Errors In Your ASP.NET Code? Don't Throw a Fit, Throw an Exception!

Error handling?everyone's favorite topic right?Even the best designed applications need to handle and properly manage errors the errors you can plan for and those you cannot.In this article, you'll learn error handling techniques in ASP.NET. Topics will range from handling common errors with the Try...Catch syntax to logging unhandled errors into the Windows Event Log.

Error handling, paying taxes, and root canals. Each one conjures up a number of negative mental images?some real, some imagined. While I cannot do much to ease the pain of paying taxes or reduce the suffering you might experience during a root canal, this article will enlighten you regarding the ins and outs of error handling in ASP.NET.

Errors you need to concern yourself with fall into two distinct error types?handled errors and unhandled errors. A handled error occurs within your application?you are ready for it and have code in place to take action upon it. For example, maybe you have coded your application to divide one number by another. What if the second number is 0? A divide by zero error will occur and hopefully you will have code in place to handle such a problem. Unhandled errors are those errors that you cannot foresee happening. An example is an incorrectly typed URL reference for a page on your Web site that results in a "404 Page Not Found" error. Hopefully you will have the pieces in place that will direct the user to a much cleaner and more refined page than the default "404 Page Not Found" page.

Let's start by discussing the language constructs available to handle errors within ASP.NET applications. We will take a look at system exception objects, the Try...Catch...Finally commands, and creating you own Exception objects.

Errors and Exceptions

While the terms error and exception are often used interchangeably, there is a distinct difference between the two. An error happens during the execution of a block of program code and alters the normal program flow, thus creating an Exception object. When the flow of a program is interrupted by an error, the program will search for exception handling code to tell the program how to react. Simply put, an error is an event. That event creates an object called an exception. This Exception object contains information about the error including when and where it occurred.

Unlike throwing a punch, you will often hear the phrase "throwing an exception." This means that an error occurred in a code block and an Exception object was created.

The generic Exception class from which all Exception objects are derived is the System.Exception class. It contains a number of properties designed to give you an easy way to capture and manage information about the Exception object. Table 1 lists System.Exception properties.

Structured vs. Unstructured Error Handling

Visual Basic .NET and ASP.NET support structured error handling. Structured error handling uses a control structure that acts as the exception handling mechanism. This structure allows your program to determine which type of exception was thrown and act accordingly.

Unstructured error handling is the type of error handling supported in prior versions of Visual Basic and VBScript. It is comprised of an Err object and commands such as On Error, Resume, and Error. This article will not discuss the details of unstructured error handling since structured error handling is the preferred method to trap and manage errors.

By: Jim Duffy

jduffy@takenote.com

Jim Duffy is founder and president of TakeNote Technologies, an award-winning training, consulting, and software development company specializing in .NET software developer training and helping clients create business solutions with Microsoft enterprise technologies. Jim’s expertise is with .NET technologies, ASP.NET, SQL Server and Visual FoxPro-to-.NET conversions. He has a BS degree in Computer and Information Systems and over 25 years of programming and training experience. He is an energetic trainer, skilled developer, and has been published in leading developer-oriented publications.

Jim is a Microsoft Regional Director, a Microsoft MVP, an ASPInsider, and is an entertaining and popular speaker at regional user groups and international developer conferences.

You can find additional information about Jim, TakeNote Technologies, links to his blog, as well as a public training class schedule, on-site training information, consulting information, and software development services at www.takenote.com.

jduffy@takenote.com