C# 0 The Complete Reference

Exceptions Generated by GetResponseStream( )

For the HTTP protocol, the 

GetResponseStream( )

 method of 

WebResponse

 can throw a 

ProtocolViolationException

, which, in general, means that some error occurred relative to 

the specified protocol. As it relates to 

GetResponseStream( )

, it means that no valid 

response stream is available. An 

ObjectDisposedException

 will be thrown if the response 

has already been disposed. Of course, an 

IOException

 could occur while reading the 

stream, depending on how input is accomplished.

Using Exception Handling

The following program adds handlers for all possible network exceptions to the example 

shown earlier:

// Handle network exceptions.

using System;

using System.Net;

using System.IO;

class NetExcDemo {

  static void Main() {

    int ch;

    try {

      // First, create a WebRequest to a URI.

      HttpWebRequest req = (HttpWebRequest)

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             WebRequest.Create("http://www.McGraw-Hill.com");

      // Next, send that request and return the response.

      HttpWebResponse resp = (HttpWebResponse)

             req.GetResponse();

      // From the response, obtain an input stream.

      Stream istrm = resp.GetResponseStream();

      /* Now, read and display the html present at

         the specified URI. So you can see what is

         being displayed, the data is shown

         400 characters at a time. After each 400

         characters are displayed, you must press

         ENTER to get the next 400. */

      for(int i=1; ; i++) {

        ch =  istrm.ReadByte();

        if(ch == -1) break;

        Console.Write((char) ch);

        if((i%400)==0) {

          Console.Write("\nPress Enter.");

          Console.ReadLine();

        }

      }

      // Close the response. This also closes istrm.

      resp.Close();

    } catch(WebException exc) {

      Console.WriteLine("Network Error: " + exc.Message +

                        "\nStatus code: " + exc.Status);

    } catch(ProtocolViolationException exc) {

      Console.WriteLine("Protocol Error: " + exc.Message);

    } catch(UriFormatException exc) {

      Console.WriteLine("URI Format Error: " + exc.Message);

    } catch(NotSupportedException exc) {

      Console.WriteLine("Unknown Protocol: " + exc.Message);

    } catch(IOException exc) {

      Console.WriteLine("I/O Error: " + exc.Message);

    } catch(System.Security.SecurityException exc) {

      Console.WriteLine("Security Exception: " + exc.Message);

    } catch(InvalidOperationException exc) {

      Console.WriteLine("Invalid Operation: " + exc.Message);

    }

  }

}

Now the exceptions that the networking methods might generate have been caught. For 

example, if you change the call to 

Create( )

 as shown here,

WebRequest.Create("http://www.McGraw-Hill.com/moonrocket");

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