Chương 8: Understanding Structed Exception Handling potx

Chapter 8. Understanding Structured Exception Handling Hoang Anh Viet VietHA@it-hut.edu.vn Hanoi University of Technology 1 Microsoft Outline “This chapter is to discuss how to handle runtime anomalies in your code base through the use of structured exception handling. Not only will you learn about the C# keywords that allow you to handle such problems (try, catch, throw, and finally), but you will also come to understand the distinction between application-level and system- level exceptions. In addition, this chapter examines various tools within Visual Studio 2008 that allow you to debug the exceptions that have escaped your view.”  Introduction  Exception Handling Overview  Example: DivideByZeroException  .NET Exception Hierarchy  finally Block  Exception Properties  Programmer-Defined Exception Classes  Handling Overflows with Operators checked and unchecked 2 Microsoft Roadmap  8.1 Introduction  8.2 Exception Handling Overview  8.3 Example: DivideByZeroException  8.4 .NET Exception Hierarchy  8.5 finally Block  8.6 Exception Properties  8.7 Programmer-Defined Exception Classes  8.8 Handling Overflows with Operators checked and unchecked 3 Microsoft 8.1 Introduction  Definitions for three commonly used anomaly-centric terms  Bugs: errors on the part of the programmer  User errors: Unlike bugs, user errors are typically caused by the individual running your application, rather than by those who created it.  Exceptions: Exceptions are typically regarded as runtime anomalies that are difficult, if not impossible, to account for while programming your application. 4 Microsoft 8.1 Introduction  Exception handling  Create application that can handle or resolve exception  Enable clear, robust and more fault-tolerant programs  .NET structured exception handling is a technique well suited to deal with runtime exceptions.  However, as for the bugs and user errors that have escaped your view, the CLR will often generate a corresponding exception that identifies the problem at hand. 5 Microsoft Roadmap  8.1 Introduction  8.2 Exception Handling Overview  8.3 Example: DivideByZeroException  8.4 .NET Exception Hierarchy  8.5 finally Block  8.6 Exception Properties  8.7 Programmer-Defined Exception Classes  8.8 Handling Overflows with Operators checked and unchecked 6 Microsoft 8.2 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 7 Microsoft 8.2 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 8 Microsoft Roadmap  8.1 Introduction  8.2 Exception Handling Overview  8.3 Example: DivideByZeroException  8.4 .NET Exception Hierarchy  8.5 finally Block  8.6 Exception Properties  8.7 Programmer-Defined Exception Classes  8.8 Handling Overflows with Operators checked and unchecked 9 Microsoft 8.3 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 10 [...]... 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... // throws exception and catches it locally public static void ThrowExceptionWithCatch() { Definition for method // try block throws exception try ThrowExceptionWithCatch( { Console.WriteLine( "In ThrowExceptionWithCatch" ); ) 24 UsingExceptions.cs (4/6) } throw new Exception( "Exception in ThrowExceptionWithCatch" ); Create a new Exception Throw statement to throwthrown in try block // catch exception. .. ThrowExceptionCatchRethrow } // end class UsingExceptions Finally block reached but program control returns to first occurrence of a try block Calling DoesNotThrowException In DoesNotThrowException Finally executed in DoesNotThrowException End of DoesNotThrowException Calling ThrowExceptionWithCatch In ThrowExceptionWithCatch Message: Exception in ThrowExceptionWithCatch Finally executed in ThrowExceptionWithCatch... Introduction 8.2 Exception Handling Overview 8.3 Example: DivideByZeroException 8.4 NET Exception Hierarchy 8.5 finally Block 8.6 Exception Properties 8.7 Programmer-Defined Exception Classes 8.8 Handling Overflows with Operators checked and unchecked Microsoft 15 8.4 NET Exception Hierarchy  Net Framework   Class Exception is base class Derived class:  ApplicationException    SystemException  ... method ToString and "exception. ToString(): \n{0}\n", properties Message, StackTrace and exception. ToString() ); InnerException to produce output Console.WriteLine( "exception. Message: \n{0}\n", exception. Message ); Console.WriteLine( "exception. StackTrace: \n{0}\n", exception. StackTrace ); 31 Properties.cs (2/4) Console.WriteLine( "exception. InnerException: \n{0}", exception. InnerException ); } // end... 8.2 Exception Handling Overview 8.3 Example: DivideByZeroException 8.4 NET Exception Hierarchy 8.5 finally Block 8.6 Exception Properties 8.7 Programmer-Defined Exception Classes 8.8 Handling Overflows with Operators checked and unchecked Microsoft 35 8.7 Programmer-Defined Exception Classes  Creating customized exception types    Should derive from class ApplicationException Should end with Exception ... 8.1 Introduction 8.2 Exception Handling Overview 8.3 Example: DivideByZeroException 8.4 NET Exception Hierarchy 8.5 finally Block 8.6 Exception Properties 8.7 Programmer-Defined Exception Classes 8.8 Handling Overflows with Operators checked and unchecked Microsoft 28 8.6 Exception Properties  Properties for a caught exception  Message  Stores the error message associated with an Exception object ... method of class UsingExceptions // process exception returned from ThrowExceptionCatchRethrow catch { Console.WriteLine( "Caught exception from " + "ThrowExceptionCatchRethrow in Main" ); } } // end method Main 23 UsingExceptions.cs (3/6) // no exceptions thrown public static void DoesNotThrowException() Definition for method { DoesNotThrowException( ) // try block does not throw any exceptions try { Console.WriteLine(... be printed" ); } // end method ThrowExceptionWithoutCatch // throws exception, catches it and rethrows it public static void ThrowExceptionCatchRethrow() { // try block throws exception try { Console.WriteLine( "In ThrowExceptionCatchRethrow" ); } throw new Exception( "Exception in ThrowExceptionCatchRethrow" ); // catch any exception, place in object error catch ( Exception error ) { Console.WriteLine(... Console.WriteLine( "\nCalling ThrowExceptionWithCatch" ); ThrowExceptionWithCatch(); Begin try // Case 3: Exception occurs, but not caught // in called method, because no catch handlers Console.WriteLine( "\nCalling ThrowExceptionWithoutCatch" ); block // call ThrowExceptionWithoutCatch try { ThrowExceptionWithoutCatch(); } 22 UsingExceptions.cs (2/6) // process exception returned from ThrowExceptionWithoutCatch . Introduction  8.2 Exception Handling Overview  8.3 Example: DivideByZeroException  8.4 .NET Exception Hierarchy  8.5 finally Block  8.6 Exception Properties  8.7 Programmer-Defined Exception. Introduction  8.2 Exception Handling Overview  8.3 Example: DivideByZeroException  8.4 .NET Exception Hierarchy  8.5 finally Block  8.6 Exception Properties  8.7 Programmer-Defined Exception. Introduction  8.2 Exception Handling Overview  8.3 Example: DivideByZeroException  8.4 .NET Exception Hierarchy  8.5 finally Block  8.6 Exception Properties  8.7 Programmer-Defined Exception