Question

I need some help trying to figure what I’m doing wrong. I’m trying to get a collection of items from the system log on a separate thread to keep the form from being frozen during the collection process. I can get the background worker to grab them all, but I am having some issues add them to the ListBox on the form.

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{

  foreach (System.Diagnostics.EventLogEntry entry in eventLog1.Entries)
  {
     listBox1.Items.Add(
        entry.EntryType.ToString() + " - " + 
        entry.TimeWritten + "     - " + 
        entry.Source);
  }
}

Obviously this doesn’t work as expected, since there are 2 separate threads, and you can’t change objects on different threads, as I have found out. So, If someone could guide me in the right direction, I would be thankful.

Answer 1

You should not access UI elements from non-UI thread. Run ReportProgress, which will be synced with UI thread.

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    foreach (System.Diagnostics.EventLogEntry entry in eventLog1.Entries)
    {
        var newEntry = entry.EntryType + " - " + entry.TimeWritten + "     - " + entry.Source;
        backgroundWorker1.ReportProgress(0, newEntry);
    }
}

void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    var newEntry = (string)e.UserState;
    listBox1.Items.Add(newEntry);
}

Make sure you enable WorkerReportsProgress.

backgroundWorker1.WorkerReportsProgress = true;

and subscribed to ProgressChanged

backgroundWorker1.ProgressChanged += backgroundWorker1_ProgressChanged;

Another approach is to call Control.Invoke inside

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    foreach (System.Diagnostics.EventLogEntry entry in eventLog1.Entries)
    {
        var newEntry = entry.EntryType.ToString() + " - " + entry.TimeWritten + "     - " + entry.Source;
        Action action = () => listBox1.Items.Add(newEntry);
        Invoke(action);
    }
}

But with this approach you don’t need BackgroundWorker as whole point of it is to use ProgressChanged and RunWorkerCompleted event handler which are synced with the UI thread.