[TestMethod]
public void Service_Login_NoInternetConnection()
{
  // Some code...
  TurnOffNetworkConnection();
  // More code...
  TurnOnNetworkConnection();
  // Blablabla code...
}

using System.Management;


    public void TurnOnNetworkConnection()
{

        try
        {
            ManagementObjectSearcher searcher = new ManagementObjectSearcher("root\CIMV2", "SELECT * FROM Win32_NetworkAdapterConfiguration");

               foreach (ManagementObject queryObj in searcher.Get())
               {
                  String Check = Convert.ToString(queryObj["DHCPLeaseObtained"]);
                  if (String.IsNullOrEmpty(Check))
                    {
                    }
                    else
                    {
                    ManagementBaseObject outParams = queryObj.InvokeMethod("RenewDHCPLease", null, null);
                    }
               }
         }
           catch (ManagementException e)
           {
           MessageBox.Show("An error occurred while querying for WMI data: " + e.Message);
           }
}

    public void TurnOffNetworkConnection()
{
        try
        {
            ManagementObjectSearcher searcher = new ManagementObjectSearcher("root\CIMV2", "SELECT * FROM Win32_NetworkAdapterConfiguration");

            foreach (ManagementObject queryObj in searcher.Get())
            {
                String Check = Convert.ToString(queryObj["DHCPLeaseObtained"]);
                if (String.IsNullOrEmpty(Check))
                {
                }
                else
                {
                    ManagementBaseObject outParams = queryObj.InvokeMethod("ReleaseDHCPLease", null, null);
                }
            }
        }
        catch (ManagementException e)
        {
            MessageBox.Show("An error occurred while querying for WMI data: " + e.Message);
        }
}