HTTP compression in the .NET Framework 1.1

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Introduction

This article will show how to request for HTTP compression and handle a compressed response from a web server (or any appliance connected to the server) without changing the client application.

The factory pattern of creating WebRequest instances

The WebRequest supplies two methods to create instances:

• CreateDefault - Initializes a new WebRequest instance for the specified URI scheme.
• Create - Initializes a new WebRequest instance.

The WebRequest class uses instances of classes that implement the IWebRequestCreate interface and are registered in the webRequestModules section in the configuration files. The Create method (called by both CreateDefault and Create) returns an initialized instance of a WebRequest descendent class capable of performing a standard request/response transaction for the protocol without needing any protocol-specific fields modified.

Because we can control the WebRequest factory using the configuration files, we can change the behavior of already built applications (our applications, or even the .NET Framework) to request and handle HTTP compression without changing them.

The WebRequest descendent instances will be responsible to create the WebResponse descendent instance that will handle the response from the web server.

The code

As shown before, to add HTTP compression to our applications, we just have to build three classes:

• CompressibleHttpRequestCreator - implementing the IWebRequestCreate interface.
• CompressibleHttpWebRequest - derived from WebRequest.
• CompressibleHttpWebResponse - derived from WebResponse.

  In order for the applications that use HttpWebRequest and HttpWebResponse to work without any changes, CompressibleHttpWebRequest has to derive from HttpWebRequest, and CompressibleHttpWebResponse has to derive from HttpWebResponse.

  Fortunately, HttpWebRequest and HttpWebResponse are not sealed (NotInheritable in VB.NET). Unfortunately, these classes don't have any public constructor, and the only protected constructor each one has is the one required by the implementation of the ISerializable interface. Due to this drawback, reflection must be used.

  CompressibleHttpRequestCreator

  This is the class that will be implementing the IWebRequestCreate interface and is used to create HttpWebRequest descendant instances.

  As mentioned before, the only way to create an instance of a HttpWebRequest derived class, is by deserialization. To accomplish this, an instance of HttpWebRequest will be created, serialized, and deserialized into an instance of CompressibleHttpWebRequest.

  public class CompressibleHttpRequestCreator : IWebRequestCreate
  {
      WebRequest IWebRequestCreate.Create(Uri uri)
      {
          ISerializable httpWebRequest = Activator.CreateInstance(
              typeof(HttpWebRequest),
              BindingFlags.CreateInstance | BindingFlags.Public | 
              BindingFlags.NonPublic | BindingFlags.Instance,
              null,
              new object[] { uri },
              null) as ISerializable;
  
          if (httpWebRequest == null)
          {
              return null;
          }
  
          SerializationInfo serializationInfo = new SerializationInfo(
              typeof(CompressibleHttpWebRequest), new FormatterConverter());
          StreamingContext streamingContext = 
              new StreamingContext(StreamingContextStates.All);
          httpWebRequest.GetObjectData(serializationInfo, streamingContext);
  
          CompressibleHttpWebRequest webRequest = new 
              CompressibleHttpWebRequest(serializationInfo, streamingContext);
          webRequest.Method = serializationInfo.GetString("_OriginVerb");
  
          return webRequest;
      }
  }

  CompressibleHttpWebRequest

  This class will handle the setup of the HTTP request and the creation of the HttpWebResponse derived instance to handle the response.

  As with CompressibleHttpWebRequest, when creating the HttpWebResponse, an instance of HttpWebResponse will be created, serialized, and deserialized into an instance of CompressibleHttpWebResponse.

  To specify what types of compression will be accepted in the response, the AcceptEncodings will be used. Its value will, also, be used to set the accept-encoding HTTP header in the request (in the BeginGetResponse). The possible values for AcceptEncodings are:

  • Identity - The default (identity) encoding; the use of no transformation whatsoever.
  • GZip - An encoding format produced by the file compression program "gzip" (GNU zip) as described in RFC 1952 [25]. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.
  • Deflate - The "zlib" format defined in RFC 1950 [31] in combination with the "deflate" compression mechanism described in RFC 1951 [29].

  [Serializable]
  public class CompressibleHttpWebRequest : HttpWebRequest
  {
      private static FieldInfo m_UsesProxySemanticsFieldInfo = 
          typeof(HttpWebResponse).GetField("m_UsesProxySemantics", 
          BindingFlags.NonPublic | BindingFlags.Instance);
      private static FieldInfo m_ConnectStreamFieldInfo = 
          typeof(HttpWebResponse).GetField("m_ConnectStream", 
          BindingFlags.NonPublic | BindingFlags.Instance);
      private AcceptEncodings acceptEncodings = AcceptEncodings.GZip | 
          AcceptEncodings.Deflate | AcceptEncodings.Identity;
  
      public AcceptEncodings AcceptEncodings
      {
          get
          {
              return this.acceptEncodings;
          }
          set
          {
              this.acceptEncodings = value;
          }
      }
  
      protected internal CompressibleHttpWebRequest(SerializationInfo 
                serializationInfo, StreamingContext 
                streamingContext) : base(serializationInfo, 
                                         streamingContext)
      {
      }
  
      public override IAsyncResult 
          BeginGetResponse(AsyncCallback callback, object state)
      {
          if (this.acceptEncodings != AcceptEncodings.Identity)
          {
              this.Headers.Add("Accept-Encoding",
                  (this.acceptEncodings & 
                     ~AcceptEncodings.Identity).ToString().ToLower());
          }
          return base.BeginGetResponse(callback, state);
      }
  
      public override WebResponse EndGetResponse(IAsyncResult asyncResult)
      {
          ISerializable httpWebResponse = 
             base.EndGetResponse(asyncResult) as ISerializable;
          if (httpWebResponse == null)
          {
              return null;
          }
  
          SerializationInfo serializationInfo = new 
              SerializationInfo(typeof(CompressibleHttpWebResponse),
              new FormatterConverter());
          StreamingContext streamingContext = new 
              StreamingContext(StreamingContextStates.All);
          httpWebResponse.GetObjectData(serializationInfo, streamingContext);
          CompressibleHttpWebResponse webResponse = new 
              CompressibleHttpWebResponse(serializationInfo,
              streamingContext);
          m_UsesProxySemanticsFieldInfo.SetValue(webResponse, 
              m_UsesProxySemanticsFieldInfo.GetValue(httpWebResponse));
          m_ConnectStreamFieldInfo.SetValue(webResponse, 
              m_ConnectStreamFieldInfo.GetValue(httpWebResponse));
  
          return webResponse;
      }
  }

  CompressibleHttpWebResponse

  This is the easiest one. All that's needed is to handle the response stream accordingly to the content-encoding HTTP response header. Unfortunately, the .NET Framework 1.1 base classes don't provide any class for handling Deflate and GZip streams. For that, the SharpZipLib will be used.

  [Serializable]
  public class CompressibleHttpWebResponse : HttpWebResponse
  {
      public CompressibleHttpWebResponse(SerializationInfo 
          serializationInfo, StreamingContext streamingContext)
          : base(serializationInfo, streamingContext)
      {
      }
  
      public override Stream GetResponseStream()
      {
          Stream stream = base.GetResponseStream();
  
          if (stream == null)
          {
              return Stream.Null;
          }
  
          if (string.Compare(ContentEncoding, "gzip", true) == 0)
          {
              return new GZipInputStream(stream);
          }
          else if (string.Compare(ContentEncoding, "deflate", true) == 0)
          {
              return new InflaterInputStream(stream);
          }
          else
          {
              return stream;
          }
      }
  }

  Configuration

  Now, to add HTTP compression support to any application, all that's needed is to add the corresponding entry to the webRequestModules section in the configuration file.

  <?xml version="1.0" encoding="utf-8" ?>
  <configuration>
      <system.net>
          <webRequestModules>
                <add prefix="http" 
                  type="PaJoCoMo.Net.CompressibleHttpRequestCreator, PaJoCoMo" />
            </webRequestModules>
      </system.net>
  </configuration>