How can I multithread (effectively) on Windows Forms?

This one is giving me a hard time. The thing is that I have a code that plays some notes in MIDI, and I wanted to be able to pause it, so I made a simple Form like this:

namespace Music
{
    public partial class Form1 : Form
    {

        static BackgroundWorker _bw = new BackgroundWorker
        {
            WorkerSupportsCancellation = true
        };

        private void button1_Click(object sender, EventArgs e)
        {
            if (!Playing)
            {
                Playing = true;
                _bw.DoWork += Start_Playing;
                _bw.RunWorkerAsync("Hello to worker");
            }
            else
            {
                Playing = false;
                _bw.CancelAsync();
            }
        }

        static void Start_Playing(object sender, DoWorkEventArgs e)
        {
            //Plays some music
        }
    }
}

And when I click it 开发者_JAVA技巧starts playing, but no matter what I do, it can't stop. But the thing is that if I do the same thing in the console it works perfect.

Did I miss something? How can I control a separate thread from the form?

This seems to work...

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

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        private BackgroundWorker _bw = new BackgroundWorker { WorkerSupportsCancellation = true, 
            WorkerReportsProgress = true};
        public Form1()
        {
            InitializeComponent();

        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (_bw.IsBusy)
            {
                _bw.CancelAsync();
            }
            else
            {
                _bw.ProgressChanged += new ProgressChangedEventHandler(_bw_ProgressChanged);
                _bw.DoWork += new DoWorkEventHandler(_bw_DoWork);
                _bw.RunWorkerAsync();
            }
        }

        void _bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            textBox1.Text += (string)e.UserState;
        }

        void _bw_DoWork(object sender, DoWorkEventArgs e)
        {
            int count = 0;
            while (!_bw.CancellationPending)
            {
                _bw.ReportProgress(0, string.Format("worker working {0}", count));
                ++count;
                Thread.Sleep(2000);
            }
        }
    }
}