collection processing in BackgroundWorker - windows-phone-7

Question

I try to make my ListBox connected to ObservaleCollection be more efficient so for the DB query I implemented a BackgroundWorker to do the job. Then whithin this backgroundworker I want to add every lets say 70 ms 3 entries to the UI, so the UI on larger number of entries (lets say 100) does not get blocked.
Here is the code:
void updateTMWorker_DoWork(object sender, DoWorkEventArgs e)
{
var MessagesInDB = from MessageViewModel tm in MessagesDB.Messages
where tm.Type.Equals(_type)
orderby tm.Distance
select tm;
// Execute the query and place the results into a collection.
Dispatcher.BeginInvoke(() => { MessagesClass.Instance.Messages = new ObservableCollection<MessageViewModel>(); });
Collection<MessageViewModel> tempM = new Collection<MessageViewModel>();
int tempCounter = 0;
foreach (MessageViewModel mToAdd in MessagesInDB)
{
if (MessagesClass.Instance.Messages.IndexOf(mToAdd) == -1)
{
tempM.Add(mToAdd);
tempCounter = tempCounter + 1;
}
if (tempCounter % 3 == 0)
{
tempCounter = 0;
Debug.WriteLine("SIZE OF TEMP:" + tempM.Count());
Dispatcher.BeginInvoke(() =>
{
// add 3 messages at once
MessagesClass.Instance.Messages.Add(tempM[0]);
MessagesClass.Instance.Messages.Add(tempM[1]);
MessagesClass.Instance.Messages.Add(tempM[2]);
});
tempM = new Collection<MessageViewModel>();
Thread.Sleep(70);
}
}
// finish off the rest
Dispatcher.BeginInvoke(() =>
{
for (int i = 0; i < tempM.Count(); i++)
{
MessagesClass.Instance.Messages.Add(tempM[i]);
}
});
}
The output is:
SIZE OF TEMP:3
A first chance exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll
in the line: MessagesClass.Instance.Messages.Add(tempM[0]); where the code tries to access the first element of tempM
Any hints whats wrong? Why can't I access the tempM elements, although the collection size is > 0?

Answer 1

You forget about thread synchronization. Look at your code:
1: Debug.WriteLine("SIZE OF TEMP:" + tempM.Count());
Dispatcher.BeginInvoke(() =>
{
// add 3 messages at once
3: MessagesClass.Instance.Messages.Add(tempM[0]);
MessagesClass.Instance.Messages.Add(tempM[1]);
MessagesClass.Instance.Messages.Add(tempM[2]);
});
2: tempM = new Collection<MessageViewModel>();
tempM already will be null when MessagesClass.Instance.Messages.Add(tempM[0]); is executed. So, use some sort or synchronization objects, for example:
EventWaitHandle Wait = new AutoResetEvent(false);
Debug.WriteLine("SIZE OF TEMP:" + tempM.Count());
Dispatcher.BeginInvoke(() =>
{
// add 3 messages at once
MessagesClass.Instance.Messages.Add(tempM[0]);
MessagesClass.Instance.Messages.Add(tempM[1]);
MessagesClass.Instance.Messages.Add(tempM[2]);
Wait.Set();
});
// wait while tempM is not in use anymore
Wait.WaitOne();
tempM = new Collection<MessageViewModel>();