网络连接状态来判断，网线是不是被拔掉了

添加引用：
System.Management.dll


using System.Management;

写个线程执行下面的代码，你也可以1秒钟检查一次

ManagementClass mc = new ManagementClass(@"Win32_NetworkAdapter");
ManagementObjectCollection moc = mc.GetInstances();
foreach (ManagementObject mo in moc)
{
    string name = (string)mo["Name"];
    object val = mo["NetConnectionStatus"];
    if (val != null)
        Console.WriteLine(" {0}/n/tConnection Status:{1}", name, (ushort)val);
    else
        Console.WriteLine(" {0}/n", name);
}