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Tip of the Day
Language: VB5,VB6
Expertise: Advanced
Jun 3, 2000

Determine whether an ActiveX DLL is being used by an interpreted program

While determine whether the current program is running as interpreted or compiled code is relatively easy, determine whether an ActiveX DLL is serving an interpreted or compiled application is a bit more difficult. This information might be useful to disable or enable special features, or just to prevent from other programmers to reuse the code in your own DLL.

The following solution is based on the EnumThreadWindows API function. This API function enumerates all the windows that belong to a given thread, and the code below uses it to check whether there is a window of class "IDEOwner" in the same thread as the DLL. If this is the case, it means that the DLL is running in the same thread as the Visual Basic IDE, hence its client is an interpreted program.

This function doesn't check for other possible interpreted clients, but you can do that simply by checking for additional window class names in the EnumThreadWindows routine.


Private Declare Function EnumThreadWindows Lib "user32" (ByVal dwThreadId As _
    Long, ByVal lpfn As Long, ByVal lParam As Long) As Long
Private Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal _
    hWnd As Long, ByVal lpClassName As String, ByVal nMaxCount As Long) As Long

' this variable is shared between the two routines below
Dim m_ClientIsInterpreted As Boolean

' return True if the client application of this DLL
' is an interpreted Visual Basic program running in the IDE
'
' NOTE: this code is meant to be inserted in a BAS module
'       inside an ActiveX DLL project

Function ClientIsInterpreted() As Boolean
    EnumThreadWindows App.ThreadID, AddressOf EnumThreadWindows_CBK, 0
    ClientIsInterpreted = m_ClientIsInterpreted
End Function

' this is a callback function that is executed for each
' window in the same thead as the DLL

Public Function EnumThreadWindows_CBK(ByVal hWnd As Long, _
    ByVal lParam As Long) As Boolean
    Dim buffer As String * 512
    Dim length As Long
    Dim windowClass As String

    ' get the class name of this window    
    length = GetClassName(hWnd, buffer, Len(buffer))
    windowClass = Left$(buffer, length)
    
    If windowClass = "IDEOwner" Then
        ' this is the main VB IDE window, therefore
        ' the client application is interpreted
        m_ClientIsInterpreted = True
        ' return False to stop evaluation
        EnumThreadWindows_CBK = False
    Else
        ' return True to continue enumeration
        EnumThreadWindows_CBK = True
    End If
End Function

Francesco Balena
 
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