Callbacks from a DLL to its calling form

Posted on 1998-07-05
Last Modified: 2010-05-03

I have just started writing ActiveX DLLs, and I have the following question:-

I start my project off by showing Form1.  Then it calls an ActiveX DLL called DLL1.  Then I want the DLL to be able to access the contents of a textbox in Form1.

Is this what they call a "callback" problem?  When I attempt to refer to the textbox in Form1 from the DLL, I get an error message saying that it does not recognise the object (Form1) being referred to.

Must I pass Form1 as an argument when I call the procedure inside the DLL?  I am not sure how to phrase this.  I would be very grateful for a code example.


Question by:trimtrom
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Accepted Solution

tomook earned 100 total points
ID: 1464963
In VB5, you can set up a callback only to a regular function in a regular module. A form is technically a class, so no callback is possible directly to a form.

I don't think this is really your question. If you could pass a reference to the form you could do what you want. You can't do this in VB5. It will not let you pass a reference to a "User Defined Type" (at least this is the error you get). Of course, you might be able to do this in VB6, but since I signed a confidentiality agreement I cannot tell you.

So back to callbacks. If you can't wait a few months (wink wink), you will have to subclass the form or control so you can catch the windows messages coming to it. You would then send a Windows message (using the API) and do your own processing. A good reference for this is "Hardcore Visual Basic, Second Edition" by Bruce MxKinney, from Microsoft Press, pages 350-362. It also comes on the MSDN Library Edition. If you are serious about VB programming, this is an excellent investment.


Expert Comment

ID: 1464964
Sorry,  tomook, I have to disagree with you - we interpret this question differently. I'll give my version of the answer for trimtron.

First of all, no, this is not what is normally called a "callback". A callback is when you call an external API function and give it the address of a sub or function in your program to "call back" . One example of this is the windows API "EnumWindows" function to enumerate windows: you call it with the address of one of your functions, it then calls that function once for every window in the system. ActiveX DLLs written in VB can not call callbacks.

Next, you are correct that you can't pass a form to an ActiveX DLL in VB5, but you *can* pass a control such as the textbox itself. The ActiveX DLL can then access and change the contents of the text control. As an example, in the called ActiveX DLL you could declare the following member of Class1:

Public Function GetSetText(ctrl As Variant) As String
    GetSetText = ctrl.Text
    ctrl.Text = "the text changed!"
End Function

You could then call it from your VB program as:

Dim testObj As Class1
Set testObj = New TestDLL.Class1
t$ = testObj.GetSetText(Form1.Text1)

I hope this helps, good luck!

Author Comment

ID: 1464965
Thanks.  Alamo has the closer answer to what I was really intending.  I will try passing the text control to the DLL for it to access.
Many thanks for both answers.
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Expert Comment

ID: 1464966
Please explain trimtron, if you liked my answer, why did you accept tomook's answer as correct?

Expert Comment

ID: 1464967
Hmm. I sure got the points, but if you liked alamo's answer you should have rejected mine. How do we fix this?

Author Comment

ID: 1464968
basically your answer involved subclassing and the API, which I want to understand.  So I thought I would accept your advice to invest in the next version of VB, and also the Hardcore VB book.
Where you are coming from, I want to go.  So you got the points!


Expert Comment

ID: 1464969
OK, works for me. Thanks.

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