c# - Cancel Async Task from a button

Question

What I need to do is be able to cancel a task that is running async.

I have been searching and cannot seem to wrap my head around it. I just cant seem to discern how it would be implemented into my current setup.

Here is my code that fires my task off. Any help on where or how to implement a cancellation token would be greatly appreciated.

    private async void startThread()
    {
        //do ui stuff before starting
        ProgressLabel.Text = String.Format("0 / {0} Runs Completed", index.Count());
        ProgressBar.Maximum = index.Count();

        await ExecuteProcesses();

        //sort list of output lines
        outputList = outputList.OrderBy(o => o.RunNumber).ToList();

        foreach (Output o in outputList)
        {
            string outStr = o.RunNumber + "," + o.Index;
            foreach (double oV in o.Values)
            {
                outStr += String.Format(",{0}", oV);
            }

            outputStrings.Add(outStr);
        }

        string[] csvOut = outputStrings.ToArray();

        File.WriteAllLines(settings.OutputFile, csvOut);
        //do ui stuff after completing.

        ProgressLabel.Text = index.Count() + " runs completed. Output written to file test.csv";
    }

    private async Task ExecuteProcesses()
    {
        await Task.Factory.StartNew(() =>
        {
            int myCount = 0;
            int maxRuns = index.Count();
            List<string> myStrings = index;
            Parallel.ForEach(myStrings,
                new ParallelOptions()
                {
                    MaxDegreeOfParallelism = settings.ConcurrentRuns
                }, (s) =>
                {
                    //This line gives us our run count.
                    int myIndex = myStrings.IndexOf(s) + 1;

                    string newInputFile = Path.Combine(settings.ProjectPath + "files/", Path.GetFileNameWithoutExtension(settings.InputFile) + "." + s + ".inp");
                    string newRptFile = Path.Combine(settings.ProjectPath + "files/", Path.GetFileNameWithoutExtension(settings.InputFile) + "." + s + ".rpt");

                    try
                    {
                        //load in contents of input file
                        string[] allLines = File.ReadAllLines(Path.Combine(settings.ProjectPath, settings.InputFile));


                        string[] indexSplit = s.Split('.');

                        //change parameters here
                        int count = 0;
                        foreach (OptiFile oF in Files)
                        {
                            int i = Int32.Parse(indexSplit[count]);
                            foreach (OptiParam oP in oF.Parameters)
                            {
                                string line = allLines[oP.LineNum - 1];
                                if (oP.DecimalPts == 0)
                                {
                                    string sExpression = oP.Value;
                                    sExpression = sExpression.Replace("%i", i.ToString());
                                    EqCompiler oCompiler = new EqCompiler(sExpression, true);
                                    oCompiler.Compile();
                                    int iValue = (int)oCompiler.Calculate();

                                    allLines[oP.LineNum - 1] = line.Substring(0, oP.ColumnNum - 1) + iValue.ToString() + line.Substring(oP.ColumnNum + oP.Length);
                                }
                                else
                                {
                                    string sExpression = oP.Value;
                                    sExpression = sExpression.Replace("%i", i.ToString());
                                    EqCompiler oCompiler = new EqCompiler(sExpression, true);
                                    oCompiler.Compile();
                                    double dValue = oCompiler.Calculate();
                                    dValue = Math.Round(dValue, oP.DecimalPts);

                                    allLines[oP.LineNum - 1] = line.Substring(0, oP.ColumnNum - 1) + dValue.ToString() + line.Substring(oP.ColumnNum + oP.Length);
                                }
                            }
                            count++;
                        }
                        //write new input file here
                        File.WriteAllLines(newInputFile, allLines);
                    }
                    catch (IOException ex)
                    {
                        MessageBox.Show(ex.ToString());
                    }


                    var process = new Process();
                    process.StartInfo = new ProcessStartInfo("swmm5.exe", newInputFile + " " + newRptFile);
                    process.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
                    process.Start();
                    process.WaitForExit();

                    Output output = new Output();
                    output.RunNumber = myIndex;
                    output.Index = s;
                    output.Values = new List<double>();

                    foreach(OutputValue oV in OutputValues) {
                         output.Values.Add(oV.getValue(newRptFile));
                    }

                    outputList.Add(output);

                    //get rid of files after run
                    File.Delete(newInputFile);
                    File.Delete(newRptFile);

                    myCount++;
                    ProgressBar.BeginInvoke(
                        new Action(() =>
                            {
                                ProgressBar.Value = myCount;
                            }
                    ));
                    ProgressLabel.BeginInvoke(
                        new Action(() =>
                            {
                                ProgressLabel.Text = String.Format("{0} / {1} Runs Completed", myCount, maxRuns);
                            }
                    ));
                });
        });
    }

Answer 1

The best way to support cancellation is to pass a CancellationToken to the async method. The button press can then be tied to cancelling the token

class TheClass
{
  CancellationTokenSource m_source;

  void StartThread() { 
    m_source = new CancellationTokenSource;
    StartThread(m_source.Token);
  }

  private async void StartThread(CancellationToken token) { 
    ...
  }

  private void OnCancelClicked(object sender, EventArgs e) {
    m_source.Cancel();
  }
}

This isn't quite enough though. Both the startThread and StartProcess methods will need to be updated to cooperatively cancel the task once the CancellationToken registers as cancelled