Adding ActiveX Event Callback Function in Visual Basic 6.0

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Visual Basic (VB) has long been one of the most popular choices for developing Windows-based applications due its ease of use and powerful features set compared to other languages like C++ or Delphi.

In this article, we will cover how to add an ActiveX event callback function in Visual VB 6.0. This is a fundamental concept that every developer should know when working with ActiveX controls since many third-party components rely on events for communication between the main application and their respective controls.

Prerequisites

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Before proceeding further, make sure you have:

1. Installed Microsoft Visual Studio Professional or Enterprise edition (Express Edition does not support creating ActiveX projects).

2. Familiarized yourself with basic VB6 concepts such as forms, class modules, User Interface (UI) design, and event-driven programming.

Step 1: Creating a New ActiveX Project

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To begin:

1. Launch Visual Studio.

2. From the 'File' menu, select 'New -> Project...'

3. In the 'Visual Basic Projects' dialog box under 'ActiveX Controls', choose 'Windows Forms Control Library (COM)' and click Next.

Step 2: Adding an ActiveX Reference

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In your new project:

1. Right-click on References in Solution Explorer.

2. Click Add Reference...

3. Check the checkbox next to the ActiveX control you want to reference and then OK.

Wiring up Events with VB6

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### Step A: Declaring WithEvents

To wire an event handler for a specific event from your referenced ActiveX control:

1. Double-click on a new form in your project (if not already open).

2. In the 'Properties' window, go to the 'Events' tab.

3. Find and double-click on the desired event under its respective event list.

### Step A: Creating an Event Procedure

Visual Basic will now automatically generate a stub procedure for you with a signature similar to this:

```vb

Private Sub YourControlName_YourEventname() Handles YourControlName.YourEventname

' Add code here that should run when the specified event occurs.

End Sub

```

### Step B: Adding Event Handlers

Now, in your form's `Load` event (double-click on 'Form1_Load' if it doesn't exist):

```vb

Private WithEvents m_oYourControl As YourActiveXClassName

Public Sub Form_Load() Handles MyBase.Load

' Create an instance of the ActiveX control here.

Me.Controls.Add(m_oYourControl)

End Sub

Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) _

Handles Button1.Click

' Call a method on your activeX object to trigger the event we wired up earlier.

m_oYourControl.YourMethod()

End Sub

```

In this example:

* We declared an instance variable `m_oYourControl` as type `YourActiveXClassName`.

* In the form's `Load` event, we created a new instance of our ActiveX control and added it to the form's controls collection.

* When button1 is clicked, its click event will be fired which in turn calls method `YourMethod()` on our ActiveX object.

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By following these simple steps, you can easily add an ActiveX event callback function to your Visual Basic 6.0 applications. Remember that proper error handling and debugging are essential when working with third-party components or complex codebases.

This article covered the basics of wiring up events between a VB6 application and an ActiveX control using `WithEvents` keyword, but there's much more you can learn about event-driven programming in .NET such as implementing custom interfaces for your own classes or using IObservable/IObserver pattern. Happy coding!

Visual basic 6.0