Error Handing in .Net FAQ

1. How to prevent errors at design time?

VB.Net offers the Option Strict and Option Explicit statements to prevent errors at design time.

• Option Explicit :

Prevent errors at design time. It forces explicit declaration of all variables at the module level

Syntax Option

Option Explicit On.

By default it is off.

We can set option explicit in .aspx page in the page directive




• Option strict :

Enabling Option Strict causes errors to be raised if you attempt a data type conversion that leads to a loss in data.


2.What Precisely is an Exception?

The Exception Class is a member of System NameSpace and base class for all the exceptions.

2 subclasses of Exception class are System Exception Class and Application Exception class.

Exception Class
System Exception Application Exception

The SystemException class defines the base class for all .NET predefined exceptions

3.What are the properties of Exception Object ?

Source : Gets the naem of the application or the object that causes the error
Message :Gets a message that describes the current exception.
StackTrace : Gets a string representation of the frames on the call stack at the time the current exception was thrown.
InnerException : Gets the Exception instance that caused the current exception
TargetSite : Gets the method that throws the current exception.
HelpLink :Gets or sets a link to the help file associated with this exception.

4.What is Structured and UnStructured Error Handling?

Unstructured Error Handling:


Unstructured error handling is implemented with the On Error statement, which is placed at the beginning of a code block to handle all possible exceptions that occur during the execution of the code. All Visual Basic 6.0 error handlers in .NET are objects that can be accessed by using the Microsoft.VisualBasic.Information.Err namespace. The handler is set to Nothing each time the procedure is called. You should place only one On Error statement in each procedure, because additional statements disable all previous handlers that are defined in that procedure.

On Error { GoTo [ line 0 -1 ] Resume Next }

Structured Error Handling :

With structured error handling, Visual Basic now has an effective way to prevent unexpected errors from terminating the execution of the application. Structured error handling also provides the programmers with a simpler way to create robust applications that are easier to maintain. Structured error handling is implemented in Visual Basic. NET or Visual Basic 2005 with a Try...Catch...Finally block of statements. The Try...Catch...Finally block provides, for the first time to Visual Basic, the capability of nested error handling.

5. Mention Predefined exceptions provided by .net run time?


Exception --Object--Base class for all exceptions.
SystemException--Exception--Base class for all runtime-generated errors.
IndexOutOfRangeException--SystemException--Thrown by the runtime only when an array indexed improperly.
NullReferenceException--SystemException--Thrown by the runtime only when a null object
referenced.

InvalidOperationException--SystemException--Thrown by methods when in an invalid state.

ArgumentException --SystemException--Base class for all argument exceptions.

ArgumentNullException --ArgumentException--Thrown by methods that do not allow
an argument to be null.
ArgumentOutOfRangeException --ArgumentException --Thrown by methods that verify
that arguments are in a given range.

ExternalException--SystemException--Base class for exceptions that occur or are targeted
at environments outside the runtime.

ComException--ExternalException--Exception encapsulating COM HRESULT information.

SEHException --ExternalException--Exception encapsulating Win32 structured exception handling information

6.What class we need to inherit to create custom Excepton class.
To create a custom exception we must inherit from the ApplicationException class

7.How to redirect the user to the friendly error-handler page when an Application error occurs?

Modify web.config as

customErrors mode="On" defaultRedirect="errorpage.aspx"

The configuration section supports an inner tag that associates HTTP status codes with custom error pages. For example:

--< mode="On" defaultredirect="genericerror.htm">
-- < statuscode="404" redirect="pagenotfound.aspx">
--< statuscode="403" redirect="noaccess.aspx">
--< /customerrors>