Self-updating collection concurrency issues
- by DEHAAS
I am trying to build a self-updating collection. Each item in the collection has a position (x,y). When the position is changed, an event is fired, and the collection will relocate the item.
Internally the collection is using a “jagged dictionary”. The outer dictionary uses the x-coordinate a key, while the nested dictionary uses the y-coordinate a key. The nested dictionary then has a list of items as value.
The collection also maintains a dictionary to store the items position as stored in the nested dictionaries – item to stored location lookup.
I am having some trouble making the collection thread safe, which I really need.
Source code for the collection:
public class PositionCollection<TItem, TCoordinate> : ICollection<TItem>
where TItem : IPositionable<TCoordinate>
where TCoordinate : struct, IConvertible
{
private readonly object itemsLock = new object();
private readonly Dictionary<TCoordinate, Dictionary<TCoordinate, List<TItem>>> items;
private readonly Dictionary<TItem, Vector<TCoordinate>> storedPositionLookup;
public PositionCollection()
{
this.items = new Dictionary<TCoordinate, Dictionary<TCoordinate, List<TItem>>>();
this.storedPositionLookup = new Dictionary<TItem, Vector<TCoordinate>>();
}
public void Add(TItem item)
{
if (item.Position == null)
{
throw new ArgumentException("Item must have a valid position.");
}
lock (this.itemsLock)
{
if (!this.items.ContainsKey(item.Position.X))
{
this.items.Add(item.Position.X, new Dictionary<TCoordinate, List<TItem>>());
}
Dictionary<TCoordinate, List<TItem>> xRow = this.items[item.Position.X];
if (!xRow.ContainsKey(item.Position.Y))
{
xRow.Add(item.Position.Y, new List<TItem>());
}
xRow[item.Position.Y].Add(item);
if (this.storedPositionLookup.ContainsKey(item))
{
this.storedPositionLookup[item] = new Vector<TCoordinate>(item.Position);
}
else
{
this.storedPositionLookup.Add(item, new Vector<TCoordinate>(item.Position)); // Store a copy of the original position
}
item.Position.PropertyChanged += (object sender, PropertyChangedEventArgs eventArgs) => this.UpdatePosition(item, eventArgs.PropertyName);
}
}
private void UpdatePosition(TItem item, string propertyName)
{
lock (this.itemsLock)
{
Vector<TCoordinate> storedPosition = this.storedPositionLookup[item];
this.RemoveAt(storedPosition, item);
this.storedPositionLookup.Remove(item);
}
}
}
I have written a simple unit test to check for concurrency issues:
[TestMethod]
public void TestThreadedPositionChange()
{
PositionCollection<Crate, int> collection = new PositionCollection<Crate, int>();
Crate crate = new Crate(new Vector<int>(5, 5));
collection.Add(crate);
Parallel.For(0, 100, new Action<int>((i) => crate.Position.X += 1));
Crate same = collection[105, 5].First();
Assert.AreEqual(crate, same);
}
The actual stored position varies every time I run the test. I appreciate any feedback you may have.
