如何检查网络连接？

你可以使用 GetIsNetworkAvailable()在.NET 2.0中检查网络连接:

System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable()

To monitor changes in IP address or changes in network availability use the events from the 网络改变 class:

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

The marked answer is 100% fine, however, there are certain cases when the standard method is fooled by virtual cards (virtual box, ...). It's also often desirable to discard some network interfaces based on their speed (serial ports, modems, ...).

下面是检查这些情况的一段代码:

    /// <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7使用 NCSI (网络连接状态指示器)技术:

1. NCSI 对 www.msftncsi.com 执行 DNS 查找，然后请求 http://www.msftncsi.com/ncsi.txt。此文件为纯文本文件，仅包含文本“ Microsoft NCSI”。
2. 国家犯罪现场调查发送一个域名解析查找请求 DNS.msftncsi.com。此 DNS 地址应解析为131.107.255.255。如果地址不匹配，则假定 Internet 连接运行不正常。

Call this method to check the network Connection.

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


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

将此代码放在下面的代码行中。

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