c# - 在 C# 中取消任务

Question

我有一个带有两个按钮(开始、停止)的表单。

当我按下 Start 按钮时，一个 Task 被初始化并调用一个函数，该函数一直运行直到按下 Stop 按钮。

但是当我按下停止 按钮时，表单会卡住。为什么？我从 StackOverflow 复制了片段但它仍然卡住表格。那么请告诉我如何正确取消任务？

public partial class Form1 : Form
{
    private readonly CancellationTokenSource _cts = new CancellationTokenSource();
    private Task _task;

    public Form1()
    {
        InitializeComponent();
    }

    //Funtion that runs when Task is initialized.
    private void EventLoop(CancellationToken token)
    { 
        //Code here..
        while (!token.IsCancellationRequested)
        {
            //Code here..
        }
        if (token.IsCancellationRequested)
        {
            MessageBox.Show("Operation Complete..!!");
        }

    }

    //Function for Start Button.
    private void Start_Click(object sender, EventArgs e)
    {
        _task = Task.Factory.StartNew(() => EventLoop(_cts.Token), _cts.Token);
    }

    //Function for Stop button.
    private void Stop_Click(object sender, EventArgs e)
    {
         _cts.Cancel();
    }
}

来自 MSDN 的类似示例:

var compute = Task.Factory.StartNew(() =>
{
    return SumRootN(j);
}, tokenSource.Token);`

在按下停止 按钮后形成。token.IsCancellationRequested 为 true。

完整的 EventLoop() 函数。

private void EventLoop(CancellationToken token)
    {
        SerialPort sp = new SerialPort();
        string text, batch, part, courseName;
        text = batch = part = courseName = "";
        int courseId = 0;

        this.Invoke((MethodInvoker)delegate()
        {
            text = portCB.SelectedItem.ToString();
            batch = comboBox2.SelectedItem.ToString();
            part = comboBox3.SelectedItem.ToString();
            courseName = comboBox1.SelectedItem.ToString();
            progressBar1.Value = 20;
            using (Model1 db = new Model1())
            {
                courseId = db.Courses.Where(c => c.Course_name.ToUpper() == courseName.ToUpper()).Select(c => c.Course_Id).Single();
            }
        });

        sp.PortName = text;
        sp.BaudRate = 9600;
        sp.Open();

        while (!token.IsCancellationRequested)
        {
            text = sp.ReadLine();

            if (text.Contains("Found ID #"))
            {
                this.Invoke((MethodInvoker)delegate()
                {
                    textBox2.Clear();
                    textBox2.Text = "Getting Registation ID.\n";
                    progressBar1.Value = 60;
                });

                string splitText = text.Split('#')[1];
                int end = splitText.IndexOf(' ');
                int id = int.Parse(splitText.Substring(0, end));

                using (Model1 db = new Model1())
                {
                    var result = db.Students.Where(s => s.Reg_Id == id && s.Batch == batch && s.Class == part).Select(s => s).SingleOrDefault();
                    if (result != null)
                    {
                        Attendance a = new Attendance();
                        a.Course_Id = courseId;
                        a.Student_Id = id;
                        a.Status = "P";
                        a.Date = DateTime.Today.Date;
                        a.Batch = batch;
                        a.Part = part;

                        db.Attendances.Add(a);
                        string message = "";

                        if (db.SaveChanges() != 0)
                        {
                            message = "Attendance Uploaded..!!\n";
                        }
                        else
                        {
                            message = "Attendance Not Uploaded ..!!\n";
                        }

                        this.Invoke((MethodInvoker)delegate()
                        {
                            progressBar1.Value = 100;
                            textBox2.AppendText(message);
                        });
                    }
                    else
                    {
                        this.BeginInvoke((MethodInvoker)delegate()
                        {
                            textBox2.AppendText("Student Does not belong to Specified Batch Or Part..\n");
                        });
                    }
                }
            }
            else
            {
                this.Invoke((MethodInvoker)delegate()
                {
                    textBox2.AppendText("No Match Found..!! \n");
                });
            }
            this.Invoke((MethodInvoker)delegate()
             {
                 textBox1.AppendText(text + "\n");
             });
        }
        sp.Close();

        // This exception will be handled by the Task
        // and will not cause the program to crash
        if (token.IsCancellationRequested)
        {
            //MessageBox.Show("Operation Comptele..!!");
        }
    }

Answer 1

您在取消过程中调用 MessageBox.Show("Operation Complete..!!");。强烈不建议这样做，这不是说您正在从 UI 线程以外的地方调用 UI 操作。

注释 MessageBox.Show("Operation Complete..!!"); 行

* 编辑 *

问题作者对他的原始问题的评论，发现错误，从帖子中删除了哪一行。这是我的结论:

始终尝试隔离问题，并以最纯粹的形式重现。在此过程中，您可能会自行诊断并发现问题 :-)。

因此，如果有问题的代码很长，则绝对不是删除行然后发布的方法。方法是删除行并查看问题是否存在，换句话说:以最纯粹的可重现形式隔离问题