Release/dispose of a UIViewController in MonoTouch

didreceivememorywarning, garbage-collection, objective-c, uiviewcontroller, xamarin.ios

Solution

As I understand it, we need to hold references to our Cocoa objects when dealing with them in MonoTouch.

Not quite. MonoTouch managed instances will keep a reference to native instances. As long as the managed instance exists the native instance will be alive (since they are reference counted and MonoTouch won't release it's reference).

IOW your need to hold references to the MonoTouch managed instances as long as their native part are required.

The reason for this is that the ObjC runtime might still hold references to the objects ... they could be garbage collected,

Native (Objective C) instances are reference counted, not garbage collected. Native instances won't be released until their reference count reach 0 (which won't happen while an associated managed instance exists);

Also native instances can hold references to other native instances. Not every native instances have a corresponding managed instance.

which results in a EXC_BAD_ACCESS as soon as the ObjC runtime tries to access them.

That won't happen, at least not this way. OTOH it's hard to tell you what's happening in your case (without seeing the code and/or the crashes).

I suspect you're disposing (manually or not) your managed instances before they have completed their jobs. Here's a simplified of what could happen:

- you create a managed MT.X instance (e.g. an `UIView`);

- this creates and reference a native X (native reference count == 1);

- you override an event (or add a delegate...) `ViewWillUnload' on MT.X (which also exists natively);

- you assign the MT.X instance to another (managed) instance, e.g. an `UIViewController`;

- the native `UIViewController` will add a reference to the native X (native reference count == 2);

- application executes happilly...

- you stop having a reference to the `MT.X` instance (e.g. set the variable to `null` or a different instance);

- since there's no reference to `MT.X` anymore the Garbage Collector will dispose the managed instance, calling `Dispose` which will reduce the reference to native X (native reference count == 1). But the native instance won't be freed since it's still referenced (not 0) by the view controller;

- the `UIViewController` does something that triggers, natively, `X.ViewWillUnload` (e.g. it tries to load a new `UIView`);

- since `X` still exists natively (ref count == 1) it will call it's `ViewWillUnload` which will try to go back to the managed instance... that was disposed.

The solution to this problem is to ensure ensure you're not disposing managed instance until their native part have completed their jobs.

Problem

Original source