Exceptions Generated by Create()
The Create() method defined by WebRequest can generate three exceptions. If the protocol specified by the URI prefix is not supported, then NotSupportedException is thrown. If the URI format is invalid, UriFormatException is thrown. It can also throw an ArgumentNullException if it is called with a null reference, but this is not an error generated by networking.
Exceptions Generated by GetResponse()
If an error occurs when obtaining a response by calling GetResponse(), a WebException is thrown. In addition to the members defined for all exceptions, WebException adds two properties that relate to network errors: Response and Status.
You can obtain a reference to the WebResponse object inside an exception handler through the Response property. This is the object that would have been returned by GetResponse() if an exception had not occurred. It is defined like this:
public WebResponse Response { get; }
When an error occurs, you can use the Status property of WebException to find out what went wrong. It is defined like this:
public WebExceptionStatus { get; }
WebExceptionStatus is an enumeration that contains the following values:
ConnectFailure
ConnectionClosed
KeepAliveFailure
NameResolutionFailure
Pending
PipelineFailure
ProtocolError
ProxyNameResolutionFailure
ReceiveFailure
RequestCanceled
SecureChannelFailure
SendFailure
ServerProtocolViolation
Success
Timeout
TrustFailure
Once the cause of the error has been determined, your program can take appropriate action.
Exceptions Generated by GetResponseStream()
The GetResponseStream() method of GetResponse 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. Also, an IOException could occur while reading the stream.
Example:
// Handling Network Exceptions.
using System;
using System.Net;
using System.IO;
class Exception
{
public static void Main()
{
int ch;
try
{
//Create a WebRequest to a URI.
HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://dotnetspider.com");
//send that request and return the response.
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
//obtain an input stream from the response.
Stream stream = response.GetResponseStream();
//Read & Display the html, press ENTER after 400 characters are displayed.
for(int i = 1; ; i++)
{
ch = stream.ReadByte();
if (ch == -1) break;
Console.WriteLine((char)ch);
if((i % 400) == 0)
{
Console.Write("\nPress ENTER");
Console.Read();
}
}
response.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 Error: " +exc.Message);
}
catch(NotSupportedException exc)
{
Console.WriteLine("Unkown Protocol: " +exc.Message);
}
catch(IOException exc)
{
Console.WriteLine("I/O Error: " +exc.Message);
}
Console.Read();
}
}
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://dotnetspider.com/yellow.aspx");
and then recompile and run the program, you will see the output:
Network Error: The remote server returned an error: (404) Not Found.
Status Code: ProtocolError
Since the dotnetspider.com web site does not have a directory called “yellow“, this URI is not found, as the output confirms.
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| Author: Ashish Jaiman 14 Jun 2004 | Member Level: Bronze Points : 0 |
you should have called response.close in finally
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