All .NET developers should know that .NET is pretty reference-happy. There’s an interesting phenomena with Structures and Classes that I can illustrate later, but the issue I wanted to point out here involves references and garbage collection. As we know, .NET manages object cleanup through the garbage collector when there are no more references to the object. This is kind of nice because you can let objects just fall out of scope and the GC will take care of everything. That is, if nothing else has a reference to those objects.
One thing to pay special attention to is event handlers. These create references and can keep objects alive MUCH longer than you want; maybe for the running life of the application. As a potential scenario, you have a form with a form-level event. That form has a method that creates some objects that listen for that event. The method does its stuff and finishes. You call the method again and suddenly you have twice as many responses to the event. The objects you set up from the first run still exist and listen for that event.
Here’s some demo code to illustrate this.
The class that responds to the event:
Public Class HangingObject Public Sub SayHello() MsgBox("Hello from " & Me.GetHashCode.ToString) End Sub End Class
The method that creates the objects and raises the event:
Private Sub ShowHanging() Dim o As HangingObject For i As Integer = 1 To 5 o = New HangingObject AddHandler Me.SayHelloEvent, AddressOf o.SayHello Next RaiseEvent SayHelloEvent() End Sub
And the form-level event:
Public Event SayHelloEvent()
So if you call ShowHanging, you get 5 Messageboxes. If you call it again, you get 10, and so on.
The reason for this is the AddHandler statement. AddHandler creates a reference to the instance of the object “o” and stores it with the form-level event SayHelloEvent. When do these references get removed? When the form is disposed. If that form is the main form of the application, that will be when the application ends.
Can you get around this? Maybe by implementing Dispose and disposing the objects?
Public Class HangingDisposableObject Implements IDisposable Public Sub SayHello() MsgBox("Hello from " & Me.GetHashCode.ToString) End Sub Private disposedValue As Boolean Protected Overridable Sub Dispose(ByVal disposing As Boolean) If Not Me.disposedValue Then If disposing Then End If End If Me.disposedValue = True End Sub Public Sub Dispose() Implements IDisposable.Dispose Dispose(True) GC.SuppressFinalize(Me) End Sub End Class
Private Sub ShowHanging() Dim o As HangingDisposableObject For i As Integer = 1 To 5 o = New HangingDisposableObject AddHandler Me.SayHelloEvent, AddressOf o.SayHello o.Dispose() Next RaiseEvent SayHelloEvent() End Sub
You’d be surprised. You still get 5 Messageboxes even though all five were Disposed inside the loop. Ah, but the garbage collector hasn’t run yet. So let’s force it to do a collection.
Private Sub ShowHanging() Dim o As HangingDisposableObject For i As Integer = 1 To 5 o = New HangingDisposableObject AddHandler Me.SayHelloEvent, AddressOf o.SayHello o.Dispose() Next GC.Collect() RaiseEvent SayHelloEvent() End Sub
Still surprised? You shouldn’t be. The GC won’t collect (Finalize) the objects because there is still a reference to that object (the event handler). So how do you manage this? You have to call RemoveHandler to remove the reference. And that means you have to keep your own references to the objects until you’re done with them and you do the cleanup yourself.
Private Sub ShowHanging() Dim oCollection As New Generic.List(Of HangingObject) Dim o As HangingObject For i As Integer = 1 To 5 o = New HangingObject AddHandler Me.SayHelloEvent, AddressOf o.SayHello oCollection.Add(o) Next RaiseEvent SayHelloEvent() For Each o In oCollection RemoveHandler Me.SayHelloEvent, AddressOf o.SayHello Next End Sub
So IDispose is not the answer. Keeping track of your objects and the references they hold is the answer. And that should be the obvious answer anyway.