Logging an Exception

Article
05/03/2010

A frequently required exception-handling task is to log the information associated with the exception. The Exception Handling Application Block, in conjunction with the Logging Application Block, lets you log formatted exception information in locations specified in the configuration file. For example, client applications typically log application information in the event log, while a server component of an e-commerce application may log exceptions in a database.

Typical Goals

You want to log an exception that occurred in one of the application layers before any other exception handling occurs. Your application has code in its catch blocks similar to the following. The code does not include the definition of the FormatException and Logger.Log methods. These methods represent typical application code to create a formatted exception message and write it to a log destination.

catch(SomeException e)
{
  string formattedInfo = FormatException(e);
  Logger.Log(e);
  throw e;
}

'Usage
Catch e As SomeException
  Dim formattedInfo As String = FormatException(e)
  Logger.Log(e)
  Throw e
End Try

Solution

Designate the logging handler as the first handler in your policy's exception handling chain. The Exception Handling Application Block includes the logging handler.

Using the Logging Handler

To use the logging handler, you must configure the application exception policies and the Logging Application Block. You must then modify your application to use the appropriate exception handling policy.

To configure the application exception policies

Use the configuration tools to create an exception handling policy for your application layer. For more information, see Entering Configuration Information.
Specify the exception types to be processed and logged.
Add the logging handler as the first handler for each exception type.
Configure the logging handler:
- Enter the event ID.
- Enter the logging categories.
- Select the severity.
- Enter the title of the log entry.
- Select the formatter type name.
- Enter the priority.
Add any additional exception handlers to be invoked after the logging handler.

To configure the Logging Application Block

Add the Logging Application Block to your application configuration (adding a logging handler will automatically add the Logging Application Block to your application configuration). For more information, see Adding Application Code in the documentation for The Logging Application Block.
Configure the Logging Application Block.

Modify Your Application

Modify your application code to execute the new policy when an exception occurs. The following code demonstrates how to do this. It assumes you have resolved an instance of the ExceptionManager class through the Enterprise Library container and saved it in a variable named exManager. Substitute the name "Logging Policy" with the name of your own policy. For more information on instantiating objects, see Creating and Referencing Enterprise Library Objects.

try
{
  // Run code.
}
catch(Exception ex)
{
  bool rethrow = exManager.HandleException(ex, "Logging Policy");
  if (rethrow)
    throw;
}

'Usage
Try
  ' Run code.
Catch ex As Exception
  Dim rethrow As Boolean = exManager.HandleException(ex, "Logging Policy")
  If (rethrow) Then
    Throw
  End If
End Try

Usage Notes

Consider the following information when you configure the block to use a logging handler:

Exceptions can be logged any time during processing; they do not have to be logged at the beginning of the event handling sequence. For example, you can log an exception after it is wrapped or replaced.
The Exception Handling Application Block provides a logging exception handler that depends on the Logging Application Block. You can also incorporate custom logging functionality into your own exception handler to use instead of the Logging Application Block.
If the post handling action is set to ThrowNewException, the exception thrown by the block is the final exception that results from running the entire chain of handlers. For example, if the chain specifies that exception A is to be wrapped with exception B, and a later handler replaces exception B with exception C, the block throws exception C when the post handling action is set to ThrowNewException.
If the post handling action is set to NotifyRethrow, the block returns true to the calling code. This allows the code to throw the original exception.
The preceding code example uses a single generic handler for all managed exceptions (these are exceptions derived from the System.Exception class. Your own application could have other catch statements that call the HandleException method with different policies or that do not use the Exception Handling Application Block. The code in this example uses the policy name "Logging Policy" to emphasize its behavior. Generally, a policy name reflects the application layer or component that uses it, instead of its function. For example, a policy used by the data access layer would be named "Data Access Layer Policy".

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