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14. Networking
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Configuring WebClient
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The WebClient class has various properties that can be used to configure the request and get information about the response.
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Request headers
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By default, all requests contain the following headers:
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Connection: Keep-Alive Host: <name of host derived from the URL used to create the request>
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Requests to send data to the server also contain the following headers:
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Content-Length: <length of data being sent> Expect: 100-continue
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Headers can be set through the Headers property, specifying key/value pairs through an indexer. The default headers just listed don't appear in the WebHeaderCollection that the property returns. WebHeaderCollection is derived from System.Collections.Specialized.NameObjectCollectionBase. For more information, see 9, "Collections." Setting a header for an instance of WebClient affects all subsequent requests to the server. If requests to the same server require different headers, different instances of WebClient should be used, or the headers should be modified between requests.
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Response headers
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After each call to send or receive data, the ResponseHeaders property will return a WebHeaderCollection that contains name/value pairs for each of the headers contained in the response. The response headers reflect the result of the most recent request and are overwritten with each subsequent request.
Setting a base address
The WebClient class allows a base address to be set. The request strings passed to the upload and download methods will be appended to the base address to form fully qualified URLs. If passed a fully formed request string, the WebClient base address is not used but remains in place for future requests. The following example demonstrates the use of the BaseAddress property. First the base address is set to http://www.microsoft.com. A subsequent DownloadData call specifying ms.htm results in a call to the URI http://www.microsoft.com/ms.htm:
WebClient x_client = new WebClient(); x_client.BaseAddress = "http://www.microsoft.com"; byte[] x_data = x_client.DownloadData("ms.htm");
14. Networking
WebRequest and WebResponse
The WebRequest and WebResponse classes represent the next level down in the network class hierarchy, placing the programmer closer to the protocol implementation. The main advantage of using the WebRequest and WebResponse classes over the WebClient class is the level of control available to the programmer. Instances of these abstract types can be cast to protocolspecific concrete types, taking advantage of the methods and properties that expose more control over the request. For example, the WebResponse class can be cast to HttpWebResponse, which supports features such as cookies and protocol versions. Here's a simple example using the WebRequest and WebResponse classes to write the contents of the Microsoft homepage to the console:
WebRequest x_req = WebRequest.Create("http://www.microsoft.com"); WebResponse x_response = x_req.GetResponse(); StreamReader x_reader = new StreamReader(x_response.GetResponseStream()); String x_str; while ((x_str = x_reader.ReadLine()) != null) { Console.WriteLine(x_str); } x_reader.Close();
WebRequest
WebRequest is an abstract class that encapsulates functionality common to all Internet requests. Implementation classes that represent a specific protocol are not required to deal with all of the features, so many of the properties in the class can throw a System.NotSupportedException to indicate that the feature has no meaning for the protocol that will be used to handle the request.
Creating a request
Instances of WebRequest are created using the static WebRequest.Create factory method. The Create method takes either a System.Uri or a String containing a URI that specifies the destination of the request. The factory method approach enables the Create method to return a subclass of WebRequest designed to handle the protocol specified in the URI argument. Currently only two protocolspecific handlers are provided, HttpWebRequest for http and https scheme requests and FileWebRequest for file scheme requests.
Configuring a request
Table 14-4 lists the properties of the WebRequest class. Not all of these properties apply to all protocol implementations, and many of the following items are clearly derived from an HTTP-centric view of Internet requests.
14. Networking
Table 14-4. The WebRequest Properties
Property Description Gets or sets the type of content being sent to the server. ContentType ConnectionName Sets a name for the connection. Connections with different names will use a different ServicePoint, discussed in the "HTTP Connection Features" section later in this chapter. Gets or sets the security credentials that will be used in the request. See the Credentials "Request Credentials" section coming up for more information. Gets or sets the collection of name/value pairs that will be used as the Headers headers for the request. Gets or sets the protocol method to use in the request. Method PreAuthenticate Specifies whether the available authentication information should always be sent with the request or only in response to an authorization challenge. Gets or sets the System.Net.IWebProxy that will be used for the request. Proxy Returns the URI of the request. RequestUri Gets or sets the amount of time that will elapse before the request times out. Timeout
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