如何测试网络连通性

Introduction

Some of the functions of our applications may require a run-time test of internet connectivity. Once internet connectivity is detected, the functions that require internet access may temporarily be disabled and/or the user can be notified via an alert message. Otherwise, the application may result in error during operation or it may cause annoying problems for the user. In this article, I will try to demonstrate a couple of ways to overcome this problem.

Method 1: WebRequest

We may send a web request to a website which assumed to be online always, for example google.com. If we can get a response, then obviously the device that runs our application is connected to the internet.

public static bool WebRequestTest(){
string url = "http://www.google.com";
try{
System.Net.WebRequest myRequest = System.Net.WebRequest.Create(url);
System.Net.WebResponse myResponse = myRequest.GetResponse();
}catch (System.Net.WebException){
return false;
}
return true;
}

Method 2: TCP Socket

There can be some delay in response of web request therefore this method may not be fast enough for some applications. A better way is to check whether port 80, default port for http traffic, of an always online website.

public static bool TcpSocketTest(){
try{
System.Net.Sockets.TcpClient client = new System.Net.Sockets.TcpClient("www.google.com", 80);
client.Close();
return true;
}catch (System.Exception ex){
return false;
}
}

Method 3: Ping

There can be some delay in response of web request, therefore this method may not be fast enough for some applications. A better way is to check whether port 80, default port for http traffic, of an always online website.

public bool PingTest(){
System.Net.NetworkInformation.Ping ping = new System.Net.NetworkInformation.Ping();
System.Net.NetworkInformation.PingReply pingStatus = ping.Send(IPAddress.Parse("208.69.34.231"),1000);
if (pingStatus.Status == System.Net.NetworkInformation.IPStatus.Success){
return true;
}else{
return false;
}
}

You cannot use this method in .NET Compact Framework because there is no NetworkInformation namespace that comes with Compact Framework. However, you can use Smart Device Framework (http://www.opennetcf.com[^], Community Edition is free for download) provided by OpenNETCF. This framework comes with many other useful tools that .NET Compact Framework does not contain.
Notice that I used Google’s IP address 208.69.34.231. We could use Google’s web address www.google.com:

System.Net.NetworkInformation.PingReply pingStatus = ping.Send("www.google.com",1000)

However, that will require DNS lookup which causes extra delay.

Method 4: DNS Lookup

Alternatively you can use DNS lookup to check internet connectivity. This method is faster than Ping method.

public static bool DnsTest(){
try{
System.Net.IPHostEntry ipHe = System.Net.Dns.GetHostByName("www.google.com");
return true;
}catch{
return false;
}
}

Method 5: Windows Internet API (WinINet)

WinINet API provides functions to test internet connectivity, such as InternetCheckConnection and InternetGetConnectedState. I do not have any idea about what these fucntions do exactly to test internet connectivity. You may refer to http://msdn.microsoft.com/en-us/library/aa384346(v=VS.85).aspx and http://msdn.microsoft.com/en-us/library/aa384702(v=VS.85).aspx for details.

An example usage can be:

[DllImport("wininet.dll")]
private extern static bool InternetGetConnectedState(out int connDescription, int ReservedValue);
//check if a connection to the Internet can be established
public static bool IsConnectionAvailable()
{
int Desc;
return InternetGetConnectedState(out connDesc, 0);
}

Summary

In this article, we have seen a couple of different ways to test internet connectivity. Each method has its own advantages and disadvantages. So, it is up to you to choose the best way for your platform and application.