Using callbacks from VBA/Access

Posted on 2004-10-25
Last Modified: 2012-08-13
I am following the instructions in
for using callbacks. It works absolutely fine from VFP8 but I need to use this method from Microsoft Access (vba). When I try the same thing from Access I get the following error message. ".SETCALLBACK: callback must be object." It would appear that the parameter being passed instead of the callback object is an empty character string.
Please help I really need to get this working from Access.
Question by:Caltor
    LVL 83
    Can you post the code you're currently using? Also, what exactly are you trying to accomplish with the callback?
    LVL 3

    Author Comment

    Code is posted below. The idea of the callback is to allow me a core product COM/DLL and then create extensions using an additional COM/DLL. The two are then bound together using the callbacks.
    The entire project is huge and very specific so I have created a sample of the problem.
    You can download the entire thing from
    You will need to register proj1.dll and proj2.sll using regsvr32
    eg regsvr32 proj1.dll
    regsvr32 proj2.dll
    After you have done this you can look at the db1.mdb that has a demo of the problem.
    The problems are logged in c:\app.log

    Alternatively if you like typing you can use all the code below....

    Access Code attached to a command button
    Option Compare Database

    Private Sub Command1_Click()
        Dim objCallback As Object
        Dim objMain As Object
        Set objMain = CreateObject("proj1.class1")
        Set objCallback = CreateObject("proj2.eventhandler")
        objMain.SetCallback (objCallback)
        MsgBox ("Program completed!")
    End Sub

    Create a program called program2 in Visual Foxpro 8 and paste the following code
    Define Class EventHandler As Session OlePublic

          Implements Iapplicationevents In "proj1.ApplicationEvents"

          Procedure Iapplicationevents_Event1() AS VARIANT
                *-- Perform the callback code
                StrToFile("Dummy File","c:\dummy.txt",0)


    Add this program to a project called proj2 and build into a COM/DLL

    Create a program called program1 in vfp8 and paste the following code:
    Define Class Class1 As Session OLEPublic

          oCallback = .Null.
          Function Error(tnError, tcMethod, tnLine)
                Local lcMessage As String, lnObjectName As String
                lcObjectName = This.Name

                lcMessage = "ERROR: "+lcObjectName+"."+tcMethod+" "+"Line: "+AllTrim(Str(tnLine))+" "+Message()


          Function Function1
                *-- Do some pre callback code...
                *-- Issue the callback
                *-- Do some post callback code...

          Function Init
                *-- Initialise Callback Object
                This.WriteLog("Initialise callback object")
                This.WriteLog("Callback object has been initialised")
          Function SetCallback(toCallback As Object)
                Local lcType As Character
                If IsNull(toCallback)
                      *-- dummy instance that does nothing
                      This.oCallback = NewObject("ApplicationEvents")
                      *-- Check callback variable is an object
                      lcType = Vartype(toCallBack)
                  If lcType != 'O'
                         This.WriteLog(PROGRAM()+": callback must be object. Parameter passed is "+lcType)
                      This.oCallback = GetInterface(toCallback, "IApplicationEvents", "proj1.class1")

          Function WriteLog(tcString)
                *-- Write expression to default log file
                *-- Prefix with date and time
                *-- Suffix with file name
                *-- Always append to file
                StrToFile(Dtoc(Date())+" "+Time()+" - "+tcString+Chr(13)+Chr(10),"c:\app.log",1)

    Define Class ApplicationEvents As Session OLEPublic
    *-- Template for Callback Interface --*

          Function Event1()

    Add this program to a project called proj1 and build into a COM/DLL

    You will then be able to try the access sample above.

    LVL 3

    Author Comment

    I have solved this problem myself with the help of Microsoft.
    I changed the object types in Access to type Variant instead of type Object.

    Accepted Solution

    Question answered by asker or dialog valuable.
    Closed, 500 points refunded.
    ee_ai_construct (replacement part #xm34)
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