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ASP.NET Programming with C# and SQL Server First Edition

ASP.NET Programming with C# and SQL Server First Edition. Chapter 6 Debugging and Error Handling. Objectives. In this chapter, you will: Study debugging concepts Use basic debugging techniques Trace errors with the Visual Studio Debugger Handle exceptions and errors. Introduction.

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ASP.NET Programming with C# and SQL Server First Edition

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  1. ASP.NET Programming with C# and SQL Server First Edition Chapter 6 Debugging and Error Handling

  2. Objectives In this chapter, you will: • Study debugging concepts • Use basic debugging techniques • Trace errors with the Visual Studio Debugger • Handle exceptions and errors ASP.NET Programming with C# and SQL Server, First Edition

  3. Introduction • Regardless of experience, all programmers incorporate errors in programs at one time or another • It is therefore essential to master the art of debugging ASP.NET Programming with C# and SQL Server, First Edition

  4. Introduction to Debugging • Syntax: rules of a programming language • Logic: the order in which parts of a program run • Bug: an error in a program that causes it to function incorrectly • Three types of errors: • Syntax errors • Logic errors • Run-time errors • Debugging: the act of tracing and resolving errors in a program ASP.NET Programming with C# and SQL Server, First Edition

  5. Understanding Syntax Errors • Syntax errors: occur when the statements are not recognized by the Visual Studio IDE • Can be caused by incorrect usage of C# code or references to objects, methods, and variables that do not exist • Most syntax errors are caused by incorrectly spelled or mistyped words ASP.NET Programming with C# and SQL Server, First Edition

  6. Interpreting Error and Warning Messages • Error List window: displays two types of messages • Compiler error messages • Warning messages • Compiler error messages: occur for any syntax errors • Specify the incorrect line of code • Logic errors do not generate compiler errors ASP.NET Programming with C# and SQL Server, First Edition

  7. Interpreting Error and Warning Messages (cont’d.) Figure 6-2 Compiler error message for a statement that does not end with a semicolon ASP.NET Programming with C# and SQL Server, First Edition

  8. Interpreting Error and Warning Messages (cont’d.) • WarningLevel attribute: a page directive attribute specifying the types of warnings to report during compilation • Can be assigned values of 0 through 4 • Higher values generate more warnings • 0 disables warnings entirely • Changing the WarningLevel attribute does not prevent warnings from occurring • Only determines whether ASP.NET will report them during compilation ASP.NET Programming with C# and SQL Server, First Edition

  9. Interpreting Error and Warning Messages (cont’d.) Figure 6-3 Build error messages in the Move Estimator program ASP.NET Programming with C# and SQL Server, First Edition

  10. Handling Run-Time Errors • Run-time error: an error that occurs while a program is executing • Does not necessarily represent C# language errors • Occurs when the program encounters code it cannot execute • Common run-time errors involve numeric calculations, such as divide by 0 • Different numeric data types behave differently when dividing by 0 • Floating-point data types are assigned a value of “infinity” • Integer data types display an error ASP.NET Programming with C# and SQL Server, First Edition

  11. Figure 6-4 Divide-by-zero error ASP.NET Programming with C# and SQL Server, First Edition

  12. Identifying Logic Errors • Logic errors: flaws in a program’s design that prevent the program from producing correct results • Logic refers to the execution of program statements in the correct order to produce the desired results • Examples of logic errors: • Performing statements in the wrong order • Using the wrong arithmetic operator • Creation of an infinite loop ASP.NET Programming with C# and SQL Server, First Edition

  13. Using Basic Debugging Techniques • Most advanced programming languages provide advanced features for debugging code • First learn basic debugging techniques, and then learn the advanced features • The best weapon against bugs is to write good code! ASP.NET Programming with C# and SQL Server, First Edition

  14. Writing Good Code • The first and most important step in creating bug-free programs is to write good code • More discipline = fewer bugs • Use good syntax • Syntax errors may be difficult to pinpoint if you have a deeply nested set of control structures ASP.NET Programming with C# and SQL Server, First Edition

  15. Enabling Debugging • By default, ASP.NET displays a generic error message that includes only a description and exception details about run-time errors • Set Debug=“true” in the @Page directive to print detailed information about errors in the browser window ASP.NET Programming with C# and SQL Server, First Edition

  16. Figure 6-5 Detailed debugging information about a divide-by-zero error ASP.NET Programming with C# and SQL Server, First Edition

  17. Enabling Debugging (cont’d.) • Use the ASP.NET Web Site Server Administration Tool to enable debugging for all pages in the project • Click the Application Configuration link, and then the Configure Debugging and Tracing link, and then the Enable debugging box • Note that the page directive overrides the application-level debugging setting • Can manually start the application without debugging if desired ASP.NET Programming with C# and SQL Server, First Edition

  18. Tracing Errors with Response.Write() Statements • Can manually trace your code by including Response.Write()statements at different points in the program • Display contents of a variable, array, or function return value • Can monitor values as they change during execution • Be sure to remove Response.Write() statements used for debug before deploying the application • Driver program: a simplified, temporary program used for testing functions and other code ASP.NET Programming with C# and SQL Server, First Edition

  19. Tracing Errors with Response.Write()Statements (cont’d.) • Stub function: an empty function that serves as a placeholder for a program’s actual function • Typically returns a hard-coded value that represents the result of the actual function • Allows you to check for errors in the program from the ground up by isolating each function until it is correct ASP.NET Programming with C# and SQL Server, First Edition

  20. Using Comments to Locate Bugs • Can comment out problematic lines by transforming them into comments • Helps to isolate where the problem is occurring • Restores lines incrementally to see where the problem is occurring ASP.NET Programming with C# and SQL Server, First Edition

  21. Combining Debugging Techniques • It can be helpful to combine debugging techniques when searching for errors • Use Response.Write()to monitor values • Comment out problematic code • Use stubs for functions that may have problems to test the other parts of the program ASP.NET Programming with C# and SQL Server, First Edition

  22. Checking XHTML Elements • If you cannot locate a bug using debugging methods, the error may be in the XHTML elements • Must perform a line-by-line analysis of the XHTML code • Ensure that: • All tags have opening and closing brackets • All necessary opening and closing tag pairs are included • Look at the IDE’s Error List window for validation errors ASP.NET Programming with C# and SQL Server, First Edition

  23. Analyzing Logic • Code errors stemming from logic problems may be difficult to spot using tracing techniques • Must analyze each statement on a case-by-case basis • With logic errors, the code is syntactically correct but does not function as expected ASP.NET Programming with C# and SQL Server, First Edition

  24. Viewing Diagnostic Trace Information • Tracing: the examination of individual statements in an executing program • Set Trace=“true” in the @Page directive to view diagnostic trace information • Prints details of the page’s execution in the browser • Diagnostic trace information can be stored in a log by enabling application-level tracing • Use Web Site Administration Tool • Click Application Configuration link, and then Configure debugging and tracing link, and then Capture tracing information box ASP.NET Programming with C# and SQL Server, First Edition

  25. Figure 6-7 Diagnostic trace output ASP.NET Programming with C# and SQL Server, First Edition

  26. Viewing Diagnostic Trace Information (cont’d.) • After enabling application-level tracing, you can view the trace log using Trace Viewer • Open the Trace.axd file in your project’s root folder • It contains links to the trace details • Trace info shows request information such as events and object collection variables • Does not show code details • Trace.Write() and Trace.Warn() methods of the TraceContext class can be used to add custom messages to the trace diagnostic output ASP.NET Programming with C# and SQL Server, First Edition

  27. Figure 6-8 Trace Viewer ASP.NET Programming with C# and SQL Server, First Edition

  28. Viewing Diagnostic Trace Information (cont’d.) • Debugging code should only run if tracing is enabled • Use Trace.IsEnabled property to determine if tracing is enabled • Ensure that tracing is turned off for production environments ASP.NET Programming with C# and SQL Server, First Edition

  29. Figure 6-9 Custom trace message ASP.NET Programming with C# and SQL Server, First Edition

  30. Tracing Errors with the Visual Studio Debugger • Basic debugging techniques are useful with smaller programs • Finding errors in larger programs is difficult using error messages and manually checking your code • Debugger: a tool in the Visual Studio IDE that contains options for tracing lines of code interactively ASP.NET Programming with C# and SQL Server, First Edition

  31. Stepping Through Your Code • Use the Debug menu to start the debugger • Step Into command: executes a single line of code and pauses • Step Over command: treats a function call as a single line of code (does not step into the function) • Breakpoint: a statement that switches program execution to break mode • Break mode: a state in which the program execution is temporarily suspended, allowing you to monitor values and trace execution one command at a time ASP.NET Programming with C# and SQL Server, First Edition

  32. Stepping Through Your Code (cont’d.) • Continue command: executes the rest of the program normally or until another breakpoint is encountered • Stop Debugging command: ends a debugging session without executing the rest of the program • Step Out command: executes all remaining code within the current function • Run to Cursor command: program runs normally until it reaches the statement where your cursor is located, and then enters break mode ASP.NET Programming with C# and SQL Server, First Edition

  33. Stepping Through Your Code (cont’d.) • When you enter break mode, the Locals and Watch windows appear at the bottom of the IDE • To insert a breakpoint, click in the statement and select Toggle Breakpoint from the Debug menu • A red circle appears in the margin to identify the breakpoint • Changes made to code during break mode will not take effect while the program is executing • Clear breakpoints individually using Toggle Breakpoint, or use Delete All Breakpoints in the Debug menu ASP.NET Programming with C# and SQL Server, First Edition

  34. Working with the Debugger Windows • Four debugging windows are available when in break mode: • Locals • Watch • Immediate • Call Stack • You can display these windows using the Debug menu – Windows submenu • Locals window: displays all local variables within the currently running function and their values • Right-click the value, and select Edit Value to change the value while the program is in break mode ASP.NET Programming with C# and SQL Server, First Edition

  35. Figure 6-10 Locals window in break mode ASP.NET Programming with C# and SQL Server, First Edition

  36. Working with the Debugger Windows (cont’d.) • Watch window: monitors variables and expressions in break mode • Enter a variable or expression in the first column in an empty row • Useful for determining when a variable changes value • Immediate window: used to monitor specific variables and expressions in break mode • Enter the variable or expression in the window and press Enter to see its current value • Enter a variable, and then equals, and then a value to change the variable’s value during break mode ASP.NET Programming with C# and SQL Server, First Edition

  37. Figure 6-11 Variables in the Watch window ASP.NET Programming with C# and SQL Server, First Edition

  38. Figure 6-12 Variables in the Immediate window ASP.NET Programming with C# and SQL Server, First Edition

  39. Working with the Debugger Windows (cont’d.) • Call Stack window: appears automatically when you enter break mode • Call stack: refers to the order in which procedures (such as functions, methods, or event handlers) execute within a program • When a procedure is called, the procedure is added to the top of the call stack • When a procedure ends, it is removed from the top of the call stack • Call stack window allows you to see the flow of the program within procedure calls ASP.NET Programming with C# and SQL Server, First Edition

  40. Handling Exceptions and Errors • Your goal is to write code that anticipates problems and gracefully handles them • Bulletproofing: writing code to anticipate and handle potential problems • Exception handling: allows a program to handle problems as they occur during execution • Exception: indicates that something unexpected occurred during program execution • May or may not be an error • C# includes exception-handling capabilities ASP.NET Programming with C# and SQL Server, First Edition

  41. Handling Exceptions and Errors (cont’d.) • Use exception handling to test any type of input or functionality that is external to the program • Three levels at which you can handle exceptions: • Code level • Page level • Application level • Always strive to handle exceptions at the code level with try…catch blocks • If you cannot handle the error at the code level, handle it at the page level ASP.NET Programming with C# and SQL Server, First Edition

  42. Throwing Exceptions • try statement: used to enclose code that might contain an exception • Throwing an exception: the process by which a try statement generates an exception class object • Syntax: try { statements; } ASP.NET Programming with C# and SQL Server, First Edition

  43. Throwing Exceptions (cont’d.)‏ • In C#, exceptions are represented by classes based on the Exception class • ArithmeticException class: thrown for errors in an arithmetic, casting, or conversion operation • MissingMemberException: thrown when attempting to access a class method or property that does not exist • throw statement: used to manually throw an exception and an Exception object ASP.NET Programming with C# and SQL Server, First Edition

  44. Catching Exceptions • Exception objects contain properties and methods that describe what went wrong • Message property: contains a message describing the current exception • ToString() method: returns a more detailed description of the exception • catch statement: used after an error is thrown to handle the exception object • Syntax: catch(exception_type identifier) { statements; } ASP.NET Programming with C# and SQL Server, First Edition

  45. Catching Exceptions (cont’d.) • When a try statement throws an exception, the compiler executes the nearest catch statement that matches the exception type • If no match in the current try block, the compiler checks for a matching catch statement in the next function or method in the call stack • If no matches are found, the compiler relies on any existing page-level exception handling • If no page-level exception handling, the compiler looks to application-level exception handling • If none found, a default error message is displayed ASP.NET Programming with C# and SQL Server, First Edition

  46. Catching Exceptions (cont’d.) • A try statement can contain more than one catch block • catch blocks are examined for a match of the exception type in the order in which they appear in the try statement • List the most specific exception types first, and then the more general types • Exception class: used to catch any type of exception that is thrown ASP.NET Programming with C# and SQL Server, First Edition

  47. Executing Final Exception-Handling Tasks • finally statement: executes regardless of whether the associated try block throws an exception • Used to perform clean-up tasks • If there is no catch statement, the finally statement executes before the compiler begins searching for a higher-level catch statement ASP.NET Programming with C# and SQL Server, First Edition

  48. Handling Page-Level Errors • Most basic page-level error handling is to assign an error page to the ErrorPage attribute of the @Pagedirective • To programmatically control page-level error-handling, you must create an event handler • Use the System.EventHandler() function in the Page_Load() event handler to designate a function as the error event handler • To redirect the browser to an error page from the error event handler, use System.Transfer() method ASP.NET Programming with C# and SQL Server, First Edition

  49. Handling Page-Level Errors (cont’d.) • Must clear an error after handling it with Server.ClearError() method • To pass an error to a page error event handler, you must throw it from the code level ASP.NET Programming with C# and SQL Server, First Edition

  50. Handling Application-Level Errors • Use the System.Transfer()method to redirect the browser to a generic error page • Use the Server.Clear()method to clear errors • Use the Server.GetLastError()method to access the last exception that occurred • Application errors are handed in the Global.asax file • Use Website menu – Add New Item to add a Global.asax file • Application_Error(): an event handler created by default in the Global.asax file ASP.NET Programming with C# and SQL Server, First Edition

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