Exceptions Syntax, semantics, and pragmatics Exceptions1

Syntax, semantics, and pragmatics Syntax How it looks, i.e. how we have to program to satisfy the compiler. Semantics What it means / how it works Pragmatics How to use it in the proper way. Exceptions2

Introduction Exceptions are thrown from methods when something exceptional (un-expected) happens. Whether something is exceptional or not is a design issue. Is it exceptional to search a list for a non-existing element? Probably not Don’t throw an exception, return null Is it exceptional to open a non-existing file? Probably yes Throws FileNotFoundException Exceptions3

Exception related syntax try { … } catch (SomeException ex) { … } In the try block you call methods that might throw exceptions In the catch block you handle the exceptions thrown in the try block (if any) try { … } catch (SomeException ex) { … } finally The finally block is executed after the try block (and the catch block) no matter if there was an exception, or not. Usually used to close resources like files, database connections, network connections, etc. try { … } finally Sometimes you omit the catch block If you only want proper close of resources Exceptions4

The using statement With the using statements resources will automatically be opened and closed. No need for finally { …. resource.close() … } Example: TryingUsing Syntax Using (declare + open the resource) { … use the resource … } // resource is automatically closed. Works on all resources (classes) implementing the interface System.Idisposable Has a single method void dispose() Source Exceptions5

Throwing an exception If something un-expected happens your method can throw an exception. You really throw an exception object, i.e. an object of a class that extends the class Exception Syntax: throw new SomeException(…) Semantics: You leave the method. More syntax: No exception declarations in method signatures Public int divide(int a, int b) The return type is declared, in this case int. However, the type of exceptions that might be thrown is not declared In this case DiviceByZeroException Exceptions6

Exceptions and the call stack Every program has a call stack. The main() method is the bottom of the call stack Every method call pushes a new element on the call stack Method return pops an element of the call stack Throw an exception pops an element of the call stack The popping continues until the exception is caught Or if it is never caught the main() is popped from the stack, and the program terminates Exceptions7

Some exception properties Message property Text message explaining what is wrong StackTrace property Shows all the methods that was popped of the call stack InnerException property Used for exception chaining (more on that later on) Exceptions8

Sequence of catch blocks A single try statement may have several catch blocks. Each catch block handling a separate exception Catch more specific exceptions before general exceptions Example: catch the specific FileNotFoundException before the general IOException Otherwise your program does not compile And it does not make sense! Example: TryUsing -> ReadFirstLine -> Main Exceptions9

Different kinds of exception handling Ignore (an empty catch block) Generally a bad idea Does it make sense to continue program execution after ignoring the exception? If you really want the catch block to be empty, you should leave a comment for the human reader /* this catch block is intentionally left empty */ Handle Catch and do something. If you can’t do anything better than Console.print(…), don’t do it! Does it make sense to continue program execution after the catch block? Partly handle Catch the exception Do something (could be Console.print(…)) Throw the exception (or another exception) Exceptions10

Some commonly used exception classes NullReferenceException You tried to call a method on a object reference which is null Student st = null; … St.someMethod(); // NullReferenceException New throw a NullReferenceException ArgumentException You have a problem with a parameter (argument) to a method ArgumentNullException Parameter with value null is not allowed ArgumentOutOfRangeException Parameter value is out of range IndexOutOfRangeException You have a problem with our array index OutOfMemoryException You have created to many object StackOverflowException To many method calls. Normally because a method is calling itself to many times. More detailed exception More detailed information about the problem Easier /better handling of the problem Exceptions11

Making your own exception types It is really easy to make another exception type, just extend the class Exception directly or in-directly Class MyException : Exception { … } Normally you your exception class will have a few constructors. Example: TryingUsing The model layer should include a homemade exception Example: A hotel reservation system should have a HotelReservationException Technical exceptions should stay at the technical layers Catch (TechException ex) { throw new ModelException(ex); } Exceptions12

Exception chaining + exception translation If you want to catch a technical (low level) exception and instead throw a model exception (high level) you have two possibilities Exception translation Catch (TechException ex) { throw new ModelException(ex.Message); } The ModelException holds the same message as the TechException Exception chaining Catch (TechException ex) { throw new ModelException(ex); } The ModelException holds a reference to the TechException The property InnerExceptions holds the reference to the inner exception Example: LayeredLibrary Exceptions13

References and further reading MSDN Handling and Throwing Exceptions Exceptions14