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2007-12-20

Best Practice to Handle Exceptions

Modern programming languages come with try/catch/finally blocks and many times an Exception Hierarchy of classes is provided also.
Exceptions are a powerful tool that can be used to handle both system and user errors.
When exceptions are thrown in some part of the code, the system keeps popping ( removing the head ) of the stack until it finds a catch statement that can handle that exception type.
One common way to capture exceptions is to write several catches from the most specific to the most generic exception class until you can capture the exception type appropriately according to its type.
Check code below:

try
{
// code block
}
catch ( SpecialSystemException e)
{
// code to handle SpecialSystemException
}
catch ( SystemException e)
{
// code to handle SystemException
}
catch ( Exception e)
{
// code to handle Exception
}
finally
{
// finishes code block
}


However in many cases it is desired to treat each exception same way according to its type in the entire system. In this case the code template shown above has some disadvantages:
  • Programmers must write more code to implement try/catch/finally code blocks to due the several "catch" statements for each exception type that is intended to treat
  • Code to treat exception classes are replicated in the software code
  • And finally if it demands more work and it is repeated than it is error prone
In order to solve this problem a better approach is to create a separate centralized class to treat all exceptions thrown from the system internally. Like the .NET pseudo-code example below:

public class ExceptionHandler
{
public static void Handler(Exception exception)
{
if (exception.GetType() == typeof(SpecialSystemException))
{
// Code that deals with SpecialSystemException
}
else if (exception.GetType() == typeof(SystemException))
{
// Code that deals with SystemException
}
else if (exeception.GetType() == typeof(Exception))
{
// Code that deals with Exception
}
}


Replacing the code of the first example:

try
{
// code block
}
catch ( Exception e)
{
// Exceptions are now treated internally by the method below
ExceptionHandler.Handler(exception);
}
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