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WebRequest and WebResponse
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WebRequest and WebResponse are abstract base classes for a family of classes that provide the most detailed level of control over web requests. The concrete HttpWebRequest and HttpWebResponse classes add details specific to HTTP, and .NET also offers specialized FtpWebRequest/Response and FileWebRequest/Response classes. This section will mainly
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focus on the HTTP classes. The main limitation with the WebClient-based mechanisms we ve explored so far is that they focus on the content of the request or the response. They don t provide any way to work with standard HTTP features such as the content type header, the UserAgent string, cache settings, or proxy configuration. But if you use HttpWebRequest and HttpWebResponse, all the detailed aspects of HTTP are available to you. The cost of this power is additional verbosity. The main difference is that you end up with one object to represent the request and one to represent the response, in addition to streams representing the data being sent or received. Moreover, the only way to access the data with these classes is through streams. To do the same job as Example 13-11 fetching the data from a particular URL into a string requires the rather more complex code shown in Example 13-13.
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HttpWebRequest req = (HttpWebRequest) WebRequest.Create("http://oreilly.com/"); using (HttpWebResponse resp = (HttpWebResponse) req.GetResponse()) using (Stream respStream = resp.GetResponseStream()) using (StreamReader reader = new StreamReader(respStream)) { string pageContent = reader.ReadToEnd(); Console.WriteLine(pageContent); }
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The two casts on the first two lines of Example 13-13 are a little messy, but are, unfortunately, usually necessary. The WebRequest family of classes is extensible to multiple protocols, so most of the methods are declared as returning the abstract base types, rather than the concrete types the exact type returned depends on the kind of URL you use. So if you need access to a protocol-specific feature, you end up with a cast. In fact, Example 13-13 isn t using anything protocol-specific, so we could have avoided the casts by declaring req and resp as WebRequest and WebResponse, respectively. However, the usual reason for using these classes is that you do in fact want access to HTTPspecific information. For example, you might want to simulate a particular web browser by setting the user agent string, as shown in Example 13-14.
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HttpWebRequest req = (HttpWebRequest) WebRequest.Create("http://oreilly.com/"); req.UserAgent = "Mozilla/5.0 (iPod; U; CPU iPhone OS 2_2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Mobile/5H11a"; ... as before
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This code has been split across multiple lines, as the user agent string is too wide to fit. This would let you discover what response a website would send if the request came from an Apple iPhone. (Many websites adapt their content for different devices.) As you d expect, asynchronous operation is available so that you can avoid blocking the current thread while waiting for network operations to complete. But it looks slightly different from the WebClient mechanisms we ve seen so far, because of the way in which the methods you call can change when the request gets sent. No network communication happens at the point where you create the request, so there is no asynchronous method for that. Remember, the request object represents all the settings you d like to use for your HTTP request, so it won t actually attempt to send anything until you ve finished setting the request s properties and tell it you re ready to proceed. There are two ways in which you can cause an HttpWebRequest to send the request. Asking for the response object will cause this, but so will asking for a request stream the request s GetStream method returns a write-only stream that can be used to supply the body of the request for POST or similar verbs (much like WebClient.OpenWrite). This stream will start sending data over the network as soon as your code writes data into the stream it doesn t wait until you close the stream to send the data all in one go. (For all it knows, you might be planning to send gigabytes of data.) This means that by the time it returns the stream, it needs to be ready to start sending data, which means that the initial phases of the HTTP request must be complete for example, if the request is going to fail for some reason (e.g., the server is down, or the client machine has lost its network connection), there s no point in attempting to provide the data for the request. So you ll be notified of failures of this kind when you ask for the stream. The upshot of all this is that GetStream is a blocking method it won t return until the server has been contacted and the request is underway. So there s an asynchronous version of this. But WebRequest doesn t support the event-based pattern that WebClient uses. Instead, it uses the more complex but slightly more flexible methodbased Asynchronous Programming Model, in which you call BeginGetRequestStream, passing in a delegate to a method that the request will call back once it s ready to proceed, at which point you call EndGetRequestStream. This Begin/End pattern is very common in .NET and will be discussed in 16. The second way in which the sending of the request can be triggered is to ask for the response object if you haven t already asked for the request stream (e.g., because you re doing a GET, so there is no request body) the request will be sent at this point. So GetResponse also has an asynchronous option. Again, this uses the method-based asynchronous pattern. Example 13-15 shows a version of Example 13-13 modified to get the response object asynchronously.
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HttpWebRequest req = (HttpWebRequest) WebRequest.Create("http://oreilly.com/"); req.BeginGetResponse(delegate(IAsyncResult asyncResult) {
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using (HttpWebResponse resp = (HttpWebResponse) req.EndGetResponse(asyncResult)) using (Stream respStream = resp.GetResponseStream()) using (StreamReader reader = new StreamReader(respStream)) { string pageContent = reader.ReadToEnd(); Console.WriteLine(pageContent); } }, null);
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This example uses an anonymous method as the completion callback, which allows the code to retain a similar structure to the original, synchronous version. But you need to be mindful that the code that handles the response in Example 13-15 is now a separate method, and could run some considerable length of time after the call to Begin GetResponse returns, and probably on a different thread. So as with the event-based pattern, you ll need to ensure that your application runs for long enough for the operation to complete having some outstanding asynchronous operations in progress will not keep your process alive if all of the nonbackground threads exit.
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This asynchronous pattern does not take care of UI threading issues (unlike the event-based pattern seen previously). The completion callback will usually occur on some random thread, and attempting to update the user interface from that code will fail. We ll see how to handle this in 16.
Example 13-14 shows just one of the HTTP protocol features you can customize the UserAgent string. Many similar settings are available, many of which are quite obscure, so we won t go through all of them here. That s what the MSDN reference is for. But we will cover the most common cases.
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