Global Exception Handling

[[MSFT]]

Exceptions thrown by an XML Web service method created using ASP.NET are
sent back to the client in the form of a SOAP fault. A SOAP fault is a
<Fault> XML element within a SOAP message that specifies when an error
occurred. When passing a SOAP fault, ASP.NET follows the method prescribed
for propagating errors back to a client by the SOAP specification. The SOAP
<Fault> XML element contains details such as the exception string and the
source of the exception. For details on SOAP faults, see the W3C Web site
([url]http://www.w3.org/TR/SOAP[/url]).

Fortunately, both clients and XML Web services created using ASP.NET do not
populate or parse the <Fault> XML element directly, but rather use the
common design pattern for throwing and catching exceptions in the .NET
Framework. An XML Web service can either throw an exception specific to the
problem, such as an ArgumentOutOfRangeException or SoapException. Either
way, ASP.NET serializes the exception into a valid SOAP message by placing
the exception into a SOAP fault element. When the SOAP message is
deserialized on an ASP.NET client, the SOAP fault is converted to a
SoapException exception, with the exception details placed in the Message
property. A client can thus set up a Try/Catch block to catch a
SoapException.

A Web application can be comprised of multiple XML Web services, however
the Application_Error event within the Global.asax file cannot be used for
global exception handling. The HttpHandler for XML Web services consumes
any exception that occurs while an XML Web service is executing and turns
it into a SOAP fault prior to the Application_Error event is called. Build
a SOAP extension to process XML Web service exceptions in a global
exception handler. A SOAP extension can check for the existence of an
exception in the ProcessMessage method. Within the ProcessMessage method,
check the Exception property of the SoapMessage passed when the Stage
property is set to AfterSerialize.

For more informaiton abour soap extensions, you can refer to:

[url]http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/htm[/url]
l/cpconAlteringSOAPMessageUsingSOAPExtensions.asp

Generally, we can catch the error in AfterSerialize with following code:

public void WriteOutput(SoapMessage message)
{


if (message.Exception != null)
{
MemoryStream ErrMsg= new MemoryStream();
StreamWriter ErrWriter = new StreamWriter(ErrMsg);

ErrWriter.WriteLine("...");
ErrWriter.Flush();
ErrWriter.Close();
Copy(ErrMsg, oldStream);
}





}


Hope this help,

Luke
Microsoft Online Support

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rights.)

[[MSFT]]

Hello,

With classic ASP, wen can catch the exceptions in Application_Error. Could
you please tell me how you implement web service with classic ASP? When we
mention Web Service, we always mean ASP.NET web service. Is the ASP
application is a client of your web service?

If I have any misunderstanding, please feel free to let me know.

Luke
Microsoft Online Support

Get Secure! [url]www.microsoft.com/security[/url]
(This posting is provided "AS IS", with no warranties, and confers no
rights.)

[Cindy]

Hi Luke

Sorry for the confusion. The web service is written in C#, it is a classic ASP application that is consuming the web service. I have a good handle on catching errors in the classic ASP side, it is the throwing of the errors from the C# web service where I am having problems

When there is an error in the C# web service there is no error bubbled down to classic ASP. This is OK, I am completely comfortable with checking the return string of the web method for an error. The problem is that the return from the web service, when there is an error in the web service, does not contain the error in an XML document. It is being returned as pure text, no XML tags

What I would like to do is within the C# web service, in a global manner, be able to format the error in XML so that the classic ASP application can check the return from the web service for an error in the string. I am hoping to be able to handle this globally within the webservice so I can avoid having to try, catch, and rethrow in every webmethod just to get the error to return as an XML representation of itself. I was under the impression that an error from the webservice would be returned as XML, but it isn't. It is just returning as plain text (no XML tags)

Thanks again
Cindy

[[MSFT]]

Hi Cindy,

Thank you for the information. Normally, when there is a exception in web
method, it will return a SOAP message like:

<soap:fault>
...
</soap:fault>

It is well formed XML string. Did you use SOAP client in classic ASP to
access C# web service? If so, you will get the exception message directly
like:

System.Web.Services.Protocols.SoapException: Server was unable to process
request. ---> System.DivideByZeroException: Attempted to divide by zero.

Is this why you tell you got plain text instead of XML?

Luke
Microsoft Online Support

Get Secure! [url]www.microsoft.com/security[/url]
(This posting is provided "AS IS", with no warranties, and confers no
rights.)

[Cindy]

Hi Luke

I did not use SOAP directly from classic ASP to access the C# web service. Here is a code snippet from the ASP application that accesses the web service. If there is an error, it is returned in the xmlhttp.ResponseText as plain text (no XML tags)

xmlhttp.Open "POST", serviceUrl, Fals
xmlhttp.setRequestHeader "Content-Type", "application/x-www-form-urlencoded
xmlhttp.send queryStringValue
rtnValue = xmlhttp.ResponseTex

[[MSFT]]

In this way, the ResponseText will only contains the exception message
indeed. To make sure if there is an error from web service, you may check
xmlhttp's Status and StatusText. If there is an error from web service, the
Status should be an error number other than 200.

If you need customized error message from web service, you may refer to my
first message. The SoapExtesion should be able to achieve this.

Luke
Microsoft Online Support

Get Secure! [url]www.microsoft.com/security[/url]
(This posting is provided "AS IS", with no warranties, and confers no
rights.)

[Cindy]

Thank you! That will work great for centralizing the error handling in the classic ASP code. :)

[Unregistered]

Very informative post. It helped me lot in understanding Exception handling in c#.net. Thanks for sharing with us and I would like to appreciate you please keep writing. While I am searching article on this topic over the internet then I was found another article too which also explained very well on exception handling in c#.Net, please check that url post at once......
http://mindstick.com/Articles/de21d4fc-c6ea-4ae8-91db-874b727e5e6c/?Exception%20Handling%20in%20C%20Sharp

Thanks everyone for your precious post.