Question

A control is accessed by two worker threads, the 2nd before the first has completed it’s work on the control. The second thread (9) gets InvokeRequired == false, and the first thread (17) then gets the exception when calling .Refresh on a child-control.

Is this expected behavior? What exactly causes a thread to see a control’s InvokeRequired as true/false?
And finally, what would be a good solution.. Put a lock on all invoke statements, and make them call a separate method instead (to avoid deadlock obviously) ?

private void OnHistoryUpdate(object sender)
{
    Console.WriteLine("<< Invoke? " + this.InvokeRequired   + " " + Thread.CurrentThread.ManagedThreadId );

    if (this.InvokeRequired)
        this.Invoke(new Action<object>(OnHistoryUpdate), sender);  

    LoadTimeSeries(this.Interval);
    Console.WriteLine(">> Invoke? " + this.InvokeRequired   + " " + Thread.CurrentThread.ManagedThreadId);

}

output:

<< Invoke? True Thread: 17
<< Invoke? False Thread:  9
>> Invoke? False Thread:  9

Answer 1

Your code updates the control twice:

• Using the call to Invoke, which delegates another call to OnHistoryUpdate. This is safe because Invoke marshals execution to the UI thread.
• After the call to Invoke. Invoke executes synchronously and when it returns your code will call LoadTimeSeries a second time, this time unsafely (it doesn’t check InvokeRequired again).

I would change the method as below, and also consider using BeginInvoke instead so the worker thread doesn’t block.

private void OnHistoryUpdate(object sender)
{
    Console.WriteLine("<< Invoke? " + this.InvokeRequired   + " " + Thread.CurrentThread.ManagedThreadId );

    if (this.InvokeRequired)
        this.Invoke(new Action<object>(OnHistoryUpdate), sender);
    else  
        LoadTimeSeries(this.Interval);

    Console.WriteLine(">> Invoke? " + this.InvokeRequired   + " " + Thread.CurrentThread.ManagedThreadId);    
}