将参数传递到backgroundWorker（用作取消按钮）

Question

一般来说，我是 C# 和面向对象编程的新手。我一直在尝试在 GUI 中实现“取消”按钮，以便用户可以在过程中停止它。

我读过这个问题：如何实现停止/取消按钮？并确定backgroundWorker对我来说应该是一个不错的选择，但给出的示例没有解释如何将参数传递给backgroundWorker。

我的问题是我不知道如何将参数传递给backgroundWorker以使其停止进程；我只能让backgroundWorker 自行停止。

我创建了以下代码来尝试学习这一点，其中我的表单有两个按钮（buttonStart 和buttonStop）和一个backgroundWorker（backgroundWorkerStopCheck）：

using System;
using System.ComponentModel;
using System.Windows.Forms;
using System.Threading;
using System.Timers;

namespace TestBackgroundWorker
{
    public partial class Form1 : Form
    {
        public Form1()
        {         
            InitializeComponent();

            // Set the background worker to allow the user to stop the process. 
            backgroundWorkerStopCheck.WorkerSupportsCancellation = true;
        }

        private System.Timers.Timer myTimer;

        private void backgroundWorkerStopCheck_DoWork(object sender, DoWorkEventArgs e)
        {
            //If cancellation is pending, cancel work.  
            if (backgroundWorkerStopCheck.CancellationPending)
            {
                e.Cancel = true;
                return;
            }
        }

        private void buttonStart_Click(object sender, EventArgs e)
        {
            // Notify the backgroundWorker that the process is starting.
            backgroundWorkerStopCheck.RunWorkerAsync();
            LaunchCode();
        }

        private void buttonStop_Click(object sender, EventArgs e)
        {
            // Tell the backgroundWorker to stop process.
            backgroundWorkerStopCheck.CancelAsync();
        }

        private void LaunchCode()
        {
            buttonStart.Enabled = false; // Disable the start button to show that the process is ongoing.
            myTimer = new System.Timers.Timer(5000); // Waste five seconds.
            myTimer.Elapsed += new ElapsedEventHandler(myTimer_Elapsed);
            myTimer.Enabled = true; // Start the timer.
        }

        void myTimer_Elapsed(object sender, ElapsedEventArgs e)
        {
            buttonStart.Enabled = true; // ReEnable the Start button to show that the process either finished or was cancelled.
        }
    }
}

如果代码正常工作，则在用户单击“Start”后，该代码将在那里停留五秒钟在重新启用“开始”按钮之前，或者如果用户单击“停止”，将快速重新激活“开始”按钮。

这段代码有两个问题我不确定如何处理：

1）“myTimer_Elapsed”方法在尝试启用“开始”按钮时会导致 InvalidOperationException，因为“跨线程操作无效”。如何避免跨线程操作？

2）现在backgroundWorker没有完成任何事情，因为我不知道如何向它提供参数，这样当它被取消时，它会停止计时器。

我将不胜感激任何帮助！

Answer 1

首先，避免“跨线程操作无效”的问题是使用 调用控件。您不能使用来自不同线程的控件。

关于第二个问题，我会按照以下方式实现。这是具有取消支持的最小后台工作实现。

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication5
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();

            // Set the background worker to allow the user to stop the process. 
            backgroundWorkerStopCheck.WorkerSupportsCancellation = true;
            backgroundWorkerStopCheck.DoWork += new DoWorkEventHandler(backgroundWorkerStopCheck_DoWork);
        }

        private void backgroundWorkerStopCheck_DoWork(object sender, DoWorkEventArgs e)
        {
            try
            {
                for (int i = 0; i < 50; i++)
                {
                    if (backgroundWorkerStopCheck.CancellationPending)
                    {
                        // user cancel request
                        e.Cancel = true;
                        return;
                    }

                    System.Threading.Thread.Sleep(100);
                }
            }
            finally
            {
                InvokeEnableStartButton();
            }
        }

        private void buttonStart_Click(object sender, EventArgs e)
        {
            //disable start button before launch work
            buttonStart.Enabled = false;

            // start worker
            backgroundWorkerStopCheck.RunWorkerAsync();
        }

        private void buttonStop_Click(object sender, EventArgs e)
        {
            // Tell the backgroundWorker to stop process.
            backgroundWorkerStopCheck.CancelAsync();
        }

        private void InvokeEnableStartButton()
        {
            // this method is called from a thread,
            // we need to Invoke to avoid "cross thread exception"
            if (this.InvokeRequired)
            {
                this.Invoke(new EnableStartButtonDelegate(EnableStartButton));
            }
            else
            {
                EnableStartButton();
            }
        }

        private void EnableStartButton()
        {
            buttonStart.Enabled = true;
        }
    }

    internal delegate void EnableStartButtonDelegate();
}

关于向工作线程传递参数，您可以在 RunWorkerAsync() 方法中传递任何对象，并在 backgroundWorkerStopCheck_DoWork 方法中接收它：

  ...
  backgroundWorkerStopCheck.RunWorkerAsync("hello");
  ...

  private void backgroundWorkerStopCheck_DoWork(object sender, DoWorkEventArgs e)
  {
      string argument = e.Argument as string;
      // argument value is "hello"
      ...
  }

希望它有所帮助。

Answer 2

尝试这个示例，您将看到如何将数据传入和传出BackgroundWorker：

public partial class Form1 : Form
{
    BackgroundWorker bw = new BackgroundWorker();

    public Form1()
    {
        InitializeComponent();

        bw.DoWork += new DoWorkEventHandler(bw_DoWork);
        bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
        bw.WorkerSupportsCancellation = true;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        btnStart.Enabled = false;
        btnCancel.Enabled = true;

        double[] data = new double[1000000];
        Random r = new Random();
        for (int i = 0; i < data.Length; i++)
            data[i] = r.NextDouble();

        bw.RunWorkerAsync(data);
    }

    void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        btnStart.Enabled = true;
        btnCancel.Enabled = false;

        if (!e.Cancelled)
        {
            double result = (double)e.Result;
            MessageBox.Show(result.ToString());
        }
    }

    void bw_DoWork(object sender, DoWorkEventArgs e)
    {
        double[] data = (double[])e.Argument;

        for (int j = 0; j < 200; j++)
        {
            double result = 0;
            for (int i = 0; i < data.Length; i++)
            {
                if (bw.CancellationPending)
                {
                    e.Cancel = true;
                    return;
                }
                result += data[i];
            }
            e.Result = result;
        }
    }

    private void btnCancel_Click(object sender, EventArgs e)
    {
        bw.CancelAsync();
        btnStart.Enabled = true;
        btnCancel.Enabled = false;
    }
}