Exception Handling in .NET Framework

Overview

The .NET Framework has enhanced the ways in which you deal with errors. C#'s mechanism for handling error conditions allows you to provide custom handling for each type of error condition as well as to separate code that identifies errors from the code that handles them.

What is an Exception?

Exceptions are unusual error conditions that occur during execution of a program or an application. Whenever an exception is occurred the .NET runtime throws an object of type "XXXException" All the Exceptions thrown by .NET runtime have their base class as Exception. There are varieties of system exceptions that are thrown. Please find some of the Exceptions thrown by the .NET runtime.

OutOfMemoryException
NullReferenceException
InvalidCastException
ArrayTypeMismatchException
IndexOutOfRangeException
ArithmeticException
DivideByZeroException
OverFlowExceptio

Be Proactive rather than reactive: It is always a better practice to write code in a manner to avoid exceptions rather than catching and handling the same. This approach may not be possible for all kinds of Exception since it is tough to anticipate OutOfMemoryException and avoid the same.

How to Handle Exceptions in .NET 2.0?

In .NET we have the try and catch block that enables us to handle the exceptions

Example 1:.

try
{
// Code where we are anticipating Exceptions
}
catch (Exception)
{
// Code to Handle the Exception
}
finally
{ 
// Code that needs to be executed in any case
// typically cleaning up code
}

The try and catch block are mandatory for handling exceptions. Finally, the block is optional. It is always a better practice to handle exceptions that are anticipated. If we are dealing with a database we need to handle the SQL Exception. The code on try should be the code that anticipates any kind of exception. It is a common practice that is observed that all the code in a function is moved to a try. Such programming should not be encouraged.

Example 2:

try
{
    int i = 0;
    String str = String.Empty;
    Hashtable ObjHashTable = new Hashtable();
    // Here the actual code
}
catch (Exception)
{ 
    // Code Handle the Exception
}

Exceptions are caught in the Catch Block. Once an Exception is caught, there are number of ways for handling the same.

Catching an Exception (eating away the exception)

This kind of handling mechanism is the least encouraged one. The problem with the approach given below is that if an exception is caught, it is not logged. This can have serious problems to the application while diagnosing in case of any problem.

Example 3:

try
{
    // Code where we are anticipating the Exception
}
catch (Exception)
{ 
    //Do Nothing
    // Just Eat Away the Exception
}

Re-Throwing an Exception

The handling mechanism involved here will catch an exception and will re-throw another exception. As we can see from Example 4, UnauthorizedAccessException is caught and another exception ApplicationAccessFailedException is thrown. ApplicationAccessFailedException is a custom exception. We will deal about the custom exceptions in later sections.

Example 4:

try
{// Code where we are anticipating the Exception
}catch (UnauthorizedAccessException ObjUnAuthorizedException)
{
    ApplicationAccessFailedException ObjApplicationAccessFailedException = new ApplicationAccessFailedException();
    ObjApplicationAccessFailedException.InnerException = ObjUnAuthorizedEnxception;
    throw ObjUnAuthorizedException;
}

Showing an Error Message to User

This kind of Exception Handling mechanism is the most commonly used mechanism. As we can see from Example 5, in the catch block a message is shown to the user..

Example 5

try
{
    // Code where we are anticipating the Exception
}
catch (Exception)
{
    System.Windows.Forms.MessageBox.Show("Error Message: This is the Message shown to the User");
}

The message shown to the user can be an error message, warning message or an Information message.

What are Custom Exceptions? Why do we need them?

Custom Exceptions are exceptions that are created for applying the Business rules of the Application. These exceptions are used to implement the business rules violations of the application.

Definition of Business Rules:

Business rules or business rule sets describe the operations, definitions and constraints that apply to an organization in achieving its goals. These rules are then used to help the organization to better achieve goals, communicate among principals and agents, communicate between the organization and interested third parties, demonstrate fulfillment of legal obligations, operate more efficiently, automate operations, perform analysis on current practices, etc.

For example: The minimum balance set for Savings A/C in a bank would be different from a Current A/C. Hence, while developing the application for the same the Business rule would be kept as the same.

Violation of Business Rule: Whenever a Violation of Business rule is done, it should be notified and the proper message should be shown to the user. This will be handled by a Custom Exception. Let us illustrate the same with an example.

Example 6

private void ValidateBeforeDebit(float Amount)
{
    {
        if((Balance â€“ Amount ) < MimimumBalanceNeedToHave)
        {
            throw new MimimumBalanceViolationForSavingsAccount("Current TransactionFailed: Mimimum Balance not available");
        }
         Else
        {
            Debit(Amount);
        }
    }
}

We have a method ValidateBeforeDebit (). This method ensures the minimum balance. If there is no minimum balance then a Custom Exception of the type MimimumBalanceViolationForSavingsAccount is thrown. We need to handle the method whenever we call the method.