Coding Defensively

1. Coding Defensively Coding Defensively Chapter 4 - Coder to Developer by Mike Gunderloy Instructor : Dr.James Fawcett Presented by Priyaa Nachimuthu priyaa@syr.edu

2. Agenda • Assertions • Exceptions • Comments

3. Defensive Coding • Reusable code • Easy maintenance • Paves way to locate problems easily • Allows modifications

4. Assertions • Problem indicators • Design time debugging • Assert a boolean condition • Microsoft .Net runtime

5. Using Assertions • Add a reference to System. Diagnostics namespace • Use Debug. Assert method or Trace. Assert method // This method should never be called // without a source Url Debug. Assert (((d.SourceURL != string. Empty) && (d.SourceUrl != null)),”Empty SourceUrl”,”Can’t download a file unless a Source Url is supplied”);

6. Assertion Failed Alert

7. Mechanics of .Net Assertions

8. Debug Configuration

9. Release Configuration

10. Listener classes • TraceListener – abstract class in System. Diagnostics namespace • DefaultTraceListener – An object of this class is automatically added to Listeners collection of Trace and Debug classes. Writes output to output window or message boxes. • TextWriterTraceListener – writes messages to any class that derives from the Stream class. • EventlogTraceListener – writes messages to Windows event log. • Inherit from TraceListener - custom behavior

11. Guidelines for good assertions • Hide assertions from end – users • Use Debug. Assert • Assertions shouldn't have side effects // Make sure path is ok Debug. Assert ((newpath = Path.Combine(foldername,filename))!= string.Empty,”Bad path to download” ); • Don’t verify compiler operation Int[] intSizes = new int[3]; Debug.Assert(intSizes.GetUpperBound(0) == 2,”Failed to initialize array “);

12. Guidelines for good assertions • Reasonable input data • Checks in the method that uses the value • Check assumptions after complex code execution

13. Exceptions • Exceptions are for exceptional situations • Run-time checking public void GetDownload ( Download d ) { string filename = string.Empty; string foldername = string.Empty; WebClient wc = new WebClient(); try { // code to do download } catch ( Exception e ) { // bubble to any exception up to caller with custom info throw new DownloadException(“Unable to download”,d,e); } Finally { wc.Dispose(); } }

14. Exception Mechanics • try – Exception handling block • catch – start of the actual exception handling • finally – start of the code that will always run. Used for cleanup • throw – generates an exception

15. Custom Exceptions • There is no existing exception class that correctly represents the exceptional condition. • To pass additional information to the parent code.

16. Guidelines for good exceptions • Exceptions are for exceptional situations • Use exception constructors with string value for additional information • While passing error information from lower level code to higher level code use one of the constructors that accepts an Exception object. • Don’t create custom exceptions when the built –ones will suffice.

17. Comments or Self-Documenting Code • Code should be its own documentation, comments are superfluous. • Well written comments make code easier to read. • Writing good comments is no excuse to writing bad code.

18. Noise Comments • Comments that make code longer with no value addition // DefaultDownloadFolder property public string DefaultDownloadFolder { // public getter get { return _D; } // public setter set { _D = value; } }

19. Noise Comments • Formal procedure header // Procedure:GetDownload // Module: DownloadEngine.cs // Author : Mike // Creation Date: 21 October 2003 // // Inputs : // d - A Download variable with download file information // Outputs: // None // // Revision History: // 05 Nov 2003 : Initial Creation // 07 Nov 2003 : Download code implemented // Public void GetDownload(Download d) { // Method body goes here … }

20. Placeholder comments // HACK: Brute- force search, can replace this if this becomes a perf issue // TODO: Add code to make sure user has access to folder // POSTPONE : This is where we want to create custom properties • Tools -> Options -> Environment -> Task List

21. Visual Studio.NET Task List

22. Summary and Intent Comments • Intent comment // GetDownload uses the information in a download object // to get the specified file from the internet to a local hard // drive. Any download errors are wrapped in a custom // exception and returned to the caller. Public void GetDownload(Download d) { // Method body goes here … } • Summary comment public void GetDownload(Download d) { try { // This method should never be called // without a source Url Debug. Assert (((d.SourceURL != string. Empty) && (d.SourceUrl != null)),”Empty SourceUrl”,”Can’t download a file unless a Source Url is supplied”); } catch(Exception e) { // bubble to any exception up to caller with custom info throw new DownloadException(“Unable to download”,d,e); } }

23. Defensive Coding Checklist • Use assertions to catch design –time errors • Use exceptions to handle runtime errors • Use exceptions only for exceptional situations • Avoid peppering your code with noise comments • Use comments as placeholders for future work • Use comments to summarize and document the intent of the code • When you change code, make sure to keep the comments up-to-date.