Chapter Eight

1. Chapter Eight Exception Handling

2. Objectives • Learn about exceptions and the Exception class • How to purposely generate a SystemException • Learn about traditional error-handling methods • Learn about object-oriented exception-handling methods

3. Objectives • How to use the Exception class’s ToString() method and Message field • How to catch multiple Exceptions • How to use the finally block • How to handle an Exception with a loop

4. Objectives • How to throw an Exception • How to trace Exceptions through the call stack • How to create your own Exception classes

5. Understanding Exceptions • An exception is any error condition or unexpected behavior in an executing program • Certain errors are called exceptions because they are not usual occurrences • The object-oriented technique used to manage exceptions make up the group of methods known as exception handling • In C#, all exceptions are objects

6. Understanding Exceptions • Most exceptions you will use derive from two classes that in turn derive from the Exception class • The predefined Common Language Runtime exception classes derived from SystemException • The user-defined application exception classes you derive from ApplicationException

7. Purposely Generating a SystemException • You can deliberately generate a SystemException exception by forcing a program to contain an error

8. Purposely Generating a SystemException • The DivideByZeroException object below was generated automatically by C# • Just because an Exception occurs when an Exception object is created, you don’t necessarily have to deal with it

9. Understanding Object-Oriented Exception-Handling Methods • In object-oriented terminology, you “try” a procedure that may not complete correctly • A method that detects an error condition or Exception “throws” an Exception • The block of code that processes the error “catches” the Exception

10. Understanding Object-Oriented Exception-Handling Methods • When you write a block of code in which something can go wrong, you can place the code in a try block, consisting of: • The keyword try • An opening curly brace • Statements that might cause Exceptions • A closing curly brace

11. Understanding Object-Oriented Exception-Handling Methods • You create a catch block with the following elements: • The keyword catch, followed by an opening parenthesis, the Exception type, a name for an instance of the Exception type, and a closing parenthesis • An opening curly brace • Statements that take the action you want to use to deal with the error condition • A closing curly brace

12. Understanding Object-Oriented Exception-Handling Methods • General form of a try…catch pair

13. Using the Exception Class’s ToString() Method and Message Field • Any Exception generated from within a try block would be caught by a catch block whose argument is an Exception type (e.g., Exception e); there is no way to confirm the origin of the Exception • You can use the ToString() method to provide a descriptive error message • The Exception class also contains a field named Message that contains useful information about an Exception

14. Using the Exception Class’s ToString() Method and Message Field • Output of UsingTheException program when user enters 0 for second number

15. Catching Multiple Exceptions • You can place as many statements as you need within a try block, and you can catch as many different Exceptions as you want • If you place more than one statement in a try block, only the first error-generating statement throws an Exception • When multiple catch blocks are present, they are examined in sequence until a match is found for the Exception that occurred

16. Catching Multiple Exceptions • TwoErrors class with two catch blocks

17. Catching Multiple Exceptions • Output of TwoErrors program

18. Catching Multiple Exceptions • If you reverse the two try statements within the TwoErrors class, the output of the program changes

19. Catching Multiple Exceptions • When you want to execute the same code, no matter what type of Exception occurs, you can use only one catch block, which receives type Exception

20. Catching Multiple Exceptions • The Exception class is the base class for all Exception objects and therefore can reference all Exception descendants • The catch block in the previous code accepts all Exception argument types • When you list multiple catch blocks following a try, you must be careful that some catch blocks don’t become unreachable

21. Catching Multiple Exceptions • Program with unreachable catch block

22. Catching Multiple Exceptions • Error message generated by UnreachableCatch program

23. Using the finally Block • The code within a finally block executes whether or not the try block identifies an Exception • Typically, the finally block is used to perform clean-up tasks • When you include a finally block, you are assured that the finally statements will execute before the program is abandoned

24. Using the finally Block • General form of a try…catch block with a finally block

25. Handling an Exception with a Loop • Different programs require different ways of handling Exceptions • In some cases, the try-catch sequence could be placed in a loop that continues to execute until the code is successful

26. Throwing Exceptions • An advantage of using object-oriented exception-handling techniques is the ability to deal with Exceptions appropriately as you make conscious decisions about how to handle them • When methods from other classes throw Exceptions, they don’t have to catch them • When you design your own classes that might cause Exceptions, you should create them to throw the Exception but not to handle it • Handling an Exception should be left to the client (the program that uses the class)

27. Throwing Exceptions • Two executions of TrySoccerPlayer1 program

28. Throwing Exceptions • Execution of TrySoccerPlayer2 program

29. Tracing Exceptions Through the Call Stack • The memory location where the computer stores the list of locations to which the system must return (after method calls) is known as the call stack • When a method throws an Exception, if the method does not catch it, then the Exception is thrown to the next method “up” the call stack • You can print the value of the StackTrace field to display a list of methods in the call stack so you can determine the location of the Exception

30. Tracing Exceptions Through the Call Stack • The StackTrace field can be a useful debugging tool

31. Creating Your Own Exception Classes • You can create your own customized Exception class for your application • To create your own Exception that you can throw, you should extend the ApplicationException class • You should not create an excessive number of special Exception types for your class because it adds a level of complexity to your program

32. Chapter Summary • An exception is any error condition or unexpected behavior in an executing program • You can purposely generate a SystemException exception by forcing a program to contain an error • When you think an error will occur frequently, it is most efficient to handle it in the traditional way, with if statements • In object-oriented terminology, you “try” a procedure that may not complete correctly

33. Chapter Summary • Every Exception object contains a ToString() method and a Message field • You can place as many statements as you need within a try block, and you can catch as many different Exceptions as you want • When you have actions to perform at the end of a try…catch sequence, you can use a finally block • When you want to keep trying a block of code until some value or state within a program is correct, you can place a try…catch block within a loop

34. Chapter Summary • When methods throw Exceptions, they don’t have to catch them; instead, the program that calls a method that throws an Exception can catch it and determine what to do • When a method throws an Exception, if the method does not catch it, then the Exception is thrown to the method that called the offending method • To create your own Exception that you can throw, you should extend the ApplicationException class