C# 0 The Complete Reference

The System.Net Members

System.Net

 is a large namespace that contains many members. It is far beyond the scope of 

this chapter to discuss them all or to discuss all aspects related to Internet programming. (In 

fact, an entire book is needed to fully cover networking and C#’s support for it in detail.) 

However, it is worthwhile to list the members of 

System.Net

 so you have an idea of what 

is available for your use.

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CHAPTER

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The classes defined by 

System.Net

 are shown here:

AuthenticationManager

Authorization

Cookie

CookieCollection

CookieContainer

CookieException

CredentialCache

Dns

DnsPermission

DnsPermissionAttribute

DownloadDataCompletedEventArgs

DownloadProgressChangedEventArgs

DownloadStringCompletedEventArgs

EndPoint

EndpointPermission

FileWebRequest

FileWebResponse

FtpWebRequest

FtpWebResponse

HttpListener 

HttpListenerBasicIdentity

HttpListenerContext

HttpListenerException

HttpListenerPrefixCollection 

HttpListenerRequest

HttpListenerResponse

HttpVersion

HttpWebRequest

HttpWebResponse

IPAddress

IPEndPoint

IPEndPointCollection

IPHostEntr y

IrDAEndPoint

NetworkCredential

OpenReadCompletedEventArgs

OpenWriteCompletedEventArgs

ProtocolViolationException

Ser vicePoint

Ser vicePointManager

SocketAddress

SocketPermission

SocketPermissionAttribute

UploadDataCompletedEventArgs

UploadFileCompletedEventArgs

UploadProgressChangedEventArgs

UploadStringCompletedEventArgs

UploadValuesCompletedEventArgs

WebClient

WebException

WebHeaderCollection

WebPermission

WebPermissionAttribute

WebProxy

WebRequest

WebRequestMethods

WebRequestMethods.File

WebRequestMethods.Ftp 

WebRequestMethods.Http WebResponse

System.Net

 defines the following interfaces:

IAuthenticationModule

ICer tificatePolicy

ICredentialPolicy

ICredentials

ICredentialsByHost

IWebProxy

IWebProxyScript

IWebRequestCreate

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It defines these enumerations:

AuthenticationSchemes

DecompressionMethods

FtpStatusCode

HttpRequestHeader

HttpResponseHeader

HttpStatusCode

NetworkAccess

SecurityProtocolType

Transpor tType

WebExceptionStatus

System.Net

 also defines several delegates.

Although

System.Net

defines many members, only a few are needed to accomplish most 

common Internet programming tasks. At the core of networking are the abstract classes 

WebRequest

 and 

WebResponse

. These classes are inherited by classes that support a 

specific network protocol. (A 

protocol

 defines the rules used to send information over a 

network.) For example, the derived classes that support the standard HTTP protocol are 

HttpWebRequest

 and 

HttpWebResponse

.

Even though 

WebRequest

 and 

WebResponse

 are easy to use, for some tasks you can 

employ an even simpler approach based on 

WebClient

. For example, if you only need to 

upload or download a file, then 

WebClient

 is often the best way to accomplish it.

Uniform Resource Identifiers

Fundamental to Internet programming is the Uniform Resource Identifier (URI). A 

URI

describes the location of some resource on the network. A URI is also commonly called a 

URL,

 which is short for 

Uniform Resource Locator.

 Because Microsoft uses the term 

URI

 when 

describing the members of 

System.Net

, this book will do so, too. You are no doubt familiar 

with URIs because you use one every time you enter an address into your Internet browser.

A URI has the following general form:

Protocol:

//

ServerID

/

FilePath

?

Query

Protocol

 specifies the protocol being used, such as HTTP. 

ServerID

 identifies the specific 

server, such as mhprofessional.com or HerbSchildt.com. 

FilePath

 specifies the path to a 

specific file. If 

FilePath

 is not specified, the default page at the specified 

ServerID

 is obtained. 

Finally, 

Query

 specifies information that will be sent to the server. 

Query

 is optional. In C#, 

URIs are encapsulated by the 

Uri

 class, which is examined later in this chapter.

Internet Access Fundamentals

The classes contained in 

System.Net

 support a request/response model of Internet interaction. 

In this approach, your program, which is the client, requests information from the server 

and then waits for the response. For example, as a request, your program might send to the 

server the URI of some website. The response that you will receive is the hypertext associated 

with that URI. This request/response approach is both convenient and simple to use 

because most of the details are handled for you.

The hierarchy of classes topped by 

WebRequest

 and 

WebResponse

 implement what 

Microsoft calls 

pluggable protocols.

 As most readers know, there are several different types of 

network communication protocols. The most common for Internet use is HyperText Transfer 

Protocol (HTTP). Another is File Transfer Protocol (FTP). When a URI is constructed, the 

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prefix of the URI specifies the protocol. For example, http://www.HerbSchildt.com uses the 

prefix 

http,

 which specifies hypertext transfer protocol.

As mentioned earlier, 

WebRequest

 and 

WebResponse

 are abstract classes that define the 

general request/response operations that are common to all protocols. From them are derived 

concrete classes that implement specific protocols. Derived classes register themselves, using 

the static method 

RegisterPrefix( )

, which is defined by 

WebRequest

. When you create a 

WebRequest

 object, the protocol specified by the URI’s prefix will automatically be used, if it 

is available. The advantage of this “pluggable” approach is that most of your code remains the 

same no matter what type of protocol you are using.

The .NET runtime automatically defines the HTTP, HTTPS, file, and FTP protocols. 

Thus, if you specify a URI that uses the HTTP prefix, you will automatically receive the 

HTTP-compatible class that supports it. If you specify a URI that uses the FTP prefix, you 

will automatically receive the FTP-compatible class that supports it.

Because HTTP is the most commonly used protocol, it is the only one discussed in this 

chapter. (The same techniques, however, will apply to all supported protocols.) The classes 

that support HTTP are 

HttpWebRequest

 and 

HttpWebResponse

. These classes inherit 

WebRequest

 and 

WebResponse

 and add several members of their own, which apply to 

the HTTP protocol.

System.Net

 supports both synchronous and asynchronous communication. For many 

Internet uses, synchronous transactions are the best choice because they are easy to use. 

With synchronous communications, your program sends a request and then waits until 

the response is received. For some types of high-performance applications, asynchronous 

communication is better. Using the asynchronous approach, your program can continue 

processing while waiting for information to be transferred. However, asynchronous 

communications are more difficult to implement. Furthermore, not all programs benefit 

from an asynchronous approach. For example, often when information is needed from 

the Internet, there is nothing to do until the information is received. In cases like this, the 

potential gains from the asynchronous approach are not realized. Because synchronous 

Internet access is both easier to use and more universally applicable, it is the only type 

examined in this chapter.

Since

WebRequest

 and 

WebResponse

 are at the heart of 

System.Net

, they will be 

examined next.

WebRequest

The

WebRequest

 class manages a network request. It is abstract because it does not 

implement a specific protocol. It does, however, define those methods and properties 

common to all requests. The methods defined by 

WebRequest

 that support synchronous 

communications are shown in Table 25-1. The properties defined by 

WebRequest

 are shown 

in Table 25-2. The default values for the properties are determined by derived classes. 

WebRequest

 defines no public constructors.

To send a request to a URI, you must first create an object of a class derived from 

WebRequest

 that implements the desired protocol. This is done by calling 

Create( )

, which 

is a 

static

 method defined by 

WebRequest

.

Create( )

 returns an object of a class that inherits 

WebRequest

 and implements a specific protocol.

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