Can not operate ObservableCollection in multi threads

Question

It seems the ObservableCollection only support add, remove, clear operation from the UI thread, It throw Not Support Exception if it is operated by a NO UI thread. I tried to override methods of ObservableCollection, unfortunatly, I met lots of problems. Any one can provide me a ObservableCollection sample which can be operated by multi-threads? Many thanks!

Answer 1

Using the link provided by Kent, you could use the following code to modify a collection across threads:

while (!Monitor.TryEnter(_lock, 10))
{
   DoEvents();
}

try
{
   //modify collection
}
finally
{
   Monitor.Exit(_lock);
}

If however you're just looking to modify the collection on your original thread you can try using a callback to your UI thread. I normally do something like this:

this.Dispatcher.Invoke(new MyDelegate((myParam) =>
{
  this.MyCollection.Add(myParam);
}), state);

Answer 2

Personally, I find Bob's style of answer easier to use than Mark's style of answer. Here are the C# WPF snippets for doing this:

1. in your class's constructor, get the current dispatcher as you create your observable collections. Because, as you pointed out, modifications need to be done on the original thread, which may not be the main GUI thread. So Application.Current.Dispatcher isn't alwasys right, and not all classes have a this.Dispatcher.

   _dispatcher = System.Windows.Threading.Dispatcher.CurrentDispatcher;
   _data = new ObservableCollection<MyDataItemClass>();
   

2. Use the dispatcher to Invoke your code sections that need the original thread.

   _dispatcher.Invoke(new Action(() => { _data.Add(dataItem); }));
   

That should do the trick for you. Though there are situations you might prefer .BeginInvoke instead of .Invoke.

Answer 3

You've basically got to Invoke or BeginInvoke over to the UI thread to do those operations.

Public Delegate Sub AddItemDelegate(ByVal item As T)

Public Sub AddItem(ByVal item As T)
    If Application.Current.Dispatcher.CheckAccess() Then
        Me.Add(item)
    Else
        Application.Current.Dispatcher.Invoke(Threading.DispatcherPriority.Normal, New AddItemDelegate(AddressOf AddItem), item)
    End If
End Sub

Answer 4

You might want to investigate my answer to this perhaps -but please note that the code came from here and cannot be credited to me. Though I tried to implement it in VB. : Original Site

I've used this to populate a WPF listbox from a class that has an ObservableCollectionEx populated asynchronously from an access database. It does work.

public class ObservableCollectionEx<T> : ObservableCollection<T>
{
   // Override the event so this class can access it
   public override event System.Collections.Specialized.NotifyCollectionChangedEventHandler    
 CollectionChanged;

   protected override void OnCollectionChanged    (System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
   {
  // Be nice - use BlockReentrancy like MSDN said
  using (BlockReentrancy())
  {
     System.Collections.Specialized.NotifyCollectionChangedEventHandler eventHandler = CollectionChanged;
     if (eventHandler == null)
        return;

     Delegate[] delegates = eventHandler.GetInvocationList();
     // Walk thru invocation list
     foreach (System.Collections.Specialized.NotifyCollectionChangedEventHandler handler in delegates)
     {
        DispatcherObject dispatcherObject = handler.Target as DispatcherObject;
        // If the subscriber is a DispatcherObject and different thread
        if (dispatcherObject != null && dispatcherObject.CheckAccess() == false)
        {
           // Invoke handler in the target dispatcher's thread
           dispatcherObject.Dispatcher.Invoke(DispatcherPriority.DataBind, handler, this, e);
        }
        else // Execute handler as is
           handler(this, e);
     }
  }
}
}