Robust .NET Exception Handling

After receiving an Amazon Kindle 2 for Christmas the first book I purchased was Lee Dumond's Robust ASP.NET Exception Handling. Coming to .NET from a Java background I had not bothered to look deeply into the exception handling classes available in the base library.

The book moves from the basics of exceptions through to best practices and patterns for handling exceptions in .NET. I picked out the following as generic .NET gems (i.e. not specific to ASP.NET):

• The GetBaseException method can be used to find the root cause of all exceptions in the exception chain
• ArgumentException contains a ParamName string that gets the name of the parameter that caused the exception
• Objects that implement IDisposable can be instantiated in a using statement as an alternative to a finally block
• Use throw rather than throw ex as this resets the call stack from the point of the throw

In addition some handy conventions to follow in my own implementations:

• ArgumentNullException and ArgumentOutOfException should be thrown in situations whereby the application cannot provide it's defined functionality
• InvalidOperationException is useful for wrapping original exceptions
• Safe Casting - as and is can be used to avoid cast exceptions. as returns null if the cast cannot be performed and is returns true or false whether or not the cast would succeed
• Safe Parsing - use TryParse rather than Parse for value types to avoid FormatExceptions. TryParse returns true or false and an out parameter. This TryX pattern can be reused for your own implementations

The book also provides a useful pattern for handling exceptions that occur during the execution of an AJAX postback:

<asp:ScriptManager ID=”ScriptManager1” runat=”server”
AllowCustomErrorsRedirect=”false” />
...
<script type=”text/javascript”>
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(OnEndRequest);
function OnEndRequest(sender, args) {
 var errorElement = $get(“errorMessage”);
 if (args.get_error()) {
   errorElement.innerHTML = “An error occurred while attempting to fulfill your request. “ + “Please try again.”;
   args.set_errorHandled(true);
 }
 else {
   errorElement.innerHTML = “”;
 }
}
</script>

Following on from this the book provides some brilliantly flexible patterns for handling exceptions in ASP.NET. For these you will have to go and get the book. I'm off to implement them into a utility library and a recently published ASP.NET application.