How to make a VB.NET DLL callable in VB6? - vb.net

Question

I have made a DLL in VS2017 using VB.NET:
Imports System.Runtime.InteropServices
<ComClass(ComClass1.ClassId, ComClass1.InterfaceId, ComClass1.EventsId)>
Public Class ComClass1
#Region "COM GUIDs"
' These GUIDs provide the COM identity for this class
' and its COM interfaces. If you change them, existing
' clients will no longer be able to access the class.
Public Const ClassId As String = "c67bcd70-54d0-4498-97be-a5f954790dec"
Public Const InterfaceId As String = "7ef1a8ce-bcc1-464e-8dc3-fc164bdb7ea3"
Public Const EventsId As String = "9939eabd-1102-4e34-9735-54664e3536bd"
#End Region
' A creatable COM class must have a Public Sub New()
' with no parameters, otherwise, the class will not be
' registered in the COM registry and cannot be created
' via CreateObject.
Public Sub New()
MyBase.New()
End Sub
Public Xposition As Int32
Public GoX As Int32
Public Function MoveX() As Int32
Dim target As Int32
target = Convert.ToInt32(Rnd() * 1000)
Xposition = target
Return Xposition
End Function
Public Function ReadX() As Int32
Dim target As Int32
target = Convert.ToInt32(Rnd() * 1000 - GoX)
Xposition = target
Return Xposition
End Function
End Class
I compiled the DLL as Administrator on the development Widows 10 PC.
I copied the DLL file to the Windows XP target machine running VB6 and get Run-time error 453.
The VB6 code:
Private Declare Function GSCloseServer Lib "GSWDLL32.DLL" () As Long
Private Declare Function MoveX Lib "C:\Temp\VB_Applications\My_DLL_Test\MyNewDLL.dll" () As Integer
Private Sub btnMove_Click()
Call test
End Sub
Private Sub btnStop_Click()
Dim Running As Integer
' Unload MyGraph
Set Form1 = Nothing
Running = GSCloseServer()
If Running = 0 Then
MsgBox "Close Server OK: " & Running, vbOKOnly
Else
MsgBox "Close Server Error " & Running, vbOKOnly
End If
Unload Me
End Sub
Private Sub test()
'txtbxReadBack.Text = ComClass1.Xposition
txtbxReadBack.Text = MoveX
End Sub
I am running VS2017 as administrator, so the DLL appears to be registered properly on the development PC.

Answer 1

These are the steps. I'll create a very simple "COMVisibleHelloWorld" class library with one class
Imports System.Runtime.InteropServices
'Create a manual interface so we can use a fixed and known guid for the COM interface.
<Guid("3BBA8F8E-6B35-48E0-91D4-124DC6FCE7B9"), InterfaceType(ComInterfaceType.InterfaceIsDual)>
Public Interface IHelloWorld
<DispId(1)> Function SayHello() As String
End Interface
<Guid("BCDCDA5C-DCAF-4C6C-86BA-03908DBEFF4B"), ClassInterface(ClassInterfaceType.None)>
Public Class HelloWorld
Implements IHelloWorld
Public Function SayHello() As String Implements IHelloWorld.SayHello
Return "Hello World"
End Function
End Class
In Visual Studio select "Make Assembly COM-Visible"
In Visual Studio create a strong key
Build the project. This will then create a tlb file.
You can now reference this is VB6
To use in VB6 just create an instance, and call the methods that have a COM interface
Private Sub Command1_Click()
Dim hello As New COMVisibleHelloWorld.HelloWorld
MsgBox (hello.SayHello)
End Sub
Result: