Question

Could someone please be kind enough to explain to me what the purpose of the BlockReentrancy Method is in the ObservableCollection<T> ?

MSDN shows the following as an example:

//The typical usage is to wrap an OnCollectionChanged call within a using scope, as in the following example:

using (BlockReentrancy())
{
    // OnCollectionChanged call
}

But this doesn’t seem to clarify for me what the purpose is. Anyone care to explain?

Answer 1

An ObservableCollection implements INotifyCollectionChanged and so it has a CollectionChanged event. If there is a subscriber to this event, they could further modify the collection while the collection is already in the process of notification. Since the CollectionChanged event keeps track of exactly what changed, this interaction can get very messy.

As a result, the ObservableCollection allows, as a special case, a single subscriber of the CollectionChanged event to modify the collection from its handler. But it disallows modifying the collection from the CollectionChanged handler if there are two or more subscribers to the CollectionChanged event.

The pair of methods BlockReentrancy and CheckReentancy are used to implement this logic. The BlockReentrancy is used at the start of the OnCollectionChanged method and CheckReentancy is used in all methods that modify the collection.