Csphtp1 11

Chapter 11 – Exception Handling Outline 11.1 Introduction 11.2 Exception Handling Overview 11.3 Example: DivideByZeroException 11.4 .NET Exception Hierarchy 11.5 finally Block 11.6 Exception Properties 11.7 Programmer-Defined Exception Classes 11.8 Handling Overflows with Operators checked and unchecked

11.1 Introduction Exception handling Create application that can handle or resolve exception Enable clear, robust and more fault-tolerant programs

11.2 Exception Handling Overview Keywords Try Include codes in which exceptions might occur Catch Represent types of exceptions the catch can handle Finally (Optional) codes present here will always execute Exception handling Process synchronous errors Follows the termination model of exception handling

11.2 Exception Handling Overview Catch type Must be of class Exception or one that extends it directly or indirectly Stack unwinding CLR attempt to locate an enclosing try block in a calling method

11.3 Example: DivideByZeroException Error catching Method Convert.ToInt32 will automatically detect for invalid representation of an integer Method generates a FormatException CLR automatic detection for division by zero Occurrence will cause a DivideByZeroException

DivideByZeroTest.cs 1 // Fig 11.1: DivideByZeroTest.cs 2 // Basics of C# exception handling. 3 4 using System; 5 using System.Drawing; 6 using System.Collections; 7 using System.ComponentModel; 8 using System.Windows.Forms; 9 using System.Data; 10 11 // class demonstrates how to handle exceptions from 12 // division by zero in integer arithmetic and from 13 // improper numeric formatting 14 public class DivideByZeroTest : System.Windows.Forms.Form 15 { 16 private System.Windows.Forms.Label numeratorLabel; 17 private System.Windows.Forms.TextBox numeratorTextBox; 18 19 private System.Windows.Forms.Label denominatorLabel; 20 private System.Windows.Forms.TextBox denominatorTextBox; 21 22 private System.Windows.Forms.Button divideButton; 23 private System.Windows.Forms.Label outputLabel; 24 25 // required designer variable 26 private System.ComponentModel.Container components = null ; 27 28 // default constructor 29 public DivideByZeroTest() 30 { 31 // required for Windows Form Designer support 32 InitializeComponent(); 33 } 34

DivideByZeroTest.cs 35 // main entry point for the application 36 [STAThread] 37 static void Main() 38 { 39 Application.Run( new DivideByZeroTest() ); 40 } 41 42 // Visual Studio .NET generated code 43 44 // obtain integers input by user and divide numerator 45 // by denominator 46 private void divideButton_Click( 47 object sender, System.EventArgs e ) 48 { 49 outputLabel.Text = "" ; 50 51 // retrieve user input and call Quotient 52 try 53 { 54 // Convert.ToInt32 generates FormatException if 55 // argument is not an integer 56 int numerator = Convert.ToInt32( numeratorTextBox.Text ); 57 int denominator = 58 Convert.ToInt32( denominatorTextBox.Text ); 59 60 // division generates DivideByZeroException if 61 // denominator is 0 62 int result = numerator / denominator; 63 64 outputLabel.Text = result.ToString(); 65 66 } // end try 67 Try block encloses codes that could result in a throw exception Will not be reached (executed) if an exception is thrown FormatException thrown if it cannot convert string into integer DivideByZeroException thrown if denominator is zero

DivideByZeroTest.cs 68 // process invalid number format 69 catch ( FormatException ) 70 { 71 MessageBox.Show( "You must enter two integers" , 72 "Invalid Number Format" , 73 MessageBoxButtons. OK , MessageBoxIcon. Error ); 74 } 75 76 // user attempted to divide by zero 77 catch ( DivideByZeroException divideByZeroException ) 78 { 79 MessageBox.Show( divideByZeroException.Message, 80 "Attempted to Divide by Zero" , 81 MessageBoxButtons. OK , MessageBoxIcon. Error ); 82 } 83 84 } // end method divideButton_Click 85 86 } // end class DivideByZeroTest Catch handler for FormatException Catch handler for DivideByZeroException Keyword Message box to display error message Handler uses property Message of class Exception

DivideByZeroTest.cs Program Output When incorrect format are entered into either input fields When attempting to diving by zero

11.4 .NET Exception Hierarchy .Net Framework Class Exception is base class Derived class: ApplicationException Programmer use to create data types specific to their application Low chance of program stopping execution SystemException CLR can generate at any point during execution Runtime exceptin Example: IndexOutOfRangeException

11.5 Finally Block Resource leak Improper allocation of memory Finally block Ideal for placing resource deallocation code Execute immediately after catch handler or try block Must be present if no catch block is present Is optional if more than one or more catch handler exist

11.5 Finally Block The Throw expression An exception object Must be of either class Exception or one of its derived class Customize the exception type thrown from methods

1 // Fig 11.2: UsingExceptions.cs 2 // Using finally blocks. 3 4 using System; 5 6 // demonstrating that finally always executes 7 class UsingExceptions 8 { 9 // entry point for application 10 static void Main( string [] args ) 11 { 12 // Case 1: No exceptions occur in called method. 13 Console.WriteLine( "Calling DoesNotThrowException" ); 14 DoesNotThrowException(); 15 16 // Case 2: Exception occurs and is caught 17 // in called method. 18 Console.WriteLine( "\nCalling ThrowExceptionWithCatch" ); 19 ThrowExceptionWithCatch(); 20 21 // Case 3: Exception occurs, but not caught 22 // in called method, because no catch handlers. 23 Console.WriteLine( 24 "\nCalling ThrowExceptionWithoutCatch" ); 25 26 // call ThrowExceptionWithoutCatch 27 try 28 { 29 ThrowExceptionWithoutCatch(); 30 } 31 UsingExceptions.cs Static methods of this class so main can invoke directly Begin try block

UsingExceptions.cs 32 // process exception returned from 33 // ThrowExceptionWithoutCatch 34 catch 35 { 36 Console.WriteLine( "Caught exception from " + 37 "ThrowExceptionWithoutCatch in Main" ); 38 } 39 40 // Case 4: Exception occurs and is caught 41 // in called method, then rethrown to caller. 42 Console.WriteLine( 43 "\nCalling ThrowExceptionCatchRethrow" ); 44 45 // call ThrowExceptionCatchRethrow 46 try 47 { 48 ThrowExceptionCatchRethrow(); 49 } 50 51 // process exception returned from 52 // ThrowExceptionCatchRethrow 53 catch 54 { 55 Console.WriteLine( "Caught exception from " + 56 "ThrowExceptionCatchRethrow in Main" ); 57 } 58 59 } // end method Main 60 Try block for ThrowExceptionCatchRethrow Another static method of class UsingExceptions Would process exception that were thrown with no catch handler available

UsingExceptions.cs 61 // no exceptions thrown 62 public static void DoesNotThrowException() 63 { 64 // try block does not throw any exceptions 65 try 66 { 67 Console.WriteLine( "In DoesNotThrowException" ); 68 } 69 70 // this catch never executes 71 catch 72 { 73 Console.WriteLine( "This catch never executes" ); 74 } 75 76 // finally executes because corresponding try executed 77 finally 78 { 79 Console.WriteLine( 80 "Finally executed in DoesNotThrowException" ); 81 } 82 83 Console.WriteLine( "End of DoesNotThrowException" ); 84 85 } // end method DoesNotThrowException 86 87 // throws exception and catches it locally 88 public static void ThrowExceptionWithCatch() 89 { 90 // try block throws exception 91 try 92 { 93 Console.WriteLine( "In ThrowExceptionWithCatch" ); 94 Definition for method DoesNotThrowException( ) Enters the try block, skips catch block and execute the finally block End of method, program control returns to Main Definition for method ThrowExceptionWithCatch( )

95 throw new Exception( 96 "Exception in ThrowExceptionWithCatch" ); 97 } 98 99 // catch exception thrown in try block 100 catch ( Exception error ) 101 { 102 Console.WriteLine( "Message: " + error.Message ); 103 } 104 105 // finally executes because corresponding try executed 106 finally 107 { 108 Console.WriteLine( 109 "Finally executed in ThrowExceptionWithCatch" ); 110 } 111 112 Console.WriteLine( "End of ThrowExceptionWithCatch" ); 113 114 } // end method ThrowExceptionWithCatch 115 116 // throws exception and does not catch it locally 117 public static void ThrowExceptionWithoutCatch() 118 { 119 // throw exception, but do not catch it 120 try 121 { 122 Console.WriteLine( "In ThrowExceptionWithoutCatch" ); 123 124 throw new Exception( 125 "Exception in ThrowExceptionWithoutCatch" ); 126 } 127 UsingExceptions.cs The string now becomes the exception object’s error message Throw statement to throw the exception object Create a new Exception object Try block expires because of throw command, program control continue at the first catch following the try block. Using the exception object’s Message property to access the error message Try block expires immediately because of “throw new Exception” Definition for method ThrowExceptionWithoutCatch ( ) No catch handlers exist so the program control go directly to the finally block

128 // finally executes because corresponding try executed 129 finally 130 { 131 Console.WriteLine( "Finally executed in " + 132 "ThrowExceptionWithoutCatch" ); 133 } 134 135 // unreachable code; would generate logic error 136 Console.WriteLine( "This will never be printed" ); 137 138 } // end method ThrowExceptionWithoutCatch 139 140 // throws exception, catches it and rethrows it 141 public static void ThrowExceptionCatchRethrow() 142 { 143 // try block throws exception 144 try 145 { 146 Console.WriteLine( "In ThrowExceptionCatchRethrow" ); 147 148 throw new Exception( 149 "Exception in ThrowExceptionCatchRethrow" ); 150 } 151 152 // catch any exception, place in object error 153 catch ( Exception error ) 154 { 155 Console.WriteLine( "Message: " + error.Message ); 156 157 // rethrow exception for further processing 158 throw error; 159 160 // unreachable code; would generate logic error 161 } 162 UsingExceptions.cs Finally block is reached but program control returns to main immediately after Program control continue from throw statement to the first catch block that match with the same type Rethrow the exception back to the calling method for further processing

UsingExceptions.cs Program Output 163 // finally executes because corresponding try executed 164 finally 165 { 166 Console.WriteLine( "Finally executed in " + 167 "ThrowExceptionCatchRethrow" ); 168 } 169 170 // unreachable code; would generate logic error 171 Console.WriteLine( "This will never be printed" ); 172 173 } // end method ThrowExceptionCatchRethrow 174 175 } // end class UsingExceptions Calling DoesNotThrowException In DoesNotThrowException Finally executed in DoesNotThrowException End of DoesNotThrowException Calling ThrowExceptionWithCatch In ThrowExceptionWithCatch Message: Exception in ThrowExceptionWithCatch Finally executed in ThrowExceptionWithCatch End of ThrowExceptionWithCatch Calling ThrowExceptionWithoutCatch In ThrowExceptionWithoutCatch Finally executed in ThrowExceptionWithoutCatch Caught exception from ThrowExceptionWithoutCatch in Main Calling ThrowExceptionCatchRethrow In ThrowExceptionCatchRethrow Message: Exception in ThrowExceptionCatchRethrow Finally executed in ThrowExceptionCatchRethrow Caught exception from ThrowExceptionCatchRethrow in Main Finally block reached but program control returns to first occurrence of a try block

11.6 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

11.6 Exception Properties InnerException property “ Wrap” exception objects caught in code Then throw new exception types

Properties.cs 1 // Fig 11.3: Properties.cs 2 // Stack unwinding and Exception class properties. 3 4 using System; 5 6 // demonstrates using the Message, StackTrace and 7 // InnerException properties 8 class Properties 9 { 10 static void Main( string [] args ) 11 { 12 // call Method1, any Exception it generates will be 13 // caught in the catch handler that follows 14 try 15 { 16 Method1(); 17 } 18 19 // Output string representation of Exception, then 20 // output values of InnerException, Message, 21 // and StackTrace properties 22 catch ( Exception exception ) 23 { 24 Console.WriteLine( 25 "exception.ToString(): \n{0}\n" , 26 exception.ToString() ); 27 28 Console.WriteLine( "exception.Message: \n{0}\n" , 29 exception.Message ); 30 31 Console.WriteLine( "exception.StackTrace: \n{0}\n" , 32 exception.StackTrace ); 33 Main becomes first method on the method call stack Invoked in try block, becomes second on method call stack When control returns from stack unwinding, try block is expired sending exception to catch block Catch block uses method ToString and properties Message, StackTrace and InnerException to produce output

34 Console.WriteLine( 35 "exception.InnerException: \n{0}" , 36 exception.InnerException ); 37 38 } // end catch 39 40 } // end Main 41 42 // calls Method2 43 public static void Method1() 44 { 45 Method2(); 46 } 47 48 // calls Method3 49 public static void Method2() 50 { 51 Method3(); 52 } 53 54 // throws an Exception containing an InnerException 55 public static void Method3() 56 { 57 // attempt to convert non-integer string to int 58 try 59 { 60 Convert.ToInt32( "Not an integer" ); 61 } 62 Properties.cs Method2 is then unwinds from the method-call stack and return control to Method1 When control return to method2, the CLLR determines that there is no try block Not an integer format, throws a FormatException Try block uses Convert.ToInt32 which become the fifth and final method on stack Method3 invoked by Method2 becomes fourth on the method on the stack Method2 invoked by Method1 becomes third on the method on the stack Here also, the CLLR searches for a try block, but unsuccessful it terminates and unwinds from the call stack From Method1 control is then returned to the caller which is Main

63 // catch FormatException and wrap it in new Exception 64 catch ( FormatException error ) 65 { 66 throw new Exception( 67 "Exception occurred in Method3" , error ); 68 } 69 70 } // end method Method3 71 72 } // end class UsingExceptions Properties.cs Program Output exception.ToString(): System.Exception: Exception occurred in Method3 ---> System.FormatException: Input string was not in a correct format. at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at System.Convert.ToInt32(String s) at Properties.Method3() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 60 --- End of inner exception stack trace --- at Properties.Method3() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 66 at Properties.Method2() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 51 at Properties.Method1() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 45 at Properties.Main(String[] args) in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 16 After catch block execute the exception is terminated from the method call stack Catch handler creates an Exception object, then throws it First block of output shows the exception’s string representation returned from method ToString Catches the FormatException thrown by Convert.ToInt32 Control will be returned to the statement that invoked Method3 , which is Method2 This removes Method3 from the method-call stack Method3 terminates, because the exception thrown is not caught in the method body Name of the exception class followed by the Message property value The next eight lines show the string representation of the InnerException object Output for the StackTrace for the Exception thrown in Method3

Properties.cs Program Output exception.Message: Exception occurred in Method3 exception.StackTrace: at Properties.Method3() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 66 at Properties.Method2() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 51 at Properties.Method1() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 45 at Properties.Main(String[] args) in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 16 exception.InnerException: System.FormatException: Input string was not in a correct format. at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at System.Convert.ToInt32(String s) at Properties.Method3() in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_8\ properties\properties.cs:line 60 These two line represent the Message property of the exception thrown in Method3 StackTrace property of the exception thrown in Method3 ToString representation of the InnerException property

11.7 Programmer-Defined Exception Classes Creating customized exception types Should derive from class ApplicationException 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

NegativeNumberException.cs 1 // Fig 11:4: NegativeNumberException.cs 2 // NegativeNumberException represents exceptions caused by illegal 3 // operations performed on negative numbers 4 5 using System; 6 7 // NegativeNumberException represents exceptions caused by 8 // illegal operations performed on negative numbers 9 class NegativeNumberException : ApplicationException 10 { 11 // default constructor 12 public NegativeNumberException() 13 : base ( "Illegal operation for a negative number" ) 14 { 15 } 16 17 // constructor for customizing error message 18 public NegativeNumberException( string message ) 19 : base ( message ) 20 { 21 } 22 23 // constructor for customizing error message and 24 // specifying inner exception object 25 public NegativeNumberException( 26 string message, Exception inner ) 27 : base ( message, inner ) 28 { 29 } 30 31 } // end class NegativeNumberException Class NegativeNumberException derives from ApplicationException This represent the default constructor This is a constructor that takes in a string argument This is a constructor that takes in a string argument and an Exception argument

SquareRootTest.cs 1 // Fig 11.5: SquareRootTest.cs 2 // Demonstrating a programmer-defined exception class. 3 4 using System; 5 using System.Drawing; 6 using System.Collections; 7 using System.ComponentModel; 8 using System.Windows.Forms; 9 using System.Data; 10 11 // accepts input and computes the square root of that input 12 public class SquareRootTest : System.Windows.Forms.Form 13 { 14 private System.Windows.Forms.Label inputLabel; 15 private System.Windows.Forms.TextBox inputTextBox; 16 17 private System.Windows.Forms.Button squareRootButton; 18 19 private System.Windows.Forms.Label outputLabel; 20 21 // Required designer variable. 22 private System.ComponentModel.Container components = null ; 23 24 // default constructor 25 public SquareRootTest() 26 { 27 // Required for Windows Form Designer support 28 InitializeComponent(); 29 } 30 31 // Visual Studio .NET generated code 32

SquareRootTest.cs 33 // main entry point for the application 34 [STAThread] 35 static void Main() 36 { 37 Application.Run( new SquareRootTest() ); 38 } 39 40 // computes the square root of its parameter; throws 41 // NegativeNumberException if parameter is negative 42 public double SquareRoot( double operand ) 43 { 44 // if negative operand, throw NegativeNumberException 45 if ( operand < 0 ) 46 throw new NegativeNumberException( 47 "Square root of negative number not permitted" ); 48 49 // compute the square root 50 return Math.Sqrt( operand ); 51 52 } // end class SquareRoot 53 54 // obtain user input, convert to double and calculate 55 // square root 56 private void squareRootButton_Click( 57 object sender, System.EventArgs e ) 58 { 59 outputLabel.Text = "" ; 60 61 // catch any NegativeNumberExceptions thrown 62 try 63 { 64 double result = 65 SquareRoot( Double.Parse( inputTextBox.Text ) ); 66 SqaureRoot throws a NegativeNumberException Try block invoke SqaureRoot A FormatException occurs if not a valid number from user

SquareRootTest.cs 67 outputLabel.Text = result.ToString(); 68 } 69 70 // process invalid number format 71 catch ( FormatException notInteger ) 72 { 73 MessageBox.Show( notInteger.Message, 74 "Invalid Operation" , MessageBoxButtons. OK , 75 MessageBoxIcon. Error ); 76 } 77 78 // display MessageBox if negative number input 79 catch ( NegativeNumberException error ) 80 { 81 MessageBox.Show( error.Message, "Invalid Operation" , 82 MessageBoxButtons. OK , MessageBoxIcon. Error ); 83 } 84 85 } // end method squareRootButton_Click 86 87 } // end class SquareRootTest Process the exception caused by FormatException Catch handler takes care of the NegativeNumberException Output showing correct function When attempting to take a negative square root

11.8 Handling Overflows with Operators checked and unchecked Calculation that could overflow Use a checked context when performing calculations that can result in overflow Programmer should define exception handlers to process the overflow In .NET, primitive data types are stored in fixed-size structure Example, maximum for int is 2,147,483,647 Overflow causes program to produce incorrect result C# provides operators checked and unchecked to specify the validity of integer arithmetic.

11.8 Handling Overflows with Operators checked and unchecked Checked context The CLR throws an overflowException if overflow occur during calculation Unchecked context The result is of the overflow is truncated Explicit conversions between integral data types can cause overflow

Overflow.cs 1 // Fig 11.6: Overflow.cs 2 // Demonstrating operators checked and unchecked. 3 4 using System; 5 6 // demonstrates using the checked and unchecked operators 7 class Overflow 8 { 9 static void Main( string [] args ) 10 { 11 int number1 = Int32. MaxValue ; // 2,147,483,647 12 int number2 = Int32. MaxValue ; // 2,147,483,647 13 int sum = 0 ; 14 15 Console.WriteLine( 16 "number1: {0}\nnumber2: {1}" , number1, number2 ); 17 18 // calculate sum of number1 and number2 19 try 20 { 21 Console.WriteLine( 22 "\nSum integers in checked context:" ); 23 24 sum = checked ( number1 + number2 ); 25 } 26 27 // catch overflow exception 28 catch ( OverflowException overflowException ) 29 { 30 Console.WriteLine( overflowException.ToString() ); 31 } 32 33 Console.WriteLine( 34 "\nsum after checked operation: {0}" , sum ); 35 Initialize and declare variables and assigned value to the maximum of int Sum adds number1 and number2 in a checked context Number1 and Number2 together causes an overflow, causes overflowException The catch handler gets the overflowException and prints out its string representation

Overflow.cs Program Output 36 Console.WriteLine( 37 "\nSum integers in unchecked context:" ); 38 39 sum = unchecked ( number1 + number2 ); 40 41 Console.WriteLine( 42 "sum after unchecked operation: {0}" , sum ); 43 44 } // end method Main 45 46 } // end class Overflow number1: 2147483647 number2: 2147483647 Sum integers in checked context: System.OverflowException: Arithmetic operation resulted in an overflow. at Overflow.Overflow.Main(String[] args) in f:\books\2001\csphtp1\csphtp1_examples\ch11\fig11_09\ overflow\overflow.cs:line 24 sum after checked operation: 0 Sum integers in unchecked context: sum after unchecked operation: -2 Sum of the numbers in an unchecked context Addition of number1 and number2 in unchecked context

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