System.Environment.Exit(0); 这是最彻底的退出方式，不管什么线程都被强制退出，把程序结束的很干净。
private void button6_Click(object sender, EventArgs e)
{
textBox8.Text = "";
if (!m_bNetState)
{
m_bNetState = true;
button6.Enabled = true;
StartNetwork();
pictureBox17.Image = imageList4.Images[1];
button6.Text = "停止网络服务";
textBox8.Text = "";
textBox8.Text = "与服务器连接成功......";
button7.Enabled = true;
button8.Enabled = true;
button9.Enabled = true;
}
else
{
m_bNetState = false;
button6.Enabled = true;
//stopNetwork();
pictureBox17.Image = imageList4.Images[0];
button6.Text = "启动网络服务";
textBox8.Text = "";
myTimer.Enabled = false; //关闭自动刷新
textBox8.Text = "启动自动刷新";
button7.Enabled = false;
button8.Enabled = false;
button9.Enabled = false;
client.Shutdown(SocketShutdown.Both);
client.Close();
System.Environment.Exit(0); //是最彻底的退出方式，不管什么线程都被强制退出，把程序结束的很干
}
停止网络连接的时候，加上最后一句
