尽量避免通过捕获异常来测试连接。因为我们真的预料到有时我们可能会失去网络连接。

 if (NetworkInterface.GetIsNetworkAvailable() &&
     new Ping().Send(new IPAddress(new byte[] { 8, 8, 8, 8 }),2000).Status == IPStatus.Success)
 //is online
 else
 //is offline

简介

在某些情况下，您需要在windows应用程序中使用c#代码检查internet是否可用。可能是在windows窗体中使用internet下载或上传文件，或从远程位置的数据库中获取一些数据，在这些情况下，internet检查是强制性的。

有一些方法可以从后面的代码中使用c#检查internet的可用性。这里解释了所有这些方法，包括它们的局限性。

InternetGetConnectedState(经由wininet)

“wininet”API可以用来检查本地系统是否有活跃的互联网连接。用于此操作的命名空间是'System.Runtime。并使用DllImport导入dll' wininet.dll'。在此之后，创建一个带有extern static的布尔变量，函数名为InternetGetConnectedState，带有两个参数描述和reservedValue，如示例所示。

注意:extern修饰符用于声明外部实现的方法。当您使用Interop服务调用非托管代码时，extern修饰符的常见用法是与DllImport属性一起使用。在这种情况下，方法也必须声明为静态的。

接下来创建一个名称为“IsInternetAvailable”的布尔方法。的 上面的函数将用于这个返回Internet的方法 本地系统状态

[DllImport("wininet.dll")]
private extern static bool InternetGetConnectedState(out int description, int reservedValue);
public static bool IsInternetAvailable()
{
    try
    {
        int description;
        return InternetGetConnectedState(out description, 0);
    }
    catch (Exception ex)
    {
        return false;
    }
}

GetIsNetworkAvailable

下面的示例使用GetIsNetworkAvailable方法来确定网络连接是否可用。

if (System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable())
{
    System.Windows.MessageBox.Show("This computer is connected to the internet");
}
else
{
    System.Windows.MessageBox.Show("This computer is not connected to the internet");
}

备注(根据MSDN):如果任何网络接口被标记为“向上”且不是环回或隧道接口，则网络连接被认为是可用的。

There are many cases in which a device or computer is not connected to a useful network but is still considered available and GetIsNetworkAvailable will return true. For example, if the device running the application is connected to a wireless network that requires a proxy, but the proxy is not set, GetIsNetworkAvailable will return true. Another example of when GetIsNetworkAvailable will return true is if the application is running on a computer that is connected to a hub or router where the hub or router has lost the upstream connection.

在网络上Ping一个主机名

Ping和PingReply类允许应用程序通过从主机获得回复来确定是否可以通过网络访问远程计算机。这些类可以在System.Net.NetworkInformation命名空间中找到。以ping主机为例。

protected bool CheckConnectivity(string ipAddress)
{
    bool connectionExists = false;
    try
    {
        System.Net.NetworkInformation.Ping pingSender = new System.Net.NetworkInformation.Ping();
        System.Net.NetworkInformation.PingOptions options = new System.Net.NetworkInformation.PingOptions();
        options.DontFragment = true;
        if (!string.IsNullOrEmpty(ipAddress))
        {
            System.Net.NetworkInformation.PingReply reply = pingSender.Send(ipAddress);
            connectionExists = reply.Status == 
System.Net.NetworkInformation.IPStatus.Success ? true : false;
        }
    }
    catch (PingException ex)
    {
        Logger.LogException(ex.Message, ex);
    }
    return connectionExists;
}

Remarks (As per MSDN): Applications use the Ping class to detect whether a remote computer is reachable. Network topology can determine whether Ping can successfully contact a remote host. The presence and configuration of proxies, network address translation (NAT) equipment, or firewalls can prevent Ping from succeeding. A successful Ping indicates only that the remote host can be reached on the network; the presence of higher level services (such as a Web server) on the remote host is not guaranteed.

欢迎发表意见和建议。编码……快乐!

我个人觉得Anton和moffeltje的答案最好，但我增加了一个检查，排除VMWare和其他公司设置的虚拟网络。

public static bool IsAvailableNetworkActive()
{
    // only recognizes changes related to Internet adapters
    if (!System.Net.NetworkInformation.NetworkInterface.GetIsNetworkAvailable()) return false;

    // however, this will include all adapters -- filter by opstatus and activity
    NetworkInterface[] interfaces = System.Net.NetworkInformation.NetworkInterface.GetAllNetworkInterfaces();
    return (from face in interfaces
            where face.OperationalStatus == OperationalStatus.Up
            where (face.NetworkInterfaceType != NetworkInterfaceType.Tunnel) && (face.NetworkInterfaceType != NetworkInterfaceType.Loopback)
            where (!(face.Name.ToLower().Contains("virtual") || face.Description.ToLower().Contains("virtual")))
            select face.GetIPv4Statistics()).Any(statistics => (statistics.BytesReceived > 0) && (statistics.BytesSent > 0));
}

ping google.com会引入一个DNS解析依赖。ping 8.8.8.8很好，但谷歌离我很远。我所需要做的就是在互联网上ping离我最近的东西。

我可以使用Ping的TTL功能来Ping跳#1，然后跳#2，等等，直到我从一个可路由地址上的某个东西得到回复;如果该节点在一个可路由地址上，那么它就在因特网上。对于我们大多数人来说，跳点1将是我们的本地网关/路由器，跳点2将是光纤连接或其他设备的另一侧的第一个点。

这段代码对我有用，比这个线程中的其他一些建议反应更快，因为它是在互联网上ping离我最近的任何东西。

using System.Diagnostics;
using System.Net;
using System.Net.NetworkInformation;
using System.Net.Sockets;
using System.Threading.Tasks;
    
public static async Task<bool> IsConnectedToInternetAsync()
{
    const int maxHops = 30;
    const string someFarAwayIpAddress = "8.8.8.8";
    
    // Keep pinging further along the line from here to google 
    // until we find a response that is from a routable address
    for (int ttl = 1; ttl <= maxHops; ttl++)
    {
        var options = new PingOptions(ttl, true);
        byte[] buffer = new byte[32];
        PingReply reply;
        try
        {
            using (var pinger = new Ping())
            {
                reply = await pinger.SendPingAsync(someFarAwayIpAddress, 10000, buffer, options);
            }
        }
        catch (PingException pingex)
        {
            Debug.Print($"Ping exception (probably due to no network connection or recent change in network conditions), hence not connected to internet. Message: {pingex.Message}");
            return false;
        }
    
        string address = reply.Address?.ToString() ?? null;
        Debug.Print($"Hop #{ttl} is {address}, {reply.Status}");
    
        if (reply.Status != IPStatus.TtlExpired && reply.Status != IPStatus.Success)
        {
            Debug.Print($"Hop #{ttl} is {reply.Status}, hence we are not connected.");
            return false;
        }
    
        if (IsRoutableAddress(reply.Address))
        {
            Debug.Print("That's routable, so we must be connected to the internet.");
            return true;
        }
    }
    
    return false;
}
    
private static bool IsRoutableAddress(IPAddress addr)
{
    if (addr == null)
    {
        return false;
    }
    else if (addr.AddressFamily == AddressFamily.InterNetworkV6)
    {
        return !addr.IsIPv6LinkLocal && !addr.IsIPv6SiteLocal;
    }
    else // IPv4
    {
        byte[] bytes = addr.GetAddressBytes();
        if (bytes[0] == 10)
        {   // Class A network
            return false;
        }
        else if (bytes[0] == 172 && bytes[1] >= 16 && bytes[1] <= 31)
        {   // Class B network
            return false;
        }
        else if (bytes[0] == 192 && bytes[1] == 168)
        {   // Class C network
            return false;
        }
        else
        {   // None of the above, so must be routable
            return true;
        }
    }
}

NetworkInterface。GetIsNetworkAvailable非常不可靠。只是有一些VMware或其他局域网连接，它将返回错误的结果。 还有Dns。我只是关心测试URL是否可能在我的应用程序部署的环境中被阻止。

我发现的另一种方法是使用InternetGetConnectedState方法。 我的代码是

[System.Runtime.InteropServices.DllImport("wininet.dll")]
private extern static bool InternetGetConnectedState(out int Description, int ReservedValue);

public static bool CheckNet()
{
     int desc;
     return InternetGetConnectedState(out desc, 0);         
}

而不是检查，只需执行操作(web请求，邮件，ftp等)，并为请求失败做好准备，即使检查成功，您也必须这样做。

考虑以下几点:

1 - check, and it is OK
2 - start to perform action 
3 - network goes down
4 - action fails
5 - lot of good your check did

如果网络坏了，你的动作就会像ping一样迅速失败。

1 - start to perform action
2 - if the net is down(or goes down) the action will fail