可见BackGroundWorker有三个事件，事件间通过EventArgs实现参数传递。有用到CancellationPending属性判断取消。EvnentArgs 有很多属性：Cancel、Argument、Result、ProgressPercentage、UserState属性等。

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

namespace BackGroundWorkDemo
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.backgroundWorker1.WorkerReportsProgress = true;  // 这两个属性也可以去控件的属性栏里面改
            this.backgroundWorker1.WorkerSupportsCancellation = true;
            this.StartPosition = FormStartPosition.CenterScreen;
        }

        // Backgroundworker的三个事件

        // [1]开始后台要执行的任务，可见参数都是通过 EventArgs 参数传递的。
        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker bgw = sender as BackgroundWorker;

            int result = 0;
            for (int i = 1; i <= 100; i++) {
                if (bgw.CancellationPending) {// 如果取消操作
                    e.Cancel = true; // 把取消结果传出去，传给 RunWorkerComplete事件中。
                    return;
                }
                result += i;// 累加
                bgw.ReportProgress(i, "已经完成了" + i.ToString() + "%"); // 报告进度，第二个参数是： UserState。传给 ProgressChanged 事件。
                System.Threading.Thread.Sleep((int)e.Argument);//根据外部传入参数休眠，模拟计算耗时。
                e.Result = result; // 把计算结果传出去
            }
        }
        // [2]/汇报后台工作进展
        private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            this.progressBar1.Value = e.ProgressPercentage;// 显示百分比
            this.lblProgressPercent.Text = e.UserState.ToString();// 显示进度条上面的百分信息
        }
        // [3]后台任务完成
        private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if (e.Cancelled) {
                toolStripStatusLabel1.Text = "用户取消操作";
                btnBegin.Enabled = false;
                return;
            }
            if (e.Error != null) {
                toolStripStatusLabel1.Text = e.Error.Message;
                return;
            }

            this.lblResult.Text = e.Result.ToString(); // 显示结果
            this.btnBegin.Enabled = true;
            toolStripStatusLabel1.Text = "计算完成";
        }

        // 开启任务
        private void btnBegin_Click(object sender, EventArgs e)
        {
            lblResult.Text = "???";
            lblResult.Refresh();

            // 启动后台程序(参数argument传给休眠时间)
            backgroundWorker1.RunWorkerAsync(20);
            btnBegin.Enabled = false;
            toolStripStatusLabel1.Text = "稍等，正在计算…";
        }
        // 取消
        private void btnCancel_Click(object sender, EventArgs e)
        {
            backgroundWorker1.CancelAsync();
        }
        // 关闭
        private void btnClose_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}

运行效果：