CODEWITHSUNDEEP
c#.netwpfmultithreadingbackgroundworker
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Reputation: 17055

How to handle exceptions from a BackgroundWorker thread?

In a WPF app I have a sheduled database access task, periodically run by a timer and this task have been executed in a BackgroundWorker thread.

When connection attempt failed I raise an exception by try_catch construction and I want to update a Status Bar text in a UI thread.

Is there some prebuild event construction in a BackgroundWorker for implementing this, something like DoWorkEventHandler or RunWorkerCompletedEventHandler, which can be used for this? If not, how to do it better?

Edited (added):

If I want to handle the exception inside RunWorkerCompletedEventHandler, using e.Error parameter, it doesn't work. In case I leave exception unhandled in the BackgroundWorker thread, application hangs on and debugger points to the string of code which is excuted inside BackgroundWorker thread, saying that: Exception was unhandled by user code.

So, in this case, thread doesn't just stop, signalling to RunWorkerCompletedEventHandler that it stopped with error, but the whole application stop working.

Upvotes: 9

Views: 10679

Answers (5)

Kyle Delaney
Kyle Delaney

Reputation: 12264

If I want to handle the exception inside RunWorkerCompletedEventHandler, using e.Error parameter, it doesn't work. In case I leave exception unhandled in the BackgroundWorker thread, application hangs on and debugger points to the string of code which is excuted inside BackgroundWorker thread, saying that: Exception was unhandled by user code.

So, in this case, thread doesn't just stop, signalling to RunWorkerCompletedEventHandler that it stopped with error, but the whole application stop working.

I'm very curious about this. Try running the following code in a console app and see if your program crashes or if the error shows up in the RunWorkerCompletedEventArgs.

    static void Main(string[] args)
    {
        var bw = new BackgroundWorker();
        bw.DoWork += Bw_DoWork;
        bw.RunWorkerCompleted += Bw_RunWorkerCompleted;
        bw.RunWorkerAsync();

        Console.ReadKey();
    }

    private static void Bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        Console.WriteLine("Complete.");
        Console.WriteLine(e.Error);
    }

    private static void Bw_DoWork(object sender, DoWorkEventArgs e)
    {
        throw new NotImplementedException();
    }

Upvotes: 0

IordanTanev
IordanTanev

Reputation: 6240

The RunWorkerCompletedEventArgs e of the RunWorkerCompletedEventHandler contains property Error it is of type Exception. If no exception occurred during the work of the background thread the prpery has null as value. Else it contains the error that occurred.

Upvotes: 7

Doug Ferguson
Doug Ferguson

Reputation: 2618

A WPF UI can be updated from a background thread by using Dispatcher.BeginInvoke.

For example if your background code was part of a Window then you could update a TextBlock:

this.Dispatcher.BeginInvoke((Action)(() =>
    {
        textBlock.Text = "Connection Failed!";
    }));

Edit:

If your background code were in a class other than your Window you could make an interface to help:

public interface IShowStatus
{
    void ShowStatus(string message);
}

Implement the interface in your Window

public void ShowStatus(string message)
{
   this.Dispatcher.BeginInvoke((Action)(() =>
       {
           textBlock.Text = message;
       }));
}

In your class with the background worker make a property to hold a reference to the interface.

public IShowStatus StatusDisplay { get; set; }

In your Window class initialize the background class.

public void InitBackground()
{
    BackgroundClass background = new BackgroundClass();
    background.StatusDisplay = this;
    ...

Finally in your background thread you can say:

StatusDisplay.ShowStatus("Connection Failed!");

Upvotes: 5

Sameh Deabes
Sameh Deabes

Reputation: 2973

Set the WorkerReportsProgress property of the background worker to true, then add an event handler for the event ProgressChanged. In the following code, I added an event handler for Form.Load also.

Now try the following code:

private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            MessageBox.Show(e.UserState.ToString());
        }

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            try
            {
                //some code that throws an exception
                throw new NotImplementedException();
            }
            catch (Exception ex) 
            {
                backgroundWorker1.ReportProgress(0/*percent of progress*/, ex);

            }
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            backgroundWorker1.RunWorkerAsync();
        }

Upvotes: 3

Ken
Ken

Reputation: 1880

private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
     Exception exceptionThrowDuringDoWorkEventHandler = e.Error;
}

Upvotes: 3

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