The .NET Compact Framework

This chapter is from the book

Pocket PC Network Programming

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This chapter is from the book

This chapter is from the book 

Pocket PC Network Programming

Learn More Buy

Internet Protocols and the .NET Pluggable Protocol Model

When requesting data over the Internet using a standardized protocol such as HTTP (the protocol for the Web), you use a Uniform Resource Identifier (URI) to specify the protocol, server, and name of the resource that you are attempting to access. The .NET Compact Framework provides two abstract classes for handling any Internet resource request and response: System.Net.WebRequest and System.Net.WebResponse.

Client applications use the WebRequest class to make the request for a specific URI from an Internet location over a specific protocol (such as HTTP or FTP). Instead of calling a constructor for the WebRequest class, you initialize a new request by calling the WebRequest.Create() method. This automatically instantiates a new request object based on the protocol that you used for the request. For example, if you are trying to access a resource on the Web using the HTTP protocol, you are returned an HttpWebRequest object for which you can set properties and receive a response stream.

Once your request has been configured, you can call the WebRequest.GetResponse() method to get a Stream class that is used to receive the data from the request.

The WebRequest object is an abstract class that contains the methods and properties described in Table 12.12.

Table 12.12. WebRequest Class Methods and Properties

The WebResponse object is also abstract, and contains the methods and properties described in Table 12.13.

Table 12.13. WebResponse Class Methods and Properties

Both the WebRequest and WebResponse abstract classes form the basis for what is known as pluggable protocols. The concept of pluggable protocols is fairly straightforward—a client application can make a request for any Internet resource using a URI and not have to worry about the underlying details of the network protocol being used. When a request is made using the WebRequest.Create() method, the appropriate protocol-specific class is automatically instantiated and returned to the client application.

Consider the following request for a Web resource:

// Set up the URI
System.Uri urlRequest = new System.Uri("http://www.furrygoat.com/");

// Make the request
HttpWebRequest httpReq = (HttpWebRequest)WebRequest.
  Create(urlRequest);

// Get the response
HttpWebResponse webResponse = (HttpWebResponse)httpReq.
  GetResponse();

This request will return a new object that is based on the HttpWebRequest class. The HttpWebRequest class is actually derived from WebRequest, but adds all of the protocol specifics surrounding HTTP.

What makes the pluggable protocol model extremely useful is that you can also use it to create your own classes for handling new protocols that are not native to the .NET Compact Framework.

Creating a Pluggable Protocol

Any new class that is designed to be used as a pluggable protocol is always derived from WebRequest and WebResponse. All new pluggable protocol classes must also be registered with the base WebRequest object in order for the WebRequest.Create() method to appropriately instantiate the correct object for the protocol.

To register a new protocol with the WebRequest class, you can use the following function:

public static bool WebRequest.RegisterPrefix(string prefix,
   IWebRequestCreate creator);

The first parameter, prefix, is a string that represents the protocol that will be used in URI requests for the new object. For example, if you were creating a new protocol that handled requests for resources over the File Transfer Protocol (such as ftp://ftp.microsoft.com/dir/filename.txt), you could simply use ftp for the prefix value. The creator parameter should be set to an object that implements the IWebRequestCreate interface, which is used to create the new WebRequest class.

The following code shows the basic layout for creating a new protocol-specific class that can be used by the WebRequest.Create() method:

/// <summary>Ftp request protocol handler</summary>
class FtpWebRequest: WebRequest {
   // Private internal variables.
   private NetworkCredential reqCredentials;
   private WebHeaderCollection reqHeaders;
   private WebProxy reqProxy;
   private System.Uri reqUri;
   private string reqConnGroup;
   private long reqContentLength;
   private string reqContentType;
   private string reqMethod;
   private bool reqPreAuthen;
   private int reqTimeout;

   // Constructor
   public FtpWebRequest(System.Uri uri) {
      reqHeaders = new WebHeaderCollection();
      reqUri = uri;
   }

   // Properties
   public override string ConnectionGroupName {
      get { return reqConnGroup; }
      set { reqConnGroup = value; }
   }
   public override long ContentLength {
      get { return reqContentLength; }
      set { reqContentLength = value; }
   }
   public override string ContentType {
      get { return reqContentType; }
      set { reqContentType = value; }
   }
   public override ICredentials Credentials {
      get { return reqCredentials; }
      set { reqCredentials = (System.Net.NetworkCredential)
         value; }
   }
   public override WebHeaderCollection Headers {
      get { return reqHeaders; }
      set { reqHeaders = value; }
   }
   public override string Method {
      get { return reqMethod; }
      set { reqMethod = value; }
   }
   public override bool PreAuthenticate {
      get { return reqPreAuthen; }
      set { reqPreAuthen = value; }
   }
   public override IWebProxy Proxy {
      get { return reqProxy; }
      set { reqProxy = (System.Net.WebProxy)value; }
   }
   public override Uri RequestUri {
      get { return reqUri; }
   }
   public override int Timeout {
      get { return reqTimeout; }
      set { reqTimeout = value; }
   }

   // Methods. These are just stubbed in here for this example.
   // In an actual FTP client, you would need to implement these by
   // using p/Invoke to call into the WinInet FTP functions.
   public override void Abort() {
      base.Abort();
   }
   public override IAsyncResult BeginGetRequestStream
     (AsyncCallback callback, object state) {
     return base.BeginGetRequestStream (callback, state);
   }
   public override IAsyncResult BeginGetResponse
      (AsyncCallback callback, object state) {
      return base.BeginGetResponse (callback, state);
   }
   public override Stream EndGetRequestStream(IAsyncResult
      asyncResult) {
      return base.EndGetRequestStream (asyncResult);
   }
   public override WebResponse EndGetResponse(IAsyncResult
      asyncResult) {
      return base.EndGetResponse (asyncResult);
   }
   public override Stream GetRequestStream() {
      return base.GetRequestStream();
   }
   public override WebResponse GetResponse() {
      return base.GetResponse();
   }
}
/// <summary>Ftp request registration interface</summary>
class FtpWebRequestCreate: IWebRequestCreate {
   public System.Net.WebRequest Create(System.Uri uri) {
      System.Net.WebRequest request = new FtpWebRequest
         (uri);
      return request;
   }
}

/// <summary>Ftp request response handler</summary>
class FtpWebResponse: WebResponse {
   // Private internal variables.
   private WebHeaderCollection respHeaders;
   private System.Uri respUri;
   private long respContentLength;
   private string respContentType;

   // Properties
   public override long ContentLength {
      get { return respContentLength; }
      set { respContentLength = value; }
   }
   public override string ContentType {
      get { return respContentType; }
      set { respContentType = value; }
   }
   public override WebHeaderCollection Headers {
      get { return respHeaders; }
      set { respHeaders = value; }
   }
   public override Uri ResponseUri {
      get { return reqUri; }
   }

   // Methods. These are just stubbed in here for this example.
   // In an actual FTP client, you would need to implement these by
   // using p/Invoke to call into the WinInet FTP functions.
   public override void Close() {
      base.Close();
   }
   public override Stream GetResponseStream() {
      return base.GetResponseStream();
   }
}

Remember that you also need to register the protocol with the WebRequest class in order for it to be properly instantiated:

class FtpTest {
   static void Main(string[] args) {
      // Create a pluggable protocol
      System.Uri urlRequest = new
         System.Uri("ftp://ftp.microsoft.com/developr/
            readme.txt");

      // Register it
      WebRequest.RegisterPrefix("ftp", new
         FtpWebRequestCreate());

      // Make the request
      FtpWebRequest ftpClient = (FtpWebRequest)WebRequest.
         Create(urlRequest);

      // Get the response
      FtpWebResponse ftpResponse = (FtpWebResponse)
         ftpClient.GetResponse();

      // Use a StreamReader class to read in the response
      StreamReader responseStream = new
         StreamReader(ftpResponse.GetResponseStream(),
         System.Text.Encoding.ASCII);

      // Since FTP can be binary or ASCII, you would want
      // to copy it in chunks to the destination file...

      // Close the stream
      responseStream.Close();
   }
}

Accessing Content on the Web

One of the built-in pluggable protocols available in the .NET Compact Framework for handling HTTP and HTTPS requests to the Internet is the HttpWebRequest class. As with any other protocol-specific class, it has been derived from the WebRequest class and can be created by using the WebRequest.Create() method:

HttpWebRequest httpReq =
   (HttpWebRequest)WebRequest.Create("http://www.
   furrygoat.com");

The HttpWebRequest class contains the methods and properties described in Table 12.14.

Table 12.14. HttpWebRequest Class Methods and Properties

To get the results for the request that was made by the HttpWebRequest object, you can use the GetResponse() method:

HttpWebResponse webResponse =
   (HttpWebResponse)httpReq.GetResponse();

The HttpWebResponse class supports the methods and properties described in Table 12.15.

Table 12.15. HttpWebResponse Class Methods and Properties

The following code shows how to create a new request for a Web resource, using the StreamReader class to read in the response that you receive from the Web server:

using System;
using System.Data;
using System.Net;
using System.IO;

namespace WebSample {
   class WebTest {
      static void Main(string[] args) {
         // Make a new WebRequest object
         System.Uri urlRequest = new
            System.Uri("http://www.furrygoat.com/");
         HttpWebRequest webClient = (HttpWebRequest)
            WebRequest.Create(urlRequest);

         // Get the response
         HttpWebResponse webResponse = (HttpWebResponse)
            webClient.GetResponse();

         // Use a StreamReader class to read in the response
         StreamReader responseStream = new StreamReader(
            webResponse.GetResponseStream(),
            System.Text.Encoding.ASCII);

         // Copy the stream to a string, do something with it
         // string strResponse = responseStream.ReadToEnd();

         // Close the stream
         responseStream.Close();
      }
   }
}

The response stream, strResponse, contains the HTML code that was downloaded from the Web site:

<HTML>
<title>The Furrygoat Experience</title>
<body>
<p><b><font face="Arial">This is the Furrygoat homepage!
  </font></b></p>
</body>
</HTML>