1. 可以用WMI (Win2K & XP）：
(首先要在VS.NET中创建一个项目，然后在添加引用中引用一个.net的装配件：System.Management.dll,这样你的项目才能使用WMI)

with Windows 2000 & XP, UNDOCUMENTED:
  MSNdis_MediaConnectStatus - NdisMediaConnectStatus :
   (0 = connected?, 1 = not connected?)

with Windows XP:
  Win32_NetworkAdapter - NetConnectionStatus :
    
using System.Management;

static void ReportConnection2000()
{
 ManagementClass mc = new ManagementClass( @"root\WMI", @"MSNdis_MediaConnectStatus", null );
 ManagementObjectCollection moc = mc.GetInstances();
 foreach( ManagementObject mo in moc )
 {
  string name = (string) mo["InstanceName"];
  bool active = (bool) mo["Active"];
  uint status = (uint) mo["NdisMediaConnectStatus"];
  Console.WriteLine( " {0}\n\tActive:{1}  Media Status:{2}", name, active, status );
 }
}

static void ReportConnectionXP()
{
 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 );
 }
}


如果返回0则表示连接；

如果返回1则表示没连接；

上面得到的是本地的连接状态；

==========================================================

利用ping来得取： 

			Process p = new Process();			
			p.StartInfo .WorkingDirectory ="c:\\";
			p.StartInfo.FileName = "ping.exe";
			p.StartInfo.Arguments="192.168.52.31";
			p.StartInfo.UseShellExecute = false;
			p.StartInfo.RedirectStandardOutput = true;
			p.StartInfo.CreateNoWindow = true;
			p.Start ();
			string output = p.StandardOutput.ReadToEnd();
			p.WaitForExit ();
			MessageBox.Show(output);//得到ping的输出值