C# BackgroundWorker.CancelAsync() 与 DoWorkEventArgs.Cancel

Question

I want to use the RunWorkerCompletedEventArgs.Cancelled value in my BackgroundWorker completed handler, but the documentation isn't clear how how BackgroundWorker.CancelAsync() and DoWorkEventArgs.Cancel (in the background worker do work handler) will each effect it. Are they functionally the same?

For example, is this...

private void _Worker_DoWork(object sender, DoWorkEventArgs e)
{
    (sender as BackgroundWorker).CancelAsync();
}

...equivalent to this?

private void _Worker_DoWork(object sender, DoWorkEventArgs e)
{
    e.Cancel = true;
}

Will the latter result in subsequent evaluations of CancellationPending() to evaluate as true? Also, if the background worker is cancelled externally (ie myBW.CancelAsync(); outside the do work handler), will e.Cancel = false cause an evaluation of CancellationPending() to be false?

Answer 1

BackgroundWorker.CancelAsync() will set the value of BackgroundWorker.CancellationPending to true, so the DoEvent code can check it.

DoWorkEventArgs.Cancel is there to tell RunWorkerCompleted Event that the process was Canceled. You are not supposed to use the result of a operation that was aborted or ended in Exception. Setting DoWorkEventArgs.Cancel to true will set RunWorkerCompletedEventArgs.Canceled to true. Wich will also force RunWorkerCompletedEventArgs.Result to throw an exception if accessed .

I have some pretty old example code from when I learned Multithrading with BGW. It should help you.

#region Primenumbers
private void btnPrimStart_Click(object sender, EventArgs e)
{
    if (!bgwPrim.IsBusy)
    {
        //Prepare ProgressBar and Textbox
        int temp = (int)nudPrim.Value;
        pgbPrim.Maximum = temp;
        tbPrim.Text = "";

        //Start processing
        bgwPrim.RunWorkerAsync(temp);
    }
}

private void btnPrimCancel_Click(object sender, EventArgs e)
{
    if (bgwPrim.IsBusy)
    {
        bgwPrim.CancelAsync();
    }
}

private void bgwPrim_DoWork(object sender, DoWorkEventArgs e)
{
    int highestToCheck = (int)e.Argument;
    //Get a reference to the BackgroundWorker running this code
    //for Progress Updates and Cancelation checking
    BackgroundWorker thisWorker = (BackgroundWorker)sender;

    //Create the list that stores the results and is returned by DoWork
    List<int> Primes = new List<int>();


    //Check all uneven numbers between 1 and whatever the user choose as upper limit
    for(int PrimeCandidate=1; PrimeCandidate < highestToCheck; PrimeCandidate+=2)
    {
        //Report progress
        thisWorker.ReportProgress(PrimeCandidate);
        bool isNoPrime = false;

        //Check if the Cancelation was requested during the last loop
        if (thisWorker.CancellationPending)
        {
            //Tell the Backgroundworker you are canceling and exit the for-loop
            e.Cancel = true;
            break;
        }

        //Determin if this is a Prime Number
        for (int j = 3; j < PrimeCandidate && !isNoPrime; j += 2)
        {
            if (PrimeCandidate % j == 0)
                isNoPrime = true;
        }

        if (!isNoPrime)
            Primes.Add(PrimeCandidate);
    }

    //Tell the progress bar you are finished
    thisWorker.ReportProgress(highestToCheck);

    //Save Return Value
    e.Result = Primes.ToArray();
}

private void bgwPrim_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    pgbPrim.Value = e.ProgressPercentage;
}

private void bgwPrim_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    pgbPrim.Value = pgbPrim.Maximum;
    this.Refresh();

    if (!e.Cancelled && e.Error == null)
    {
        //Show the Result
        int[] Primes = (int[])e.Result;

        StringBuilder sbOutput = new StringBuilder();

        foreach (int Prim in Primes)
        {
            sbOutput.Append(Prim.ToString() + Environment.NewLine);
        }

        tbPrim.Text = sbOutput.ToString();
    }
    else 
    {
        tbPrim.Text = "Operation canceled by user or Exception";
    }
}
#endregion

Personally I consider the BackgroundWorker class in a GUI to be good Multitasking "Training Wheels".

Answer 2

No, they are not the same. The "CancelAsync()" method runs from outside of the "backgroundworker"'s code. The "CancellationPending" can be check in the "DoWork" body and "e.Cancel" is set in "DoWork" to be used in the "Completed" method.
Please see the page below for more information:
( https://www.c-sharpcorner.com/uploadfile/mahesh/backgroundworker-in-C-Sharp/ )