Debugging and Handling Exceptions Chapter 11 Presented by Rodion Karmazin

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Quick Summary What you shall learn from this HUGE chapter are the “Try”, “Catch”, and “Finally” methods. Also you shall find out what debugging really is. I will also do my best to explain something about Exceptions

Error message does not always state the correct problem Errors Visual Studio IDE reports errors as soon as it is able to detect a problem Syntax errors Language rule violation Error message does not always state the correct problem Quick info Figure 11-1 Syntax error – extraneous semicolon

Run-Time Errors Just because your program reports no syntax errors does not necessarily mean it is running correctly One form of run-time error is a logic error Program runs, but produces incorrect results May be off-by-one in a loop Sometime users enter incorrect values Finding the problem can be challenging

“Exceptions” Some circumstances are beyond programmer’s control You have assumed nothing unusual would occur Have probably experienced unhandled exceptions being thrown While you browsed Web pages Unless provisions are made for handling exceptions, your program may crash or produce erroneous results Unhandled exception

“Exceptions” example Dialog box asks you whether you want to have an error report sent to Microsoft Figure 11-9 Microsoft error reporting

“Exceptions” example cont. Message displayed when you are creating console application and unhandled exception occurs Figure 11-11 Unhandled exception in a console application

“Unhandled Exceptions” Selecting Debug>Start to run application in Visual Studio Yellow arrow marks the error erroneous code highlighted Figure 11-12 Unhandled exception thrown – dividing by zero

Raising an “Exception” Error encountered – no recovery Raise or throw an exception Execution halts in the current method and the “Common Language Runtime” or “CLR” attempts to locate an “exception handler” Exception handler: block of code to be executed when a certain type of error occurs If no exception handler is found in current method, exception is thrown back to the calling method

Exception-Handling Techniques If event creates a problem frequently, best to use conditional expressions to catch and fix problem Execution is slowed down when CLR has to halt a method and find an appropriate event handler Exception-handling techniques are for serious errors that occur infrequently Exceptions classes integrated within the FCL Used with the try…catch…finally program constructs .NET Framework Class Library

Try…Catch…Finally Blocks Code that may create a problem is placed in the try block Code to deal with the problem (the exception handler) is placed in catch blocks Catch clause Code to be executed whether an exception is thrown or not is placed in the finally block

catch [ (ExceptionClassName exceptionIdentifier) ] try { // Statements } catch [ (ExceptionClassName exceptionIdentifier) ] // Exception handler statements : // [additional catch clauses] [ finally } ] Notice square brackets indicate optional entry One catch clause required finally clause optional

Try…Catch…Finally Blocks (continued) Generic catch clause Omit argument list with the catch Any exception thrown is handled by executing code within that catch block Control is never returned into the try block after an exception is thrown Using a try…catch block can keep the program from terminating abnormally

Use of Generic Catch Clause This example uses a generic catch block Figure 11-14 Generic catch block handles the exception

// calculate average grade for a class int totalGrades = 0; int grade = 0; int sumGrades = 0; String data = ""; Console.WriteLine("Please enter a grade [## to end]"); while (true) { try { data = Console.ReadLine(); grade = int.Parse(data); sumGrades += grade; totalGrades++; } catch (Exception e) if (data == "##") break; Console.WriteLine("Invalid data - try again"); Console.WriteLine("Please enter a grade [## to end]"); if (totalGrades > 0) Console.WriteLine("Average grade is {0:F2}", sumGrades / totalGrades); else Console.WriteLine("Average = 0 (No data was supplied)");

Comments, complaints, suggestions are also inquired for at this time! Any Questions? Comments, complaints, suggestions are also inquired for at this time! !!!!!!!! READ THE BOOK

Assignments (5 Pointer) (10 Pointer) Make a program that requests a phone number. If a knucklehead runs the program and types in a letter, display an error message. “Try” and “Catch” are required for this. (10 Pointer) Try to make out whatever is happening on slide 16 and make a program that asks for the number of scores and has the user put in the scores. If there is an error in the input, address that error immediately with a message box or Console.Writeline();