IEnumerator
  Dim _objs As ArrayList = New ArrayList()
  Dim ide As IEnumerator
  Public Sub New()
  For i As Integer = 1 To 10
  _objs.Add(New TestObj(i))
  Next
  End Sub
  Public Sub CopyTo(ByVal array As System.Array, ByVal index As Integer) Implements System.Collections.ICollection.CopyTo
  _objs.CopyTo(array, index)
  End Sub

  Public ReadOnly Property Count() As Integer Implements System.Collections.ICollection.Count
  Get
  Return _objs.Count
  End Get
  End Property

  Public ReadOnly Property IsSynchronized() As Boolean Implements System.Collections.ICollection.IsSynchronized
  Get
  Return _objs.IsSynchronized
  End Get
  End Property

  Public ReadOnly Property SyncRoot() As Object Implements System.Collections.ICollection.SyncRoot
  Get
  Return _objs.SyncRoot
  End Get
  End Property
  <DispId(-4)> _
  Public Function GetEnumerator() As <MarshalAs(UnmanagedType.CustomMarshaler, MarshalType:="System.Runtime.InteropServices.CustomMarshalers.EnumeratorToEnumVariantMarshaler,CustomMarshalers,Version=2.0.0.0, Culture=Neutral, PublicKeyToken=b03f5f7f11d50a3a")> System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator
  Debug.WriteLine("GetEnumerator")
  ide = _objs.GetEnumerator()
  Dim i As IEnumerator = Me
  Return i
  End Function

  Public ReadOnly Property Current() As Object Implements System.Collections.IEnumerator.Current
  Get
  Debug.WriteLine(ide.Current.ToString())
  Dim o As TestObj = ide.Current
  Return o
  End Get
  End Property

  Public Function MoveNext() As Boolean Implements System.Collections.IEnumerator.MoveNext
  Return ide.MoveNext()
  End Function

  Public Sub Reset() Implements System.Collections.IEnumerator.Reset
  ide.Reset()
  End Sub
  End Class

  
  I call it from VB6 via the code below.

  Private Sub Command1_Click()
  Dim obj As Enumerators2COM.TestObj
  Dim objs As New Enumerators2COM.TestObjCol
  For Each obj In objs
  Debug.Print obj.Name
  Next
  End Sub

  If you still have any concern, please feel free to post here.

  Best regards,
  Peter Huang

  
  
  

• Matt Ahlgren

  See the next post for updates:

  ----------------------------------------------

  Thanks for your reply, Peter.

  I should have mentioned that my wrapper classes are written in VB.Net and I'm having some trouble converting the decoration to a syntax that VB likes. Specifically, I can't seem to find a construct for

                  [return:
  MarshalAs(UnmanagedType.CustomMarshaler,MarshalType="System.Runtime.Interop­S
  ervices.CustomMarshalers.EnumeratorToEnumVariantMarshaler,CustomMarshalers,
  Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
                  public IDictionaryEnumerator GetEnumerator()

  that the VB compiler will accept. I've looked around on the web to see if there's an example, but I so far haven't come up with one. The closest I've come (by guessing) is

  <System.Runtime.InteropServices.MarshalAs(UnmanagedType.CustomMarshaler, MarshalType = "System.Runtime.Interop­Services.CustomMarshalers.EnumeratorToEnumVariantMarshaler,CustomMarshalers, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")> _
  Public Function GetEnumerator() As System.Collections.IEnumerator Implements System.Collections.IEnumerable.GetEnumerator

  Return New CFieldsEnumerator(moInternal_DAO_Fields)

  End Function

  but this generates a compiler error that says

  Error 5 Attribute 'MarshalAsAttribute' cannot be applied to 'GetEnumerator' because the attribute is not valid on this declaration type. 

  I have been able to apply the Dispid(-4) attribute without error, but by itself, that doesn't solve my problem.

  This is the deepest that I've gotten so far into Interop, and I'm at the very ragged edge of what I understand. Any clues are most appreciated.

  To answer your other question,

      What do you mean by "ABXDB.Index"
      Do you mean the type DAO.Index in VB6

  I have defined abstract database classes for nearly every corresponding DAO object. So I have a

      Public Class AbstractDatabase
      Public Class AbstractTableDef
      Public Class AbstractIndex

  each with suitable MustInherit interfaces to ensure a child correctly implements the right things.

  Then, I have defined a set of concrete classes for Jet

      Public Class JetDatabase
           Inherits AbstractDatabase
      Public Class JetTableDef
           Inherits AbstractTableDef

  and so on.

  Lastly, in my legacy code, any declaration that relates to a DAO object is changed to refer to the corresponding Abstract object, and I can then maintain very nearly all of the syntactical constructs that were written for the DAO objects, but now do essentially the same thing through an extra layer of indirection (i.e., through my wrappers). As I said in my earlier post, the goal of this exercise is to allow someone else to write another set of concrete classes for SQL Server using ADO.Net. The fact that they present the same interface to the legacy code means that I shouldn't have to make any other changes to connect to a new DB.

  So far, with the exception of the enumerators not working as I think they should, this approach is working well, albeit with some negative performance hits. It remains to be seen whether we can shoehorn ADO.Net into this scheme.

  Thanks for your help.

     Sean

  
   

   

  
  
• custom enumerator, com interop, and vb6 client