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WP7Contrib: Thread safe ObservableCollection<T>

Continuing with the introduction of WP7Contrib concepts, patterns & services from my previous post I thought I would explain why we have chosen to implement our own ObservableCollection.

As with the desktop version of Silverlight the idea of a 'blocking call' on the UI thread is discouraged, to point where if you do block the UI thread on a WP7 device your application could be automatically torn down - bad user experience. So to get round the issue the Silverlight framework makes use of the standard .Net 'async pattern' - IAsyncResult interface. Examples of this are the HttpWebRequest class and it's counterpart HttpWebResponse, they allow you to execute web requests without blocking the UI thread.

The problem with the ObservableCollection<T> supplied by the framework is it's not thread safe for any methods. For example, when you have a control bound to the collection and it's enumerating over the collection and another thread modifies the collection you'll receive the exception - "Collection was modified; enumeration operation may not execute.". It could be said this is an oversight of the implementation but I wouldn't go that far, the idea of copying the internal list for every call to GetEnumerator could be an expensive time & memory operation.

We required a thread safe version of an ObservableCollection<T> so that meant rolling our own. The first thing we defined where the interfaces we needed, the obvious ones being the INotifyCollectionChanged & INotifyPropertyChanged.

/// <summary>
/// Marker interface for the observable collection.
/// </summary>
public interface IObservableCollection
{
}

/// <summary>
/// Generic interface for observable collection
/// </summary>
/// <typeparam name="T">The type being used for binding.
/// </typeparam>
public interface IObservableCollection<T> : IList<T>, IObservableCollection, INotifyCollectionChanged, INotifyPropertyChanged
{
/// <summary>
/// Adds a range of items to the observable collection.
/// </summary>
/// <param name="items">
/// The items to be added to the observable collection.
/// </param>
void AddRange(IEnumerable<T> items);
}

The implementation of these interfaces are the ObservableCollection we developed.

We use the composition principle to store the items of collection internally in a List<T> implementation.

/// <summary>
/// Thread safe observable collection
/// </summary>
/// <typeparam name="T">The type being used for binding.
/// </typeparam>
public sealed class ObservableCollection<T> : BaseCollectionModel, IObservableCollection<T>
{
/// <summary>
/// The internal collection.
/// </summary>
private readonly IList<T> collection = new List<T>();

...
...
...
}

Synchronisation to all methods which could modify or enumerate the items is done using a Monitor critical section exposed via the 'lock' method.

Add Method:

/// <summary>
/// Adds an item to the collection, raises a RaiseCollectionChanged event with a NotifyCollectionChangedAction.Add
/// parameter.
/// </summary>
/// <param name="item">
/// The item being added.
/// </param>
public void Add(T item)
{
int index;

lock (this.sync)
{
this.collection.Add(item);
index = this.collection.IndexOf(item);
}

this.RaiseCollectionChanged(NotifyCollectionChangedAction.Add, item, index);
}

Remove Method:

/// <summary>
/// Removes an item from the collection. If the item does not exist in the collection the false is returned.
/// </summary>
/// <param name="item">
/// The item to be removed from the collection
/// </param>
/// <returns>
/// Returns the result of removing an item from the collection, if the item existed it returns true else false.
/// </returns>
public bool Remove(T item)
{
int index;
bool result;

lock (this.sync)
{
index = this.collection.IndexOf(item);
if (index == -1)
{
return false;
}
result = this.collection.Remove(item);
}

if (result)
{
this.RaiseCollectionChanged(NotifyCollectionChangedAction.Remove, item, index);
}

return result;
}

Clear Method:

/// <summary>
/// Clears the contents of the collection, raises a RaiseCollectionChanged event with a NotifyCollectionChangedAction.Reset
/// </summary>
public void Clear()
{
lock (this.sync)
{
this.collection.Clear();
}

this.RaiseCollectionChanged(NotifyCollectionChangedAction.Reset);
}

You will notice from the above code we don't fire the events to notify observers the collection has changed from inside the critical section - the observers are notified in a sequential manner and therefore doing this inside of the critical section would mean holding the lock for a longer period and we are trying to reduce this time to a minimum.

I mentioned above a way to get around the problem of enumerating the collection whilst another thread makes modifications, we do make use of this pattern, a copy is made for each call to the either of the GetEnumerator methods. We took the view that the collection would never contain 'to many' items that making a copy of the list would be an issue - we don't envisage have 1000's of items, what would be the point on a WP7 device.

The last major issue to consider when implementing the collection was - do we want to support the firing the events on the UI thread via the 'Dispatcher.BeginInvoke' method. We took the view this is not the responibility of the collection, it's the responsibility of the host application to make sure they are on the correct thread (if required). The responsibility of collection is to maintain and update items and notify observers, not to manage any constraints of the UI threading model - Single Responsibility Principle.

The full implementation can be found in the WP7Contrib.Collections project on CodePlex.

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