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15. Exceptions Handling - C# Fundamentals

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In this presentation, part of the C# Fundamentals course, we discuss exceptions.
Telerik Academy http://csharpfundamentals.telerik.com
The website and all video materials are in Bulgarian.
Main topics:
How to handle the exception.
How to throw the exception. Best practices.
Types of Exceptions and their Hierarchy.
Telerik Software Academy: http://www.academy.telerik.com
The website and all video materials are in Bulgarian

What are Exceptions; Handling Exceptions;
The System.Exception Class; Types of Exceptions and their Hierarchy;
Raising (Throwing) Exceptions; Best Practices;

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Transcript of "15. Exceptions Handling - C# Fundamentals"

  1. 1. Exceptions Handling Handling Errors during the Program ExecutionSvetlin NakovManager Technical trainerwww.nakov.comTelerik Software Academyacademy.telerik.com
  2. 2. Table of Contents1. What are Exceptions?2. Handling Exceptions3. The System.Exception Class4. Types of Exceptions and their Hierarchy5. Raising (Throwing) Exceptions6. Best Practices 2
  3. 3. What are Exceptions?The Paradigm of Exceptions in OOP
  4. 4. What are Exceptions? The exceptions in .NET Framework are classic implementation of the OOP exception model Deliver powerful mechanism for centralized handling of errors and unusual events Substitute procedure-oriented approach, in which each function returns error code Simplify code construction and maintenance Allow the problematic situations to be processed at multiple levels 4
  5. 5. Handling Exceptions Catching and Processing Errors
  6. 6. Handling Exceptions In C# the exceptions can be handled by the try-catch-finally construction try { // Do some work that can raise an exception } catch (SomeException) { // Handle the caught exception } catch blocks can be used multiple times to process different exception types 6
  7. 7. Handling Exceptions – Examplestatic void Main(){ string s = Console.ReadLine(); try { Int32.Parse(s); Console.WriteLine( "You entered valid Int32 number {0}.", s); } catch (FormatException) { Console.WriteLine("Invalid integer number!"); } catch (OverflowException) { Console.WriteLine( "The number is too big to fit in Int32!"); }} 7
  8. 8. HandlingExceptions Live Demo
  9. 9. The System.Exception Class Exceptions in .NET are objects The System.Exception class is base for all exceptions in CLR  Contains information for the cause of the error / unusual situation  Message – text description of the exception  StackTrace – the snapshot of the stack at the moment of exception throwing  InnerException – exception caused the current exception (if any) 9
  10. 10. Exception Properties – Exampleclass ExceptionsExample{ public static void CauseFormatException() { string s = "an invalid number"; Int32.Parse(s); } static void Main() { try { CauseFormatException(); } catch (FormatException fe) { Console.Error.WriteLine("Exception: {0}n{1}", fe.Message, fe.StackTrace); } }} 10
  11. 11. Exception Properties The Message property gives brief description of the problem The StackTrace property is extremely useful when identifying the reason caused the exception Exception caught: Input string was not in a correct format. at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at System.Int32.Parse(String s) at ExceptionsTest.CauseFormatException() in c:consoleapplication1exceptionstest.cs:line 8 at ExceptionsTest.Main(String[] args) in c:consoleapplication1exceptionstest.cs:line 15 11
  12. 12. Exception Properties (2) File names and line numbers are accessible only if the compilation was in Debug mode When compiled in Release mode, the information in the property StackTrace is quite different: Exception caught: Input string was not in a correct format. at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info) at ExceptionsTest.Main(String[] args) 12
  13. 13. Exception Properties Live Demo
  14. 14. The Hierarchy of Exceptions
  15. 15. Exception Hierarchy Exceptions in .NET Framework are organized in a hierarchy 15
  16. 16. Types of Exceptions .NET exceptions inherit from System.Exception The system exceptions inherit from System.SystemException, e.g.  System.ArgumentException  System.NullReferenceException  System.OutOfMemoryException  System.StackOverflowException User-defined exceptions should inherit from System.ApplicationException 16
  17. 17. Handling Exceptions When catching an exception of a particular class, all its inheritors (child exceptions) are caught too Example: try { // Do some works that can cause an exception } catch (System.ArithmeticException) { // Handle the caught arithmetic exception } Handles ArithmeticException and its descendants DivideByZeroException and OverflowException 17
  18. 18. Find the Mistake!static void Main(){ string s = Console.ReadLine(); try { This should be last Int32.Parse(s); } catch (Exception) { Console.WriteLine("Can not parse the number!"); } catch (FormatException) Unreachable code { Console.WriteLine("Invalid integer number!"); } catch (OverflowException) { Unreachable code Console.WriteLine( "The number is too big to fit in Int32!"); }} 18
  19. 19. Handling All Exceptions All exceptions thrown by .NET managed code inherit the System.Exception exception Unmanaged code can throw other exceptions For handling all exceptions (even unmanaged) use the construction: try { // Do some works that can raise any exception } catch { // Handle the caught exception } 19
  20. 20. Throwing Exceptions
  21. 21. Throwing Exceptions Exceptions are thrown (raised) by throw keyword in C#  Used to notify the calling code in case of error or unusual situation When an exception is thrown:  The program execution stops  The exception travels over the stack until a suitable catch block is reached to handle it Unhandled exceptions display error message 21
  22. 22. How Exceptions Work? 5. Throw an exceptionMethod N Method N … 4. Method call … 6. Find handlerMethod 2 Method 2 3. Method call 7. Find handlerMethod 1 Method 1 2. Method call 8. Find handlerMain() Main() .NET CLR 22
  23. 23. Using throw Keyword Throwing an exception with an error message: throw new ArgumentException("Invalid amount!"); Exceptions can accept message and cause: try { Int32.Parse(str); } catch (FormatException fe) { throw new ArgumentException("Invalid number", fe); } Note: if the original exception is not passed the initial cause of the exception is lost 23
  24. 24. Re-Throwing Exceptions Caught exceptions can be re-thrown again: try { Int32.Parse(str); } catch (FormatException fe) { Console.WriteLine("Parse failed!"); throw fe; // Re-throw the caught exception } catch (FormatException) { throw; // Re-throws the last caught exception } 24
  25. 25. Throwing Exceptions – Examplepublic static double Sqrt(double value){ if (value < 0) throw new System.ArgumentOutOfRangeException( "Sqrt for negative numbers is undefined!"); return Math.Sqrt(value);}static void Main(){ try { Sqrt(-1); } catch (ArgumentOutOfRangeException ex) { Console.Error.WriteLine("Error: " + ex.Message); throw; }} 25
  26. 26. Throwing Exceptions Live Demo
  27. 27. Choosing the Exception Type When an invalid parameter is passed to a method:  ArgumentException, ArgumentNullException, ArgumentOutOfRangeException When requested operation is not supported  NotSupportedException When a method is still not implemented  NotImplementedException If no suitable standard exception class is available  Create own exception class (inherit Exception) 27
  28. 28. Using Try-Finally Blocks
  29. 29. The try-finally Statement The statement: try { // Do some work that can cause an exception } finally { // This block will always execute } Ensures execution of given block in all cases  When exception is raised or not in the try block Used for execution of cleaning-up code, e.g. releasing resources 29
  30. 30. try-finally – Examplestatic void TestTryFinally(){ Console.WriteLine("Code executed before try-finally."); try { string str = Console.ReadLine(); Int32.Parse(str); Console.WriteLine("Parsing was successful."); return; // Exit from the current method } catch (FormatException) { Console.WriteLine("Parsing failed!"); } finally { Console.WriteLine( "This cleanup code is always executed."); } Console.WriteLine( "This code is after the try-finally block.");} 30
  31. 31. Try-Finally Live Demo
  32. 32. Exceptions: Best Practices
  33. 33. Exceptions – Best Practices catch blocks should begin with the exceptions lowest in the hierarchy  And continue with the more general exceptions  Otherwise a compilation error will occur Each catch block should handle only these exceptions which it expects  If a method is not competent to handle an exception, it should be left unhandled  Handling all exceptions disregarding their type is popular bad practice (anti-pattern)! 33
  34. 34. Exceptions – Best Practices (2) When raisingan exception always pass to the constructor good explanation message When throwing an exception always pass a good description of the problem  Exception message should explain what causes the problem and how to solve it  Good: "Size should be integer in range [1…15]"  Good: "Invalid state. First call Initialize()"  Bad: "Unexpected error"  Bad: "Invalid argument" 34
  35. 35. Exceptions – Best Practices (3) Exceptions can decrease the application performance  Throw exceptions only in situations which are really exceptional and should be handled  Do not throw exceptions in the normal program control flow (e.g. for invalid user input) CLR could throw exceptions at any time with no way to predict them  E.g. System.OutOfMemoryException 35
  36. 36. Summary Exceptions provide flexible error handling mechanism in .NET Framework  Allow errors to be handled at multiple levels  Each exception handler processes only errors of particular type (and its child types)  Other types of errors are processed by some other handlers later  Unhandled exceptions cause error messages Try-finally ensures given code block is always executed (even when an exception is thrown) 36
  37. 37. Exceptions Handling курсове и уроци по програмиране, уеб дизайн – безплатно BG Coder - онлайн състезателна система - online judge курсове и уроци по програмиране – Телерик академия форум програмиране, форум уеб дизайн уроци по програмиране и уеб дизайн за ученици ASP.NET курс - уеб програмиране, бази данни, C#, .NET, ASP.NET http://academy.telerik.com програмиране за деца – безплатни курсове и уроци ASP.NET MVC курс – HTML, SQL, C#, .NET, ASP.NET MVC безплатен SEO курс - оптимизация за търсачки алго академия – състезателно програмиране, състезаниякурсове и уроци по програмиране, книги – безплатно от Наков курс мобилни приложения с iPhone, Android, WP7, PhoneGap 37 уроци по уеб дизайн, HTML, CSS, JavaScript, Photoshop Дончо Минков - сайт за програмиране free C# book, безплатна книга C#, книга Java, книга C# Николай Костов - блог за програмиране безплатен курс "Качествен програмен код" безплатен курс "Разработка на софтуер в cloud среда" C# курс, програмиране, безплатно
  38. 38. Exercises1. Write a program that reads an integer number and calculates and prints its square root. If the number is invalid or negative, print "Invalid number". In all cases finally print "Good bye". Use try-catch-finally.2. Write a method ReadNumber(int start, int end) that enters an integer number in given range [start…end]. If an invalid number or non-number text is entered, the method should throw an exception. Using this method write a program that enters 10 numbers: a1, a2, … a10, such that 1 < a1 < … < a10 < 100 38
  39. 39. Exercises (2)3. Write a program that enters file name along with its full file path (e.g. C:WINDOWSwin.ini), reads its contents and prints it on the console. Find in MSDN how to use System.IO.File.ReadAllText(…). Be sure to catch all possible exceptions and print user- friendly error messages.4. Write a program that downloads a file from Internet (e.g. http://www.devbg.org/img/Logo-BASD.jpg) and stores it the current directory. Find in Google how to download files in C#. Be sure to catch all exceptions and to free any used resources in the finally block. 39
  40. 40. Free Trainings @ Telerik Academy Fundamentals of C# Programming Course  csharpfundamentals.telerik.com Telerik Software Academy  academy.telerik.com Telerik Academy @ Facebook  facebook.com/TelerikAcademy Telerik Software Academy Forums  forums.academy.telerik.com
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