Exception Handling Chapter 12 from Visual C

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Exception Handling Chapter 12 fromVisual C
2005 How to Program, Deitel Deitel
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Exceptions

• What is exceptions
• Catching and Handling Exceptions
• How to Throw Exceptions
• Unchecked Exceptions
• Creating Custom Exceptions

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What is an Exception?

• An exception is an event that occurs during the
  execution (Run Time) of a program that disrupts
  the normal flow of instructions.

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Three Exceptions Example

• int x5
• int y0
• int zx/y
• int.Parse(hello)
• public void SomeMethod(...)
• File file new File("Readme.txt")
• .
• .
• .
• file.Close()

Divide by zero
Convert hello into int!
The file doesnt exist.
BIG QUESTION. What will happen then?
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Program Normal Flow, No Exception

• static void Main()
• // get numerator and denominator
• Console.Write("Please enter an integer
  numerator ")
• int numerator int.Parse(Console.ReadLine())
  
• Console.Write("Please enter an integer
  denominator ")
• int denominator int.Parse(Console.ReadLine()
  )
• // divide the two integers, then display the
  result
• int result divide(numerator, denominator)
• Console.WriteLine("\nResult " numerator
  " / "
• denominator " " result)
• Console.WriteLine("End of the program")
• // end Main

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static int divide(int value1, int value2)
int result value1 / value2 return result
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Program Flow with Unhandled Exception

• static void Main()
• // get numerator and denominator
• Console.Write("Please enter an integer
  numerator ")
• int numerator int.Parse(Console.ReadLine())
  
• Console.Write("Please enter an integer
  denominator ")
• int denominator int.Parse(Console.ReadLine()
  )
• // divide the two integers, then display the
  result
• int result divide(numerator, denominator)
• Console.WriteLine("\nResult " numerator
  " / "
• denominator " " result)
• Console.WriteLine("End of the program")
• // end Main

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static int divide(int value1, int value2)
int result value1 / value2 return result
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Exit the program
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Program Flow with Unhandled Exception (2)

• static void Main()
• // get numerator and denominator
• Console.Write("Please enter an integer
  numerator ")
• int numerator int.Parse(Console.ReadLine())
  
• Console.Write("Please enter an integer
  denominator ")
• int denominator int.Parse(Console.ReadLine()
  )
• // divide the two integers, then display the
  result
• int result divide(numerator, denominator)
• Console.WriteLine("\nResult " numerator
  " / "
• denominator " " result)
• Console.WriteLine("End of the program")
• // end Main

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5 int.Parse method
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static int divide(int value1, int value2)
int result value1 / value2 return result
Exit the program
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Exception generation

• Exception generated with keyword throw
• must create and throw exception object
• can generated by user code, CLR, or .NET Framework

int Divide(int n, int d) if (d 0)
throw new DivideByZeroException("divide by
zero") return n / d
user code
int data ... dataindex 17
CLR
FileStream f new FileStream("MyFile.dat",
FileMode.Open)
Framework
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Information Included in the Exception

• The code location
• The type
• The reason (message)
• Stack Trace

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Catching and Handling Exceptions

• Code that may generate exception is placed inside
  try block.
• Directly followed by one or more catch blocks.
• One optional finally block can be followed by
  catch blocks.
• Each catch block includes a type filter that
  determines the types of exceptions it handles.
• If try block catches an exception it will handle
  it with the first matching catch block and skip
  the rest catch block.
• If not match, the exception is not handled.
• Code in finally block got always executed.

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DivideByZeroTest.cs

• static void Main()
• try
• // get numerator and denominator
• Console.Write("Please enter an integer
  numerator ")
• int numerator int.Parse(Console.ReadLine
  ())
• Console.Write("Please enter an integer
  denominator ")
• int denominator int.Parse(Console.ReadLi
  ne())
• // divide the two integers, then display
  the result
• int result divide(numerator,
  denominator)
• Console.WriteLine("\nResult "
  numerator " / " denominator " "
  result)
• catch (FormatException)
• Console.WriteLine("You must enter two
  integers.",
• "Invalid Number Format")

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Program Ends Normally
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DivideByZeroTest.cs

• static void Main()
• try
• // get numerator and denominator
• Console.Write("Please enter an integer
  numerator ")
• int numerator int.Parse(Console.ReadLine
  ())
• Console.Write("Please enter an integer
  denominator ")
• int denominator int.Parse(Console.ReadLi
  ne())
• // divide the two integers, then display
  the result
• int result divide(numerator,
  denominator)
• Console.WriteLine("\nResult "
  numerator " / " denominator " "
  result)
• catch (FormatException)
• Console.WriteLine("You must enter two
  integers.",
• "Invalid Number Format")

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Program Ends Normally
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Try-catch-finally syntax

• try
• //code that may cause exception
• catch(Exception var)
• catch(Exception var)
• ..more catch blocks
• finally()
• ..//this code will always occur

Must be derived from System.Exception
Optional
You may list more than on Catch block
OptionalFinally block always got executed
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Part of the exception hierarchy
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Throw and Re-Throw Exceptions

• In general you may raise exception
• throw new DivideByZeroException("Custom
  Message")
• The above statement raises DivideByZeroException
  with a Custom Message.
• throw can be use to rethrow a caught exception in
  try block
• try
• ..
• catch(Exception ex)
• throw//this will raise the Exception again.

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Exceptions the general idea

• try
• some code that might throw an exception
• more code
• catch (most specific exception)
• handle the exception
• catch (less specific exception)
• handle the exception
• catch (any exception)
• handle the exception
• finally

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• try
• some code that might throw an exception
• more code
  No exception thrown
• catch (most specific exception)
• handle the exception
• catch (less specific exception)
• handle the exception
• catch (any exception)
• handle the exception
• finally

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• try
• some code that might throw an exception
• more code
• catch (most specific exception)
• handle the exception
• catch (less specific exception)
• This exception thrown
• handle the exception
• catch (any exception)
• handle the exception
• finally

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• try
• some code that might throw an exception
• more code
• catch (most specific exception)
• handle the exception
• catch (less specific exception)
• This exception thrown
• handle the exception
• return
• catch (any exception)
• handle the exception
• finally

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• try
• some code that might throw an exception
• more code
• catch (most specific exception)
• handle the exception
• catch (less specific exception)
• This exception thrown
• handle the exception
• throw
• catch (any exception)
• handle the exception
• finally

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Handling ExceptionsAdvantages and Disadvantages

• Advantage
• Better usability
• Close resources
• Disadvantage
• Performance Overhead

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• How do you Determine Which Exceptions a Method
  Throws?

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12.7 Exception Properties

• Properties for a caught exception
• Message
• Stores the error message associated with an
  Exception object
• May be a default message or customized
• StackTrace
• Contain a string that represents the method call
  stack
• Represent sequential list of methods that were
  not fully processed when the exception occurred
• The exact location is called the throw point

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exceptiondemo.cs

• class ExceptionDemo
• static void Main()
• // call Method1 any Exception generated
  is caught
• // in the catch block that follows
• try
• Method1()
• // end try
• catch (Exception exceptionParameter)
• // output the string representation
  of the Exception, then output
• // properties InnerException, Message
  and StackTrace
• Console.WriteLine("exceptionParameter.
  ToString \n0\n",
• exceptionParameter.ToString())
• Console.WriteLine("exceptionParameter.
  Message \n0\n",
• exceptionParameter.Message)
• Console.WriteLine("exceptionParameter.
  StackTrace \n0\n",

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• // calls Method2
• static void Method1()
• try
• Method2()
• catch (Exception)
• throw
• // end method Method1
• // throws an Exception containing an
  InnerException
• static void Method2()
• // attempt to convert string to int
• try

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exceptiondemo.cs (output)
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12.8 Programmer-Defined Exception Classes

• Creating customized exception types
• Should derive from class ApplicationException.
  You may derive from Exception class (but not
  recommended)
• Should end with Exception
• Should define three constructors
• A default constructor
• A constructor that receives a string argument
• A constructor that takes a string argument and an
  Exception argument
• The Should is just best-practice.

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12.8 Programmer-Defined Exception ClassesDesign
Considerations

• When designing the Custom Exception class
• Name of the exception
• Default message (default constructor)
• Freedom of choice, how many constructor. Usually
  implement the three (mentioned in the previous
  slide)
• When using the Custom Exception class
• When you will raise the exception. Make sure all
  cases are covered.
• Use the right constructor and pass enough
  information.

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Square Class

• public class Square
• private int side_length // A Field
• public int Side_length // A Property
• get return side_length
• set
• side_length value
• public int area() // A Method
• return (side_length side_length)
• public Square(int side) // The Constructor
  method
• side_length side

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NegativeNumberException

• using System
• public class NegativeNumberException
  ApplicationException
• // default constructor
  
• public NegativeNumberException()
• base("Illegal operation for a negative
  number")
• // empty body
  
• // end default constructor
  
• // constructor for customizing error message
  
• public NegativeNumberException(string
  messageValue)
• base(messageValue)
• // empty body
  
• // end one-argument constructor
  
• // constructor for customizing the
  exception's error
• // message and specifying the InnerException
  object

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Square Class (ver. 2)

• public class Square
• private int side_length // A Field
• public int Side_length // A Property
• get return side_length
• set
• if (value lt 0)
• throw new NegativeNumberException(
  
• "Square root of negative
  number not permitted")
• else
• side_length value
• public int area() // A Method
• return (side_length side_length)

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SquareTest.cs

• class SquareTest
• static void Main()
• string mySide System.Console.ReadLine()
  
• int mySideInt int.Parse(mySide)
• Square sq new Square(mySideInt)
• int x sq.Side_length
• // Retrieve objects Side_Length
  Property
• sq.Side_length -15
• // Change objects Side_length
  Property
• int sq_area sq.area()
• System.Console.WriteLine("Area "
  sq_area.ToString())

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Finally Remember

• In a catch block, you can
• Rethrow the same exception, notifying code higher
  in the call stack
• Throw a different exception, giving additional
  information to code higher in the call stack
• Handle the exception and fall out the bottom of
  the catch block
• Remember that
• There is a performance hit for exceptions