Answer

The event model in Visual Basic allows reacting to user actions through event handlers that are linked to controls. This is critical for GUI (Graphical User Interface) applications, where each element can trigger its own events.

Background

In classic VB6, event handling was implemented through static binding of control to method. In VB.NET and WinForms, the model became more flexible: now multiple handlers can be attached to a single event, supporting dynamic binding through AddHandler/RemoveHandler.

Problem

A typical issue is the dynamic addition of controls and management of their events. If you do not unsubscribe from events or incorrectly link a handler, memory leaks, duplicate calls, and loss of control over UI behavior may occur.

Solution

When creating controls dynamically, use AddHandler for subscribing and RemoveHandler for correctly removing the handler. Pay attention to the lifespan of controls — if a control is destroyed, it is necessary to unsubscribe handlers; otherwise, "dangling" references will remain.

Example code:

Dim btn As New Button()
AddHandler btn.Click, AddressOf ButtonClickHandler
' ...
RemoveHandler btn.Click, AddressOf ButtonClickHandler

Sub ButtonClickHandler(sender As Object, e As EventArgs)
    MessageBox.Show("Button clicked!")
End Sub

Key features:

Handlers can be assigned both at design time and programmatically
Dynamic handlers allow working with elements created at runtime
It is necessary to remove unnecessary handlers in a timely manner to prevent memory leaks

Trick Questions.

What happens if the event handler is connected multiple times to the same element?

The handler will be called as many times as it was added via AddHandler, which may lead to unexpected multiple executions of logic.

AddHandler btn.Click, AddressOf Handler
AddHandler btn.Click, AddressOf Handler
' When the button is clicked, Handler will be called 2 times

Can a method with parameters that differ from the event signature be used?

No, the handler method signature must strictly match the event delegate (for example, Sub Handler(sender As Object, e As EventArgs)). The compiler will throw an error otherwise.

Does RemoveHandler remove all occurrences of the handler if it has been added multiple times?

No, RemoveHandler removes only one subscription per call. If the handler was added multiple times, it needs to be removed that many times; otherwise, one instance will remain linked.

Common mistakes and anti-patterns

Forgetting RemoveHandler when destroying controls, causing memory leaks
Assigning one handler with different logic to different events
Mismatch between handler parameters and event delegate

Real-life example

Negative case

100 buttons are dynamically created in a loop, and an event handler is assigned to all, but RemoveHandler is not called. After closing the form, the event continues to "live", which leads to frequent crashes.

Pros:

Quick connection of logic to large numbers of controls

Cons:

Memory leak, bugs when reopening the form

Positive case

An event handler is added to each dynamic control in the collection, and when a control is removed, all handlers are explicitly unsubscribed in a loop.