如何检查网络连接？

确定是否存在网络连接的最佳方法是什么？

您可以使用GetIsNetworkAvailable()以下命令在.NET 2.0中检查网络连接：

System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable()

若要监视IP地址的更改或网络可用性的更改，请使用NetworkChange类中的事件：

System.Net.NetworkInformation.NetworkChange.NetworkAvailabilityChanged
System.Net.NetworkInformation.NetworkChange.NetworkAddressChanged

标记的答案是100％正确，但是，在某些情况下，标准方法被虚拟卡（虚拟盒子，...）欺骗。通常还需要根据某些网络接口的速度（串行端口，调制解调器等）丢弃它们。

这是一段检查这些情况的代码：

    /// <summary>
    /// Indicates whether any network connection is available
    /// Filter connections below a specified speed, as well as virtual network cards.
    /// </summary>
    /// <returns>
    ///     <c>true</c> if a network connection is available; otherwise, <c>false</c>.
    /// </returns>
    public static bool IsNetworkAvailable()
    {
        return IsNetworkAvailable(0);
    }

    /// <summary>
    /// Indicates whether any network connection is available.
    /// Filter connections below a specified speed, as well as virtual network cards.
    /// </summary>
    /// <param name="minimumSpeed">The minimum speed required. Passing 0 will not filter connection using speed.</param>
    /// <returns>
    ///     <c>true</c> if a network connection is available; otherwise, <c>false</c>.
    /// </returns>
    public static bool IsNetworkAvailable(long minimumSpeed)
    {
        if (!NetworkInterface.GetIsNetworkAvailable())
            return false;

        foreach (NetworkInterface ni in NetworkInterface.GetAllNetworkInterfaces())
        {
            // discard because of standard reasons
            if ((ni.OperationalStatus != OperationalStatus.Up) ||
                (ni.NetworkInterfaceType == NetworkInterfaceType.Loopback) ||
                (ni.NetworkInterfaceType == NetworkInterfaceType.Tunnel))
                continue;

            // this allow to filter modems, serial, etc.
            // I use 10000000 as a minimum speed for most cases
            if (ni.Speed < minimumSpeed)
                continue;

            // discard virtual cards (virtual box, virtual pc, etc.)
            if ((ni.Description.IndexOf("virtual", StringComparison.OrdinalIgnoreCase) >= 0) ||
                (ni.Name.IndexOf("virtual", StringComparison.OrdinalIgnoreCase) >= 0))
                continue;

            // discard "Microsoft Loopback Adapter", it will not show as NetworkInterfaceType.Loopback but as Ethernet Card.
            if (ni.Description.Equals("Microsoft Loopback Adapter", StringComparison.OrdinalIgnoreCase))
                continue;

            return true;
        }
        return false;
    }

Microsoft Windows Vista和Windows 7使用NCSI（网络连接状态指示器）技术：

1. NCSI在www.msftncsi.com上执行DNS查找，然后请求http://www.msftncsi.com/ncsi.txt。该文件是纯文本文件，仅包含文本“ Microsoft NCSI ”。
2. NCSI为dns.msftncsi.com发送DNS查找请求。此DNS地址应解析为131.107.255.255。如果地址不匹配，则认为互联网连接运行不正常。

调用此方法以检查网络连接。

public static bool IsConnectedToInternet()
        {
            bool returnValue = false;
            try
            {

                int Desc;
                returnValue = Utility.InternetGetConnectedState(out Desc, 0);
            }
            catch
            {
                returnValue = false;
            }
            return returnValue;
        }

将其放在代码行下面。

[DllImport("wininet.dll")]
        public extern static bool InternetGetConnectedState(out int Description, int ReservedValue);