Question

I have a BindingList databound to a datgridview. I’m using it to keep track of some real-time prices. The method ‘update(Quote quote)’ is called multiple times a second by various threads. If the datagridview doesn’t contain the Quote, it is added. If it does, the values of the quote are updated. I don’t want the same quote to appear in the BindingList (or on the GUI) twice, so I tried to put a lock around the operation that checks whether the value is in the list or not. It doesn’t work! What am I doing wrong? I’ve tried two different ways of locking, and am locking on a String object rather than just an object. The problem is definitely in the BeginInvoke(new MethodInvoker(delegate() { activeQuotes.Insert(0, quote); })); call (which is probably taking some time), but if I make that synchronous, the ‘add’ method throws a ‘cross-threading’ error. . . What can I do to avoid the cross-threading error, but ensure that the lock works also??

public BindingList<Quote> activeQuotes = new BindingList<Quote>();
object lockObject = "lockObject";

dataGridViewActive.DataSource = activeQuotes;


public void update(Quote quote)
{
//lock (lockObject)
if(Monitor.TryEnter(lockObject))
{
try
   {
    if (!activeQuotes.Contains(quote))
    {
       try
       {
          activeQuotes.Add(quote);
          AddQuote(quote);
       }
       catch (Exception ex)
       {
          Console.WriteLine("Datagridview!!!!!!");
       }
     }
 else
 {
   int index = activeQuotes.IndexOf(quote);
   activeQuotes[index].Bid = quote.Bid;
   activeQuotes[index].Ask = quote.Ask;
   activeQuotes[index].Mid = quote.Mid;
   activeQuotes[index].Spread = quote.Spread;
   activeQuotes[index].Timestamp = quote.Timestamp;
}
finally
{
    Monitor.Exit(lockObject);
}
}

private void AddQuote(Quote quote)
{
     if (this.InvokeRequired)
     {
                BeginInvoke(new MethodInvoker(delegate() { activeQuotes.Insert(0, quote); }));
                BeginInvoke(new MethodInvoker(delegate() { dataGridViewActive.Refresh(); }));
                BeginInvoke(new MethodInvoker(delegate() { dataGridViewActive.AutoResizeColumns(DataGridViewAutoSizeColumnsMode.AllCells); }));
    }
    else
    {
                activeQuotes.Add(quote);
                dataGridViewActive.Refresh();
                dataGridViewActive.AutoResizeColumns (DataGridViewAutoSizeColumnsMode.AllCells);
     }
}

I’d appreciate any help at all on this.

Thanks.

Answer 1

I think you should change your BeginInvoke to just Invoke. You need to get it on the UI thread, not begin an async operation. Otherwise your lock could get released before the BeginInvoke target gets invoked because control is returned immediately upon calling BeginInvoke. Calling Invoke will block that thread on that call until the Invoke target completes then return control back to your thread, which will ensure the lock is kept.

Also, have you considered using a lock block instead of Monitor method calls? It’s basically the same thing but prevents you from needing the try/finally. I don’t see that you’re using any retry or benefit from the TryEnter, but perhaps the code sample doesn’t demonstrate that.