Question

Background

From the valuable advice I received here I have now moved all of my database intensive code to a backgroundworker, specifically the direct calls to the database. That code is executed during the backgroundworker’s DoWork event. If a DataTable is returned during the DoWork event, I set that DataTable to a class-wide variable. This is done, to avoid having to invoke the controls requiring the DataTable every time I run this code.

While that code is being executed, I have a label that is updated in the main UI thread, to let the user know that something is occurring. To update the label I use a timer, such that every 750 ms a “.” is appended to the label’s string.

The first thing that I noticed was that the backgroundworker’s RunWorkerCompleted event wasn’t triggering. To solve this I did an Application.DoEvents(); before each call I made to the backgroundworker. It was ugly, but it caused the event to trigger. If anyone has an alternative to fix this, I am all ears.

I then came across an interesting predicament. If I run the program within Visual Studio 2010, in the debugging mode, I get an InvalidOperationException error stating that the “Cross-thread operation not valid: Control ‘lblStatus’ accessed from a thread other than the thread it was created on.” This error occurs during the backgroundworker’s RunWorkerCompleted event, where I set the text of a label in the main UI thread. But, when I launch the application directly, through the executable, it works exactly as desired (i.e. the label’s text is set correctly).

Question

Can anyone explain what is going on / offer advice on how to improve upon this?

Code

I can’t post all of the code involved, but here’s some relevant stuff:

namespace Test
{
    public partial class frmMain : Form
    {
        public static Boolean bStatus = false;
        static Boolean bTimer = false;
        System.Timers.Timer MyTimer = new System.Timers.Timer();

        public frmMain()
        {
            InitializeComponent();
            MyTimer.Elapsed += new System.Timers.ElapsedEventHandler(MyTimer_Elapsed);
            MyTimer.Interval = 750; // Every 3/4 of a second
            ExampleTrigger();
        }

        /// <Insert>Lots of unshown code here</Insert>

        private void ExampleTrigger()
        {
            // This is used to simulate an event that would require the backgroundworker
            Application.DoEvents();
            bgw.RunWorkerAsync(0);
            WaitText("Example - 1");
        }

        private static void MyTimer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
        {
            bTimer = true;
        }

        // Update status text
        private void WaitText(string txt)
        {
            MyTimer.Enabled = true;
            lblStatus.Text = txt;
            bStatus = false;
            while (!bStatus)
            {
                if (bTimer)
                {
                    txt = txt + ".";
                    lblStatus.Text = txt;
                    lblStatus.Update();
                    bTimer = false;
                }
            }
            MyTimer.Enabled = false;
        }

        private void bgw_DoWork(object sender, DoWorkEventArgs e)
        {
            int iSelect = (int)e.Argument;
            switch (iSelect)
            {
                case 0:
                    // Hit the database
                    break;
                /// <Insert>Other cases here</Insert>
                default:
                    // Do something magical!
                    break;
            }
        }
        private void bgw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            bStatus = true;
            lblStatus.Text = "Ready!";  // This is where the exception occurs!
        }
    }
}

Answer 1

Never run a while() loop like that in the UI thread.
You’re freezing the UI until the loop terminates; this defeats the purpose.

In addition, System.Timers.Timer doesn’t run callbacks in the UI thread.
Use a WinForms Timer instead.

Once you switch to a WinForms timer, you can simply append to the label inside the timer callback, and disable the timer when the operation finishes.