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I'm trying to work out why some of my Silverlight controls are staying in memory. I've noticed that when I navigate away from a page with the controls on, there remain referential links to the view model. These links are a result of left-over subscriptions to the ErrorsChanged event (my view model implements INotifyDataError) created when Bindings are created between the Page and the view model. At some point some but not all of the Bindings are removed and I can't work out why this isn't happening.

At what point in the life-cycle of Silverlight UIElements do existing Bindings get removed?

Dave Clemmer
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Stephen Ellis
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1 Answers1

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I think I now know the answer to this. Please correct me if I'm wrong. Bindings are not removed. Instead referential links are made (e.g. INotifyDataError events are wired up) as a result of a particular instantiation of a Binding. In the simplest case that's when the DataContext changes.

Let's say you set the DataContext of a Page to a new, different INotifyDataError object as you load it. If there are Bindings on your Page (to the DataContext) the Page is not a candidate for garbage collection until the DataContext object is destroyed. That's because the DataContext holds a reference to the Page through its ErrorsChanged event. If you want the DataContext object to be collected you'll have to set the DataContext to null in the Page's Unloaded event.

As far as I understand it, it seems the proper pattern for implementing a DataContext (that is different from the Page) is setting the DataContext in the Loaded event on the page, and then setting the DataContext to null as the page fires its Unloaded event. This concept applies any Framework element.

As an aside, I haven't delved into element Bindings. For instance, when one element on a Page is bound to a property on another element. I'm not sure when the referential links between these objects are removed. I'm presuming it's when the page is unloaded. Anyone know the answer to this?

Stephen Ellis
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