== Why is this an issue?

Subscribing to events without unsubscribing later on can lead to memory leaks or even duplicate subscriptions, i.e. code which is executed multiple times by mistake.


Even if there is no problem right now, the code is more difficult to review and a simple refactoring can create a bug. For example the lifetime of the event publisher could change and prevent subscribers from being garbage collected.


There are patterns to automatically unsubscribe, but the simplest and most readable solution remains to unsubscribe from events explicitly using ``++RemoveHandler++``.


This rule raises an issue when a class subscribes to an even using ``++AddHandler++`` without explicitly unsubscribing with ``++RemoveHandler++``.

=== Noncompliant code example

[source,vbnet]
----
Class MyEventProcucer
    Public Shared Event EventFired As EventHandler
End Class

Public Class MyEventSubscriber
    Implements IDisposable

    Public Sub New()
        AddHandler MyEventProcucer.EventFired, AddressOf c_EventFired  'Noncompliant
    End Sub

    Private Sub c_EventFired(sender As Object, e As EventArgs)
    End Sub

    Public Sub Dispose() Implements IDisposable.Dispose
    End Sub
End Class
----

=== Compliant solution

[source,vbnet]
----
Class MyEventProcucer
    Public Shared Event EventFired As EventHandler
End Class

Public Class MyEventSubscriber
    Implements IDisposable

    Public Sub New()
        AddHandler MyEventProcucer.EventFired, AddressOf c_EventFired
    End Sub

    Private Sub c_EventFired(ByVal sender As Object, ByVal e As EventArgs)
    End Sub

    Public Sub Dispose() Implements IDisposable.Dispose
        RemoveHandler MyEventProcucer.EventFired, AddressOf c_EventFired
    End Sub
End Class
----

== Resources

* https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/events/how-to-subscribe-to-and-unsubscribe-from-events#unsubscribing[How to subscribe to and unsubscribe from events (C# Programming Guide)]
* https://michaelscodingspot.com/5-techniques-to-avoid-memory-leaks-by-events-in-c-net-you-should-know/[5 Techniques to avoid Memory Leaks by Events in C# .NET you should know]
ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Unsubscribe from this event explicitly with "RemoveHandler".


=== Highlighting

The ``++AddHandler++`` call.


endif::env-github,rspecator-view[]
