问题
When updating a collection of business objects on a background thread I get this error message:
This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread.
Ok, that makes sense. But it also begs the question, what version of CollectionView does support multiple threads and how do I make my objects use it?
回答1:
The following is an improvement on the implementation found by Jonathan. Firstly it runs each event handler on the dispatcher associated with it rather than assuming that they are all on the same (UI) dispatcher. Secondly it uses BeginInvoke to allow processing to continue while we wait for the dispatcher to become available. This makes the solution much faster in situations where the background thread is doing lots of updates with processing between each one. Perhaps more importantly it overcomes problems caused by blocking while waiting for the Invoke (deadlocks can occur for example when using WCF with ConcurrencyMode.Single).
public class MTObservableCollection<T> : ObservableCollection<T>
{
public override event NotifyCollectionChangedEventHandler CollectionChanged;
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
NotifyCollectionChangedEventHandler CollectionChanged = this.CollectionChanged;
if (CollectionChanged != null)
foreach (NotifyCollectionChangedEventHandler nh in CollectionChanged.GetInvocationList())
{
DispatcherObject dispObj = nh.Target as DispatcherObject;
if (dispObj != null)
{
Dispatcher dispatcher = dispObj.Dispatcher;
if (dispatcher != null && !dispatcher.CheckAccess())
{
dispatcher.BeginInvoke(
(Action)(() => nh.Invoke(this,
new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset))),
DispatcherPriority.DataBind);
continue;
}
}
nh.Invoke(this, e);
}
}
}
Because we are using BeginInvoke, it is possible that the change being notified is undone before the handler is called. This would typically result in an "Index was out of range." exception being thrown when the event arguments are checked against the new (altered) state of the list. In order to avoid this, all delayed events are replaced with Reset events. This could cause excessive redrawing in some cases.
回答2:
Use:
System.Windows.Application.Current.Dispatcher.Invoke(
System.Windows.Threading.DispatcherPriority.Normal,
(Action)delegate()
{
// Your Action Code
});
回答3:
This post by Bea Stollnitz explains that error message and why it's worded the way it is.
EDIT: From Bea's blog
Unfortunately, this code results in an exception: “NotSupportedException – This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread.” I understand this error message leads people to think that, if the CollectionView they’re using doesn’t support cross-thread changes, then they have to find the one that does. Well, this error message is a little misleading: none of the CollectionViews we provide out of the box supports cross-thread collection changes. And no, unfortunately we can not fix the error message at this point, we are very much locked down.
回答4:
Found one.
public class MTObservableCollection<T> : ObservableCollection<T>
{
public override event NotifyCollectionChangedEventHandler CollectionChanged;
protected override void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
{
var eh = CollectionChanged;
if (eh != null)
{
Dispatcher dispatcher = (from NotifyCollectionChangedEventHandler nh in eh.GetInvocationList()
let dpo = nh.Target as DispatcherObject
where dpo != null
select dpo.Dispatcher).FirstOrDefault();
if (dispatcher != null && dispatcher.CheckAccess() == false)
{
dispatcher.Invoke(DispatcherPriority.DataBind, (Action)(() => OnCollectionChanged(e)));
}
else
{
foreach (NotifyCollectionChangedEventHandler nh in eh.GetInvocationList())
nh.Invoke(this, e);
}
}
}
}
http://www.julmar.com/blog/mark/2009/04/01/AddingToAnObservableCollectionFromABackgroundThread.aspx
回答5:
You can also look at: BindingOperations.EnableCollectionSynchronization.
See Upgrading to .NET 4.5: An ItemsControl is inconsistent with its items source
回答6:
Sorry, can't add a comment but all this is wrong.
ObservableCollection is not thread safe. Not only because of this dispatcher issues, but it's not thread safe at all (from msdn):
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
Look here http://msdn.microsoft.com/en-us/library/ms668604(v=vs.110).aspx
There's also a problem when calling BeginInvoke with a "Reset" action. "Reset" is the only action where handler should look at the collection itself. If you BeginInvoke a "Reset" and then immediately BeginInvoke a couple of "Add" actions than handler will accept a "Reset" with already updated collection and next "Add"'s will create a mess.
Here's my implementation which works. Actually I'm thinking of removing BeginInvoke at all:
Fast performing and thread safe observable collection
回答7:
You can get wpf to manage cross thread changes to a collection by enabling collection synchronization like so:
BindingOperations.EnableCollectionSynchronization(collection, syncLock);
listBox.ItemsSource = collection;
This tells WPF that the collection may be modified off the UI thread so it knows it has to marshal any UI changes back to the appropriate thread.
There is also an overload to provide a synchronization callback if you don't have a lock object.
回答8:
If you want to update WPF UI Control periodically and at the same time use UI you can use DispatcherTimer.
XAML
<Grid>
<DataGrid AutoGenerateColumns="True" Height="200" HorizontalAlignment="Left" Name="dgDownloads" VerticalAlignment="Top" Width="548" />
<Label Content="" Height="28" HorizontalAlignment="Left" Margin="0,221,0,0" Name="lblFileCouner" VerticalAlignment="Top" Width="173" />
</Grid>
C#
public partial class DownloadStats : Window
{
private MainWindow _parent;
DispatcherTimer timer = new DispatcherTimer();
ObservableCollection<FileView> fileViewList = new ObservableCollection<FileView>();
public DownloadStats(MainWindow parent)
{
InitializeComponent();
_parent = parent;
Owner = parent;
timer.Interval = new TimeSpan(0, 0, 1);
timer.Tick += new EventHandler(timer_Tick);
timer.Start();
}
void timer_Tick(object sender, EventArgs e)
{
dgDownloads.ItemsSource = null;
fileViewList.Clear();
if (_parent.contentManagerWorkArea.Count > 0)
{
foreach (var item in _parent.contentManagerWorkArea)
{
FileView nf = item.Value.FileView;
fileViewList.Add(nf);
}
}
if (fileViewList.Count > 0)
{
lblFileCouner.Content = fileViewList.Count;
dgDownloads.ItemsSource = fileViewList;
}
}
}
回答9:
Try This:
this.Dispatcher.Invoke(DispatcherPriority.Background, new Action(
() =>
{
//Code
}));
回答10:
None of them, just use Dispatcher.BeginInvoke
回答11:
Here's a VB version I made after some googling and slight mods. Works for me.
Imports System.Collections.ObjectModel
Imports System.Collections.Specialized
Imports System.ComponentModel
Imports System.Reflection
Imports System.Windows.Threading
'from: http://stackoverflow.com/questions/2137769/where-do-i-get-a-thread-safe-collectionview
Public Class ThreadSafeObservableCollection(Of T)
Inherits ObservableCollection(Of T)
'from: http://geekswithblogs.net/NewThingsILearned/archive/2008/01/16/listcollectionviewcollectionview-doesnt-support-notifycollectionchanged-with-multiple-items.aspx
Protected Overrides Sub OnCollectionChanged(ByVal e As System.Collections.Specialized.NotifyCollectionChangedEventArgs)
Dim doit As Boolean = False
doit = (e.NewItems IsNot Nothing) AndAlso (e.NewItems.Count > 0)
doit = doit OrElse ((e.OldItems IsNot Nothing) AndAlso (e.OldItems.Count > 0))
If (doit) Then
Dim handler As NotifyCollectionChangedEventHandler = GetType(ObservableCollection(Of T)).GetField("CollectionChanged", BindingFlags.Instance Or BindingFlags.NonPublic).GetValue(Me)
If (handler Is Nothing) Then
Return
End If
For Each invocation As NotifyCollectionChangedEventHandler In handler.GetInvocationList
Dim obj As DispatcherObject = invocation.Target
If (obj IsNot Nothing) Then
Dim disp As Dispatcher = obj.Dispatcher
If (disp IsNot Nothing AndAlso Not (disp.CheckAccess())) Then
disp.BeginInvoke(
Sub()
invocation.Invoke(Me, New NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset))
End Sub, DispatcherPriority.DataBind)
Continue For
End If
End If
invocation.Invoke(Me, e)
Next
End If
End Sub
End Class
回答12:
Small mistake in the VB version. Just replace :
Dim obj As DispatcherObject = invocation.Target
By
Dim obj As DispatcherObject = TryCast(invocation.Target, DispatcherObject)
来源：https://stackoverflow.com/questions/2137769/where-do-i-get-a-thread-safe-collectionview