Debugging Your Xojo Applications

Your customers and clients expect your Xojo applications to be as bug free as possible.  What mechanisms do you have in place to handle an error and report it?  Bugs occur – that’s a fact of life – and even the best error handling in the world can’t prevent bugs from occurring.

Thoroughly testing your application is your first and best line of defense.  However, it’s very time consuming and without good testing procedures it may even be a waste of time.  I would also add the it’s very hard for the developer to be a good tester of their own code.  You programmed it to do a certain task in a certain way.  Someone else will have a different set of expectations.

Regression testing is the only way to really make sure that changes in one part of your code doesn’t change other parts of your application (or a new version of Xojo doesn’t affect you either!).  An excellent way to do regression testing on your software is to use the open source unit testing module called XojoUnit (it’s now part of Xojo).  It allows you to test your code with known inputs and test them against the actual output.

A common question from the forums is that people get an error message saying, “The application has encountered an error and must now shutdown,” and they have no idea what the error is or where the error happened.  They need to learn as much as they can about the Exception class and in particular the Stack property.  The stack was introduced way back in 2006 and is a string array that contains the methods that have run from the entry point into your code until where the exception occurred.  Be aware that the Include Function Names property has to be true in your application for the stack to be human readable.

Use the UnhandledException event in the application class to capture any errors that weren’t handled elsewhere.  The exception stack allows you to determine where the error occurred and from there it’s a simple matter to send an email, post to a web form or write the error out to a log file that includes important details such as platform, operating system version and the version of your application.

Some applications will require files be in a specific location and when debugging your application those files might not be in the proper (final) location.  Use the DebugBuild constant along with conditional compilation, #If,  to handle things differently at debug time and runtime.  For debugging purposes you can have the required files in the local project directory for convenience sake.  A feature added in 2007 allows you to place your debug build in a particular location which eliminates the need to have non-project files in your project directory.

Cross-platform applications require additional handling but now that Xojo with (or without) a desktop license can do remote debugging and it’s very easy to do.  I run the Xojo IDE on Mac OS X on an iMac and use VMWare running various versions of Windows and Linux so I can debug my applications in those environments.  The remote debugger works exactly like the regular debugger except that the debug application is running in another environment.  It’s a little slower to initiate since the app has to be transferred to the other environment but otherwise it’s the same process.

I highly recommend testing early and often on the other platforms you’re developing for.  Don’t wait until the end to do extensive testing.  While Xojo does a great job on cross-platform applications there ARE platform differences you need to be aware of.

New developers coming from Visual Basic 6 are often irritated by the perceived lack of database error in Xojo.  An incorrect SQL statement when opening a recordset results in nil recordset objects instead of a throwing a runtime error.  The unexpected nil recordset then causes NilObjectException errors.  You must get in the habit of checking your database object for errors after every database operation.  Once you catch the error you can at least be more graceful on how to recover from it.

That’s a lot of information so do your research.  Debugging your application isn’t as hard as you think.

What things do you do to make your life easier hunting down or preventing bugs?