Error Handling In ASP.NET and VB.NET

Introduction

One of the main features of ASP.NET over ASP is its new error handling features. The only way to capture errors in the ASP/VBScript was using the "On Error Resume Next" statement and checking each line for an error with "If Err.Number <> 0 Then " statements. Developers who where using JScript instead of the VBScript at the server-side where lucky. They've had access to the "Try... Catch" statements provided by JScript. Well it's all over now. Now all the .NET enabled languages support better error handling statements including VB.NET. Now, VB.NET supports the "Try...Catch" statement. On top of this ASP.NET configuration file also supports ASP.NET level error handling. Let's see all of them one by one.

VB.NET Error Handling

In the previous versions of VB we had several flavors of "On Error" statements to handle error in a non-structured way. But in VB.NET we can make use of the structured way of error handling with the Try & Catch statement. Here is an example for this.

Sub DoSomething()
Try
'Do Something
Catch e As Exception
'Catch the error and display it.
Response.Write("An Error Occurred: " & e.toString())
Finally
'Do the final cleanup such as closing the Database connection
End Try
End Sub

The above code is the simplest form of structured error handling in VB.NET. We're doing our processing in the Try Block. If anything goes wrong in the Try block, the Catch block will be called and the error will be handled. After that the "Finally" block will be called to do the cleanup.

We can also filter the error handling based on the type of exceptions. For example, Let's say we're accessing data from SQL Server 2000 and we're writing the data into a text file. In this process there could be three kids of error.

1. SQL Server related Errors. 
2. File IO Errors. 
3. Other generic errors.

We can filter the errors based on their categories in the Catch block. Let's see an example for this.

Sub DoSomething()
Try
'Do Something
Catch SQLExp As SQLException
'Catch the error and display it.
Response.Write("An SQL Server Error Occurred: " & e.toString())
Catch IOExp As IOException
'Catch the error and display it.
Response.Write("An File IO Error Occurred: " & e.toString())
Catch e As Exception
'Catch the error and display it.
Response.Write("An Error Occurred: " & e.toString())
Finally
'Do the final cleanup such as closing the Database connection
End Try
End Sub

As you can see we've categorized our error handling based on the errors. The "Catch SQLExp as SQLException" block will handle all the SQL Server errors and all File IO errors will be handled by "Catch IOExp as IOException" block. The SQLException and IOExpection classes are defined in the "System.Data.SQL" and "System.IO" namespaces. We can further customize the errors based on the error numbers,  

Sub DoSomething()
Try
'Do Something
Catch SQLExp As SQLException
If SQLExp.Number = 1212 Then
Response.Write("SQL Server Error 1212 Occurred: " & e.toString())
Else
'Catch the error and display it.
Response.Write("An SQL Server Error Occurred: " & e.toString())
End If
Catch IOExp As IOException
'Catch the error and display it.
Response.Write("An File IO Error Occurred: " & e.toString())
Catch e As Exception
'Catch the error and display it.
Response.Write("An Error Occured: " & e.toString())
Finally
'Do the final cleanup such as closing the Database connection
End Try
End Sub

or

Sub DoSomething()
Try
'Do Something
Catch When Err = 1212
Response.Write("SQL Server Error 1212 Occurred: " & e.toString())
Catch SQLExp As SQLException
'Catch the error and display it.
Response.Write("An SQL Server Error Occurred: " & e.toString())
Catch IOExp As IOException
'Catch the error and display it.
Response.Write("An File IO Error Occurred: " & e.toString())
Catch e As Exception
'Catch the error and display it.
Response.Write("An Error Occurred: " & e.toString())
Finally
'Do the final cleanup such as closing the Database connection 
End Try
End Sub

We can use the "Exit Try" statement in the "Catch" block to exit from the "Try...Catch" structure. 

ASP.NET Error Handling options

ASP.NET provides better support for tracking runtime errors and handling them. This gives us more power to display customized messages to the site users rather than seeing the error messages like "BC30311: A value of type..." .

By default, when an error occurs in ASP.NET code, ASP.NET show the error message and it'll also highlights the code line of code causing the error. For example, in the page load event, we've used the following code. 

Sub Page_Load(ByVal Src As Object, ByVal E As EventArgs)

Dim Myobj As New MyObject

Myobj.DoSomthing()

End Sub

MyObject was not defined in any of the namespaces. So, I should get a compilation error. Here is how ASP.NET shows the error message.

  

ASP.NET shows the error message and it also shows the line of code that causes the error including the line number. You'll see this error message only if you are accessing the site locally like "http://localhost/YourApp/". If you access the site with it's IP address likehttp://xx.xx.xx.xx/YourApp/ then, you'll not see the above message. Instead you'll see something like this.

  

ASP.NET hides all your application information (Your code), when your site is accessed from out side world. It only shows the application specific error message if you access the site locally. When you are developing the ASP.NET application, you may sit in a different machine than the one hosted the ASP.NET. In these cases you may want to see the error messages outside of the local machine also. Well, if you want to show the error messages outside the local machine then you can set the set the <customerrors> tags mode attribute to "off"
in WEB.CONFIG?

<configuration>
<customerrors mode=" off " />
</configuration> 

Custom Error page

The default message showed by ASP.NET may make sense in the development environment and it many not make any sense to the end users of the site. In that case, ASP.NET allows us to configure a default error-handling page for your site. We can specify the error page like this in WEB.CONFIG.

<configuration>
<customerrors mode="on" defaultredirect="error.aspx" />
</configuration>

So, when an un-handled error occurs in ASP.NET application "Error.ASPX" error page will be displayed. After we've changed the configuration in the WEB.CONFIG, here is how the above error pages looks like.
  

Of course, we can make this page prettier some meaningful content to the end user of your site.  

The mode attribute in the <customerrors> tag can have two options. The difference is shown in the following table. 

Mode attribute	Localhost	IP Address
remoteonly	The error message from the compiler and the code that is causing the error will be showed.	Custom error page will be showed.
on	Custom error page will be showed.	Custom error page will be showed.

Final Note

ASP.NET and VB.NET both are providing excellent error handling options when compared with ASP and VB6. 

Until next time, Happy Programming!