Threading using BackgroundWorker

Question

I have a scenario. I have a list of user id's displayed on the windows form. As soon as I click one of the user id's I go and get the user details from the DB. To retain the responsiveness of the application, on selection changed event of the listbox, I create new BackgroundWorker(BW) objects and hit the DB. I show 'Searching user 'abc'...' in the status bar.

Now, if the user moves between the user ids using arrow keys (4-5 times), by above design I have created multiple BW objects to make the request. But finally when the data comes back for particular user (which may not be the one where user is currently selected in the listview), since it was async call, I still end up displaying all the users in the status bar.

What I would like to do is, I want to go and get the details only for the last user. Till that time I want to display 'Searching user...' only.

Please let me know solution for this...

Answer 1

When the user switches users, you can cancel the worker processes that are currently running (check to make sure they are running). I believe that would accomplish what you are after.

Answer 2

One option would be to keep a string field in the form containing the user ID. In the background worker, after you hit the DB, check that this field is still equal to the user ID that it got passed in, EDIT: and, if it's different, make the result null. Manipulate the field using the methods on the Interlocked class to avoid the need to lock. That's wrong; reference types can be read and written atomically with Interlocked.

2^nd EDIT: You could also return the original user ID from the background worker along with the result, and check that it's the most recent one clicked in your Completed handler.

Alternatively, if you're keeping references to all of the BackgroundWorkers, you could use their cancellation support.

Answer 3

I've used this to cancel BackgroundWorkers before and it worked great.

    public void cancelWorker( BackgroundWorker worker )
    {
        if (worker != null)
        {
            if (worker.IsBusy)
            {
                worker.CancelAsync();

                while (worker.IsBusy)
                {
                    Application.DoEvents();
                }
            }
        }
    }

I've heard controversy over using Application.DoEvents(); but I had problems with endless loops if I used Thread.sleep or other.

You might want to use polling of some sort so you don't end up with backgroundWorkers fumbling over eachother - especially if the callback ends up calling something like the UI on a separate thread so they can actually cause race conditions.

Answer 4

How about waiting for a second or two before you start your Background Worker?

Once the user clicks on a user id, start a timer with 1 second interval and after that one second, start your BackgroundWorker. If user clicks on a different user id, reset the timer. This way, if user keeps clicking on different user ids in quick succession, you won't do anything. Once user has taken a break, you start your background worker.