Exceptions

• Overview
• The throws clause
• The throws clause in properties
• Simple try/catch blocks
• Multiple catch blocks for one try block
• Finally blocks
• Try blocks with a finally block but no catch blocks
• Possible syntax errors

Overview

Exception handling in ASIL is largely modeled after Java's exception handling. Exceptions must derive from one of the following:

Exception base class/interface	What it does
IThrowable	All exception classes must implement this interface
Exception	If a property or procedure throws anything of type Exception or any class derived from Exception, it must be declared with the throws clause or handled with a try/catch block.
Error	This is an abstract class whose derived types don't need to be caught. They are meant for dynamic errors.

Even though exceptions of type Error don't need to be caught, all compilers should provide an option to require instances of an Error type and its derivatives to be caught. This could be useful when the author of a exception class use Error when they should have picked Exception as their base type.

The throws clause

The throws keyword can be applied to any procedure as shown below.

command DoThis
    throws Exception
  ' Run some code that might throw Exception

The throws clause in properties

If a property needs to throw an exception, the throws clause should be applied to the accessor, not the property.

property String MyProp
  get
      throws Exception
    ' Run some code that might throw Exception

  set
  ' This accessor doesn't throw any exceptions

Simple try/catch blocks

If you need to catch rather than declare an exception, use this syntax:

try
  ' code that might throw an exception
catch Exception e
  ' handle the exception

Multiple catch blocks for one try block

This sample show how to handle multiple exception types:

try
  ' code that might throw several different types of exceptions
catch ExceptionA e
  ' Handle this exception
catch ExceptionB e
  ' Handle this one
catch Exception e
  ' Handle all other exceptions

Finally blocks

You can use a finally block to ensure a variable is initialized or to allow the garbage collector to deallocate a variable.

try
  ' Code the might throw an exception
catch Exception e
  ' Handle that exception
finally
  ' clean up

Try blocks with a finally block but no catch blocks

If you're declaring exceptions rather than catching them, you still might want a try block so you can have a finally block clean up.

command CallMe
    throws Exception
  try
    ' Do something that might throw an exception
  finally
    ' clean up, but don't handle the exception

Possible syntax errors

If you have a try block, you must either follow it with at least one catch block or a finally block. All compilers must catch this situation.