qr code generator c# asp.net Handling Network Errors in Visual C#.NET

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Handling Network Errors
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Although the program in the preceding section is correct, it is not resilient Even the simplest network error will cause it to end abruptly Although this isn t a problem for the example programs shown in this chapter, it is something that must be avoided in real-world applications To handle network exceptions that the program might generate, you must monitor calls to Create( ), GetResponse( ), and GetResponseStream( ) It is important to understand that the exceptions that can be generated depend upon the protocol being used The following discussion describes several of the exceptions possible when using HTTP
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Exceptions Generated by Create( )
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The Create( ) method defined by WebRequest that is used in this chapter can generate four 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 If the user does not have the proper authorization, a SystemSecuritySecurityException will be thrown Create( ) can also throw an ArgumentNullException if it is called with a null reference, but this is not an error generated by networking
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Exceptions Generated by GetReponse( )
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A number of errors can occur when obtaining an HTTP response by calling GetResponse( ) These are represented by the following exceptions: InvalidOperationException, ProtocolViolationException, NotSupportedException, and WebException Of these, the one of most interest is WebException WebException has 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 For the HTTP protocol, this object describes the error 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 Status {get; } WebExceptionStatus is an enumeration that contains the following values:
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CacheEntryNotFound KeepAliveFailure Pending ProxyNameResolutionFailure RequestProhibitedByCachePolicy SendFailure Timeout ConnectFailure MessageLengthLimitExceeded PipelineFailure ReceiveFailure RequestProhibitedByProxy ServerProtocolViolation TrustFailure ConnectionClosed NameResolutionFailure ProtocolError RequestCanceled SecureChannelFailure Success UnknownError
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PART II
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Once the cause of the error has been determined, your program can take appropriate action
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 network exceptions to the example shown earlier:
// Handle network exceptions using System; using SystemNet; using SystemIO;
Part II:
Exploring the C# Library
class NetExcDemo { static void Main() { int ch; try { // First, create a WebRequest to a URI HttpWebRequest req = (HttpWebRequest) WebRequestCreate("http://wwwMcGraw-Hillcom"); // Next, send that request and return the response HttpWebResponse resp = (HttpWebResponse) reqGetResponse(); // From the response, obtain an input stream Stream istrm = respGetResponseStream(); /* 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 = istrmReadByte(); if(ch == -1) break; ConsoleWrite((char) ch); if((i%400)==0) { ConsoleWrite("\nPress Enter"); ConsoleReadLine(); } } // Close the Response This also closes istrm respClose(); } catch(WebException exc) { ConsoleWriteLine("Network Error: " + excMessage + "\nStatus code: " + excStatus); } catch(ProtocolViolationException exc) { ConsoleWriteLine("Protocol Error: " + excMessage); } catch(UriFormatException exc) { ConsoleWriteLine("URI Format Error: " + excMessage); } catch(NotSupportedException exc) { ConsoleWriteLine("Unknown Protocol: " + excMessage); } catch(IOException exc) { ConsoleWriteLine("I/O Error: " + excMessage); } catch(SystemSecuritySecurityException exc) { ConsoleWriteLine("Security Exception: " + excMessage); } catch(InvalidOperationException exc) { ConsoleWriteLine("Invalid Operation: " + excMessage); } } }
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