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CHAPTER 21 NETWORKING & WCF
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string that has been retrieved from the server. Compiling the running Listing 21-5 produces the following results: Doing other work...0 Doing other work...1 Doing other work...2 Doing other work...3 Doing other work...4 Doing other work...5 Doing other work...6 Doing other work...7 Doing other work...8 Doing other work...9 Press enter to finish --- Async Result --<html><head><title>Microsoft Corporation</title><m Table 21-4 contains a list of the asynchronous methods and the corresponding events and event argument classes defined by the WebClient class. Table 21-4. WebClient Asynchronous Methods and Events
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DownloadDataAsync(Uri) DownloadFileAsync(Uri) DownloadStringAsync(Uri) OpenReadAsync(Uri) OpenWriteAsync(Uri) UploadDataAsync(Uri) UploadFileAsync(Uri) UpLoadStringAsync(Uri)
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DownloadDataCompleted DownloadFileCompleted DownloadStringCompleted OpenReadCompleted OpenWriteCompleted UploadDataCompleted UploadFileCompleted UploadStringCompleted
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DownloadDataCompletedEventArgs AsyncCompletedEventArgs DownloadStringCompletedEventArgs OpenReadCompletedEventArgs OpenWriteCompletedEventArgs UploadDataCompletedEventArgs UploadFileCompletedEventArgs UploadStringCompletedEventArgs
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The base class for the event argument classes is AsyncCompletedEventArgs from the System.ComponentModel namespace; this is also the class used by the DownloadFileCompleted event. AsyncCompletedEventArgs has two helpful properties that you can use to find out how your asynchronous request went. These properties are described in Table 21-5.
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CHAPTER 21 NETWORKING & WCF
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Table 21-5. AsyncCompletedEventArgs Properties
Property
Cancelled
Description
Returns true if the request was cancelled using the WebClient.CancelAsync method. If an error occurred while the request was being performed, this property will return the corresponding Exception. If no error has occurred, then null will be returned.
Error
It is good practice to check the Error property when handling an event before reading any other property. The classes derived from AsyncCompletedEventArgs define an additional property called Result, which returns an object of the type most suited to the asynchronous methods that was called. Table 21-6 lists the derived classes and the types that their Result property returns. Table 21-6. Derived EventArgs Classes and the Type of the Result Property
Derived Class
DownloadDataCompletedEventArgs DownloadStringCompletedEventArgs OpenReadCompletedEventArgs OpenWriteCompletedEventArgs UploadDataCompletedEventArgs UploadFileCompletedEventArgs UploadStringCompletedEventArgs
Result Property Type
byte[] string Stream Stream byte[] byte[] string
Programming with Sockets
Sockets are a low-level network programming feature that allows you to create custom network protocols that your client and server programs can use to communicate or to provide your own implementation of standard network protocols. In general, socket programming is an advanced and complex topic, but simple socket programming can be performed using some C# helper classes.
Creating a Simple TCP Client & Server
Now let s look at two helper classes, TcpListener and TcpClient. These classes allow the programmer to use the Transmission Control Protocol (TCP) to communicate between a client and server. I m not going
CHAPTER 21 NETWORKING & WCF
to go into any detail about TCP, other than to say that if you are new to networking, you probably want to use this protocol as opposed to UDP, which is covered in a later section in this chapter. The purpose of the TcpListener and TcpClient classes is to provide a facility to create a pair of Stream objects, as illustrated by Figure 21-1.
Figure 21-1. Streams and networks When the client writes data to the first Stream object, it can be read by the server using a corresponding Stream object; equally, when the server writes data to its Stream object, it can be read by the client. The entire point of the TcpClient and TcpListener classes is to create those Stream objects and connect them together. Once you have the Stream objects, you can read and write any data you like, using the methods and techniques described in 20.
Creating the Server
The TcpListener class can be used to listen for connections on a specific port, which can then be used to create the Stream object that will be used to send and receive data from the client. Listing 21-6 demonstrates a simple network server. Listing 21-6. A Simple TCP Server using using using using System; System.IO; System.Net; System.Net.Sockets;
class Listing 06 { static void Main(string[] args) { // create a new TcpListener object TcpListener myListener = new TcpListener(IPAddress.Any, 12000); // start accepting connections myListener.Start(); // define a bool that will determine if we keep accepting connections bool acceptConnections = true; while (acceptConnections) { // wait for a connection Console.WriteLine("Waiting for connection");
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