With the new AddressOf operator in Microsoft Visual Basic 5.0, you can pass
a DLL the pointer to a Microsoft Visual Basic function and have the DLL
call that function through the pointer. This article gives you some sample
code to get started and also demonstrates how to pass strings from your DLL
to Microsoft Visual Basic.
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To use this article, follow the steps below to create a Microsoft Visual
Basic 5.0 .exe project and a Microsoft Visual C++ 5.0 .dll project. Then
run your Microsoft Visual Basic program to test it.
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Steps to Create the Microsoft Visual Basic 5.0 Project
| 1. | Create a new Standard EXE project in Microsoft Visual Basic 5.0.
|
| 2. | Add a Command button to Form1.
|
| 3. | Add the following declare statement to the General Declarations section
of Form1:
Private Declare Sub ExecuteCallback Lib "vcvbdll" ( _
ByVal pFunc as Long)
|
| 4. | Add the following code to the Click event of the Command button:
Call ExecuteCallback(AddressOf MyCallback)
|
| 5. | Add a new module to the project, and add the following code to the
module.
Sub MyCallback(ByVal parm as String)
MsgBox "You are inside the VB callback function!"
MsgBox "Parameter passed in was: " & parm
End Sub
|
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Steps to Create the Microsoft Visual C++ 5.0 Project
| 1. | Create a new Microsoft Visual C++ 5.0 MFC AppWizard (dll) project
named "vcvbdll" and accept all of the default options.
|
| 2. | Add the following code to the end of the vcvbdll.cpp file:
void __stdcall ExecuteCallback(long cbAddress) {
// Declare the function pointer, with one string argument.
typedef void (__stdcall *FUNCPTR)(BSTR pbstr);
FUNCPTR vbFunc;
// Point the function pointer at the passed-in address.
vbFunc = (FUNCPTR)cbAddress;
// Call the function through the function pointer.
vbFunc(SysAllocString(L"Hi! This message came from your DLL!"));
}
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| 3. | Add the following line to the end of the vcvbdll.def file to export the
ExecuteCallback function:
ExecuteCallback
|
| 4. | Compile the project, and copy the .dll to the \Windows\System directory
to test it.
|
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Microsoft Visual Basic 5.0 Books Online: AddressOf
For additional information about using the AddressOf operator for
callbacks, see the following article in the Microsoft Knowledge Base:
171729 (http://support.microsoft.com/kb/171729/EN-US/)
: How To Do Generic Callbacks Using a Helper DLL
(c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Joe
Crump, Microsoft Corporation
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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.
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