调用Windows API

在C#中可以通过Pinvoke调用windows API方式来进行网络连接的检查：

[DllImport("wininet.dll", SetLastError=true)]
extern static bool InternetGetConnectedState(out int lpdwFlags, int dwReserved);

[Flags]
enum ConnectionStates
{
    Modem = 0x1,
    LAN = 0x2,
    Proxy = 0x4,
    RasInstalled = 0x10,
    Offline = 0x20,
    Configured = 0x40,
}

MSDN地址 https://learn.microsoft.com/en-us/windows/win32/api/wininet/nf-wininet-internetgetconnectedstate 对应的返回值Description如下：

Value Meaning
INTERNET_CONNECTION_CONFIGURED0x40 Local system has a valid connection to the Internet, but it might or might not be currently connected.
INTERNET_CONNECTION_LAN0x02 Local system uses a local area network to connect to the Internet.
INTERNET_CONNECTION_MODEM0x01 Local system uses a modem to connect to the Internet.
INTERNET_CONNECTION_MODEM_BUSY0x08 No longer used.
INTERNET_CONNECTION_OFFLINE0x20 Local system is in offline mode.
INTERNET_CONNECTION_PROXY0x04 Local system uses a proxy server to connect to the Internet.
INTERNET_RAS_INSTALLED0x10 Local system has RAS installed.

例子 ：

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication2
{
  internal class Program
  {
   [DllImport("wininet.dll", SetLastError = true)]
   private static extern bool InternetGetConnectedState(out int lpdwFlags, int dwReserved);

   private static void Main(string[] args)
   {
    int flags;
    bool isConnected = InternetGetConnectedState(out flags, 0);
    Console.WriteLine(string.Format("Is connected :{0} Flags:{1}", isConnected, flags));
   }
  }
}

使用NCSI

Registry

Windows进行网络检查的地址是 http://www.msftconnecttest.com/connecttest.txt

这些配置可以在注册表HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\NlaSvc\Parameters\Internet中找到。注册表内容可以参考这个项目。下面是通过NCSI来检查网络是否连接的代码：

首先先遍历注册表相关的键值对，因为用户可能修改相关配置。

public Dictionary<string, string> GetNCSIData()
{
    RegistryKey internetKey = Registry.LocalMachine.OpenSubKey(@"SYSTEM\CurrentControlSet\Services\NlaSvc\Parameters\Internet");
    if (internetKey != null)
    {
        Dictionary<string, string> NCSIData = new Dictionary<string, string>();
        string[] valueNames = internetKey.GetValueNames();

        foreach (string key in valueNames)
        {
            object value = internetKey.GetValue(key);
            if (value is string v)
            {
                NCSIData.Add(key, v);
            }
        }

        return NCSIData;
    }

    return null;
}

然后进行检查


public async Task<bool> IsInternetConnected()
{
    var ncsiDataDic = GetNCSIData();
    string probeHost = ncsiDataDic["ActiveDnsProbeHost"];
    string NCSIDnsIpAddress = ncsiDataDic["ActiveDnsProbeContent"];

    string webProbeHost = ncsiDataDic["ActiveWebProbeHost"];
    string WebProbePath = ncsiDataDic["ActiveWebProbePath"];
    string NCSITestResult = ncsiDataDic["ActiveWebProbeContent"];

    string NCSITestUrl = new UriBuilder()
    {
        Scheme = "http",
        Host = webProbeHost,
        Port = 80,
        Path = WebProbePath
    }.Uri.ToString();


    string NCSIDns = probeHost;

    try
    {
        using (var webClient = new WebClient())
        {
            string result = await webClient.DownloadStringTaskAsync(NCSITestUrl);
            if (result != NCSITestResult)
            {
                return false;
            }
        }

        // Check NCSI DNS IP
        var dnsIpAddresses = await Dns.GetHostAddressesAsync(NCSIDns);
        return dnsIpAddresses.Any(addr => addr.ToString() == NCSIDnsIpAddress);
    }
    catch (Exception )
    {
        return false;
    }
}

其他方式

可以参考下 Stackoverflow这个问题 https://stackoverflow.com/questions/520347/how-do-i-check-for-a-network-connection

参考连接