Exception handling: Catch, throw or let it go?

When you think about it, structured exception handling is a powerful yet reasonably straightforward mechanism for...well for handling exceptions in a structured way. You call a method and it either throws an exception or it doesn't. If it does, you have 3 choices:

• Rethrow the exception
• Throw a different exception
• Catch the exception and continue processing

There are no chiselled, marble tablets defining the universal laws of exception handling. There is no perfect strategy for dealing with every possible exceptional scenario. However, I have seen enough poorly implemented error handling strategies to at least recommend a few guidelines here.

Be careful what you catch
Any catch blocks following Catch(System.Exception) will be ignored because System.Exception is the base class for every other exception type. Luckily, the compiler does not allow this anyway and raises an exception of its own if you decide to ignore this advice.

Only catch an exception if you expected it and you know what to do about it. Never use the construct shown in Snippet 2 in a class library. The way an exception is handled should always be the responsibility of the calling application.

A System.Exception should never be caught unless it is rethrown. This rule should always be followed for class libraries. Within an application, the application policy will determine whether a System.Exception should be rethrown.

Be careful what you throw
Never throw System.Exception; take care when throwing any other exception base class types. If you are defining your own exceptions, then think carefully about its base class. For example, if you derive an exception type from ArgumentException, then any code that already catches ArgumentException will catch your new exception type. This may or may not be a problem based on how you (or someone else) is handling the exception.

All custom exception types should end with 'Exception'. e.g. NumberTooLargeException, NameNotFoundException etc.

Use the recovery position
So you have a class library and an exception occurs. You want to leave the responsibility of handling the exception to the calling application but you also want to make sure that your system is not left in an unrecoverable state. So what do you do (please see Snippet 1)?

Snippet 1  public void LibraryMethod()  {      try {          //Method logic here      }      catch {          //Handle system consistency here          throw;      }  }

Basically, all possible exceptions are caught, the system's state is verified and the same exception that originally ocurred is rethrown. It may be appropriate to restore consistency in a Finally block depending on the context of your work.

Please note that using throw ex; instead of throw; causes the stack trace to be reset here.

Avoid boilerplate code
It's easy to get into the habit of adding Try/Catch/(Finally) blocks to each method that you write but this really is not the way to approach structured exception handling. I have worked on projects where the following pattern is used with alarming regularity:

Snippet 2  public void ProcessData(int firstNum, int secondNum)  {      try {          //Method logic here      }      catch (Exception ex) {          //Log error here      }  }

So any exception is caught, logged and discarded. The application continues (most probably in an inconsistent state) and the user/developer remains blissfully unaware until they make a conscious decision to check the logging device and find entries representing errors.

Validate parameters
Where you are creating a library that will be used by other developers, it is prudent to validate the parameters of all public and protected methods. This has a number of benefits:

• The caller is informed if invalid parameters are supplied
• The code is more likely to run consistently with valid data
• Problems are identified quickly, less code is executed with invalid data
• Effort to rollback to a consistent state may be avoided

If a problem is detected, throw an exception that derives from ArgumentException; the most notable being ArgumentNullException and ArgumentOutOfRangeException. If these are not appropriate then consider throwing a custom exception derived from ArgumentNullException or ArgumentNullException itself. Snippet 3 illustrates the process.

Snippet 3  public int DivideNumbers(int numerator, int denominator)  {      //Validate denominator to ensure non-zero value.      if (denominator == 0)      {          throw new ArgumentOutOfRangeException(              "denominator", "Denominator cannot be zero");      }      return numerator/denominator;  }

Be aware that arguments passed to methods by reference may be changed after validation has occurred (e.g. in a multi-threaded application). In these cases it may be wise to create a copy of the argument(s) and operate exclusively on this copy within the method.

In the example shown, the denominator is validated against a value of zero. An alternative here would be to use a Catch block to trap this occurence. The actual method used depends on how often the situation is likely to occur. The validation as shown is fine but this check will be performed on each pass through the method. If the possibility of denominator being zero is rare then it may be more prudent to use a Catch block and rethrow the exception for improved performance.

And Finally...
A Finally block is guaranteed to run whether an exception occurs or not. It is therefore the logical place to add clean-up code. Beware that any exceptions raised within a Finally block will hide details of any exception that was raised in the corresponding Try block so try to avoid them.

Structured exception handling provides a consistent framework for dealing with exceptional situations. Taking an umbrella with you on a winter's day stroll adds a little overhead to your journey but it also prevents you from getting soaked if the heavens open. Handle exceptions in a similar way; prepare for what may happen before you start your development journey and you will never find yourself without an umbrella if something unexpected happens.