Exception handling in ASP .NET

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Exception Handling in .NET

4. What is Exception?
• An exception is an event, which occurs during
the execution of a program.
• It disrupts the normal flow of the program's
instructions
• Exception handling enables programmers to
remove error-handling code from the “main
line” of the program’s execution.

5. What is Exception? (cont.)
• Simplify code construction and maintenance
• Allow the problematic situations to be
processed at multiple levels

6. Handling Exceptions
• Catching and Processing Errors
• In C# the exceptions can be handled by the try-catch-
finally construction
• Provided by System.Exception
• Enable clear, robust and more fault-tolerant programs
• catch blocks can be used multiple times to process
different exception types

7. Handling Exceptions
• Keywords
try {
// includes code in which exception might occur
}
catch (InvalidOperationException) {
//Code to handle the exception
//Must be of class Exception or one that extends it directly or indirectly
}
catch (SomeOtherException) {
// code that recovers from an SomeOtherException
// (or any exception type derived from it)
}
catch {
// code that recovers from any kind of exception
// when you catch any exception, you usually re-throw
throw;
}
finally {
// code that cleans up any operations started
// within the try block.
// (Optional) code present here will always be executed
}

8. Types of Exceptions
• .NET exceptions inherit from System.Exception
• The system exceptions inherit from System.SystemException,
e.g.
– System.ArgumentException
– System.NullReferenceException
– System.OutOfMemoryException
– System.StackOverflowException
• User-defined exceptions should inherit from
System.ApplicationException

9. Common .NET Exceptions
 The CLR generates SystemExceptions, derived from class
Exception, which can occur at any point during program
execution.
 If a program attempts to access an out-of-range array index,
the CLR throws an exception of type
IndexOutOfRangeException.
 Attempting to use a null reference causes a
NullReferenceException.

10. try block
• A try block contains code that requires common cleanup or
exception-recovery operations.
• The cleanup code should be put in a single finally block.
• The exception recovery code should be put in one or more
catch blocks.
– Create one catch block for each kind of type you want to handle.
• A try block must have at least one catch or finally block.

11. catch block
• A catch block contains code to execute in response to an
exception.
• If the code in a try block doesn’t cause an exception to be
thrown, the CLR will never execute the code in any of its
catch blocks.
• The catch type must be of type System.Exception or a type
that derived from System.Exception
• You can also specify a variable name like catch(Exception e)
to access information specific to the exception.

12. finally block
C# provides the finally block, which is guaranteed to
execute regardless of whether an exception occurs.
If the try block executes without throwing, the finally
block executes.
If the try block throws an exception, the finally block
still executes regardless of whether the exception is
caught.
This makes the finally block ideal to release resources
from the corresponding try block.

13. finally block (conti.)
• Local variables in a try block cannot be accessed in the
corresponding finally block, so variables that must be
accessed in both should be declared before the try block.
• Placing the finally block before a catch block is a syntax
error.
• A try block does not require a finally block, sometimes
no clean-up is needed.
• A try block can have no more than one finally block.

14. System.Exception
 In .NET, only objects of class Exception and its derived classes
may be thrown and caught.
 Exceptions thrown in other .NET languages can be caught with the
general catch clause.
 Class Exception is the base class of .NET’s exception class
hierarchy.
 A catch block can use a base-class type to catch a hierarchy of
related exceptions.
 A catch block that specifies a parameter of type Exception can
catch all exceptions.

15. Benefits of Exceptions
• The ability to keep code that deals with
exceptional situations in a central place.
• The ability to locate and fix bugs in the code
• Unified error handling: all .NET Framework
classes throw exceptions to handle error
cases.

16. Benefits of Exceptions (conti.)
• Old Win32 APIs and COM returns a 32-bit error code.
Exceptions include a string description of the problem.
• Exceptions also include a stack trace that tells you the
path application took until the error occurred.
• You can also put any information you want in a user-
defined exception of your own.

17. Example
OUTPUT:

18. • HelpLink:This is empty because it was not defined on the exception.
HelpLink is a string property.
• Message:This is a short description of the exception's cause.
Message is a read-only string property.
• Source:This is the application name. Source is a string property that
can be assigned to or read from.
• StackTrace:This is the path through the compiled program's method
hierarchy that the exception was generated from.
• TargetSite:This is the name of the method where the error
occurred. This property helps simplify what part of the errors are
recorded.
KEYWORDS

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