Is this a bug in MonoTouch GC?

DanAbramovDanAbramov RUMember, Beta ✭✭
edited October 2012 in Xamarin.iOS

Note: I've created a simple project—you can see how switching types between UIButton and CustomButton in storyboard changes GC behavior.

I'm trying to get my head wrapped around MonoTouch garbage collector.
The issue is similar to the one fixed in MT 4.0, however with inherited types.

To illustrate it, consider two view controllers, parent and child.

Child's view contains a single UIButton that writes to console on tap.
Controller's Dispose method throws an exception so it's hard to miss.

Here goes child view controller:

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();

    sayHiButton.TouchUpInside += (sender, e) =>
        SayHi();
    }
}

void SayHi()
{
    Console.WriteLine("Hi");
}

protected override void Dispose (bool disposing)
{
    throw new Exception("Hey! I've just been collected.");
    base.Dispose (disposing);
}

Parent view controller just presents child controller and sets a timer to dismiss it and run GC:

public override void ViewDidLoad ()
{
    base.ViewDidLoad ();

    var child = (ChildViewController)Storyboard.InstantiateViewController("ChildViewController");

    NSTimer.CreateScheduledTimer(2, () => {
        DismissViewController(false, null);
        GC.Collect();
    });

    PresentViewController(child, false, null);
}

If you run this code, it predictably crashes inside ChildViewController.Dispose() called from its finalizer because child controller has been garbage collected. Cool.

Now open the storyboard and change button type to CustomButton. MonoDevelop will generate a simple UIButton subclass:

[Register ("CustomButton")]
public partial class CustomButton : UIButton
{
    public CustomButton (IntPtr handle) : base (handle)
    {
    }

    void ReleaseDesignerOutlets()
    {
    }
}

Somehow changing the button type to CustomButton is enough to trick garbage collector into thinking child controller is not yet eligible for collection.

How is that so?

(SO thread)

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