The WebClient Class

To get an idea of what the WebClient class can do for you, check out the following code example. It performs the same function as our original WebRequest example, but it uses less code.

ImportsSystem
ImportsSystem.Net
ImportsSystem.Text
ImportsSystem.IO

ModuleWebClientTest
SubMain()
DimclientAsWebClient=NewWebClient()
DimresStreamAsStream=_
client.OpenRead("http://www.gotdotnet.com/about_new.aspx")
DimsrAsStreamReader=NewStreamReader(resStream)
DimresChar(256)AsChar
DimcharCountAsInteger=sr.Read(resChar,0,resChar.Length)
DoWhilecharCount>0
DimstrAsString=NewString(resChar,0,charCount)
Console.Write(str)
charCount=sr.Read(resChar,0,resChar.Length)
Loop
resStream.Close()
EndSub
EndModule

As I mentioned, internally the WebClient class uses both the WebRequest and WebResponse classes. The WebClient exposes methods both for sending and reading data. However, because it is designed to keep things simple, it provides support for synchronous (blocking) calls only. Table 6-1 describes the major methods of interest for the WebClient class.

Table 6-1. Selected Methods of the WebClient Class

Apart from the fact that all WebClient method calls are synchronous, other potential disadvantages include:

• You can't maintain cookies between the calls. You must use the HttpWebRequest object's CookieContainer property.

• ResponseHeaders contains name/value header pairs, but there is no way to catch response code unless there is an error. In case of an error, a WebException will be thrown and its response property will contain the HttpWebResponse that was returned in error; there you can see the StatusCode . If you need to check nonerror status codes, you have to use the HttpWebRequest class.

• You can't know the download status (the amount of data that has been processed and copied ). You must instead use the WebRequest / WebResponse objects.

When you deal with downloads in general, and WebClient in particular, you should be aware of the classes described in Table 6-2. These classes can help your application work through proxy servers and handle authentication and credentials.

Table 6-2. Useful Classes for Making Web Requests

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Here's a sample that uses some of the other useful methods of the WebClient class. Note the use of the DownloadData and DownloadFile methods.

ImportsSystem
ImportsSystem.Net
ImportsSystem.Text
ImportsSystem.Collections.Specialized

ModuleWebClientSample
PublicSubMain()
Try
DimclientAsNewWebClient()
DimpageDataAsByte()=_
client.DownloadData("http://www.gotdotnet.com")
DimpageHtmlAsString=Encoding.ASCII.GetString(pageData)
Console.WriteLine(pageHtml)
client.DownloadFile("http://www.gotdotnet.com", "C:\Temp.html")
CatchwebExAsWebException
Console.WriteLine(webEx.ToString())
EndTry
Console.WriteLine("Hitanykeytocontinue...")
Console.Read()
EndSub
EndModule

Most of the information about the WebClient is pretty much a rehash of the WebRequest material, so I won't spend any further time on it. The MSDN documentation is quite helpful on this topic, and the information contained here should be more than enough to get you up and running.