C# 4 and .NET 4

656 ❘ ChaPTer 24 netwOrkinG
}
foreach (IPAddress ip in iphost.AddressList)
{
string ipaddress = ip.AddressFamily.ToString();
listBoxIPs.Items.Add(ipaddress);
listBoxIPs.Items.Add(" " + ip.ToString());
}
txtBoxHostName.Text = iphost.HostName;
}
catch(Exception ex)
{
MessageBox.Show("Unable to process the request because " +
"the following problem occurred:\n" +
ex.Message, "Exception occurred");
}
Notice that in this code you are careful to trap any exceptions. An exception might occur if the user types
an invalid DNS name or if the network is down.
After retrieving the IPHostEntry instance, you use the AddressList property to obtain an array containing
the IP addresses, which you then iterate through with a foreach loop. For each entry, you display the IP
address as an integer and as a string, using the IPAddress.AddressFamily.ToString() method.
loWer-leVel ProToCols
This section briefly discusses some of the .NET classes used to communicate at a lower level.
The System.Net.Sockets namespace contains the relevant classes. These classes, for example, allow you
to directly send out TCP network requests or to listen to TCP network requests on a particular port. The
following table explains the main classes:
Class
Socket
NetworkStream
SmtpClient
TcpClient
TcpListener
UdpClient
PurPose
Deals with managing connections. Classes such as WebRequest, TcpClient, and
UdpClient use this class internally.
Derived from Stream. Represents a stream of data from the network.
Enables you to send messages (mail) through the Simple Mail Transfer Protocol.
Enables you to create and use TCP connections.
Enables you to listen for incoming TCP connection requests.
Enables you to create connections for UDP clients. (UDP is an alternative protocol to TCP
but is much less widely used, mostly on local networks.)
Network communications work on several different levels. The classes you have seen in this chapter so
far work at the highest level — the level at which specific commands are processed. It is probably easiest
to understand this concept if you think of file transfer using FTP. Although today’s GUI applications hide
many of the FTP details, it was not so long ago when you executed FTP from a command-line prompt. In
this environment, you explicitly typed commands to send to the server for downloading, uploading, and
listing files.
FTP is not the only high-level protocol relying on textual commands. HTTP, SMTP, POP, and other
protocols are based on a similar type of behavior. Again, many of the modern graphical tools hide the
transmission of commands from the user, so you are generally not aware of them. For example, when you
type a URL into a web browser, and the web request goes off to a server, the browser is actually sending a
(plain-text) GET command to the server, which fulfills a similar purpose as the FTP get command. It can
also send a POST command, which indicates that the browser has attached other data to the request.
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