We've already covered the latter process - the code to do it
is essentially the same as our earlier sample to read text from a file using
the StreamReader class. Loading and requesting the file is a little more complicated
as it involves several new classes.
We're going to need a couple of classes concerned with web browsing: HttpWebRequest,
HttpWebResponse and WebRequestFactory, which are all in the System.Net namespace.
The assembly that defines this namespace is not by default loaded in C# projects
so we need to add it to our references in the solution explorer. Recall we can
do this by right-clicking on the References node in the explorer and selecting
Add Reference from the context menu.
Next we add a couple of commands to refer to some new namespaces in the C# source file:
namespace WebRequest
{
using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.WinForms;
using System.Data;
using System.Net;
using System.IO;
using System.Text;
System.Net is for the web classes just mentioned, System.IO
is because we will need to use a StreamReader, and System.Text provides a helper
enumeration value used in constructing the StreamReader. These latter two classes
are both in mscorlib.dll so no new references need to be added at compilation
time.
Now we can proceed to the code needed to make our request to the web server and
display the results:
HttpWebRequest webreq =
(HttpWebRequest)WebRequestFactory.Create
("http://localhost/postinfo.html");
HttpWebResponse webresp =
(HttpWebResponse)webreq.GetResponse();
StreamReader strm = new StreamReader(
webresp.GetResponseStream(), Encoding.ASCII);
string sLine;
do
{
sLine = strm.ReadLine();
AddItem(sLine);
}
while (sLine != null);
strm.Close();
The request is made through an instance of the class HttpWebRequest.
However, it is not possible to construct this instance directly - instead it
needs to be constructed by a static method of another class, WebRequestFactory.
The WebRequestFactory.Create() method is designed to create a general request
for a given URL - in this case we pass the URL of one of the files created on
the default web site on my local machine when IIS is installed - postinfo.html.
Note that WebRequestFactory.Create() actually returns a reference to a WebRequest
object, not an HttpWebRequest object, so we need to cast the return value. HttpWebRequest
is derived from WebRequest - the latter class is more general-purpose and able
to deal with requests using other protocols besides HTTP.
Once we have the HttpWebRequest instance, we actually make the request by calling
its GetResponse() method. GetResponse() returns a WebResponse object, which encapsulates
information returned by a web server in response to a request. In a similar manner
to WebRequestFactory.Create(), the return value is a reference to a WebResponse
rather than an HttpWebResponse, so we need to cast it to the required data type.
Once we have the HttpWebResponse, we simply need to obtain a StreamReader instance
that we can use to retrieve the contents of the file. To do this we use a StreamReader
constructor that takes two parameters: a more general stream and a value that
indicates the encoding type. The stream is obtained from the GetResponseStream()
method of the HttpWebResponse class, and the encoding type is Encoding.ASCII,
an enumerated value from the System.Text.Encoding class, which indicates that
this stream contains ASCII text data.
Although there are a lot of classes involved with this and hence a lot to take
in, the actual code is still reasonably short and simple. Running this sample
produces this result:
This indicates that the page has been successfully downloaded and displayed.
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