Exception Handling

Exception Handling:

Exception handling is process of providing an alternative path of execution when the application is unable to execute as desired.

You need to handle an exception in your program by using an exception-handler. This handler processes the exception. In exception handling, the application is divided into blocks of code. A block that shows the probability of raising an error contains one or more exception handler. These handlers follow a control structure and a uniform way of handling the system level and application level errors.

C# provides the structured solution to error-handling problems in the form of try and catch blocks. Using these blocks the core program statements can be separated from the error-handling statements. The core program statements handle normal program flow. You can also use the finally block to handle errors. These blocks for exception-handling can be implemented using the following keywords:

Ø try

Ø catch

Ø finally

The try Block:

The try block guards statements that may throw an exception.

Try

{

//statements that may cause an exception

}

The try block governs statements that are enclosed within it and defines the scope of the exception-handlers associated with it. In other words, if an exception occurs within the try block, an appropriate exception-handler that is associated with the try block handles the exception. A try block must have at least one catch block.

The catch Block:

You can associate an exception-handler within the try block by providing one or more catch handlers, immediately after the try block.

Try

{

//statements that may cause an exception

}

Catch (…)

{

//error handling code

}

The catch statement of the catch block takes an object of the exception class as parameter, which refers to the raised exception. When the exception is caught, the statements within the catch block are executed.

The finally Block:

The finally block is used to execute a given se of statements, whether an exception is thrown or not thrown. For example, a file has to be closed whether an exception is raised or not. You can place code to close the file in both the try and catch block,

Try

{

//statements that may cause an exception

}

Catch (…)

{

//error handling code

}

Finally

{

//statements to be executed

}

The catch block is used to handle exceptions that occur in a try block. The finally block is used to guarantee the execution of statements, regardless of whether an exception has occurred or not occurred. You can have only one finally block for each try block. It however not mandatory to have finally block after a try block.

User-defined exception handling:

To distinguish between error conditions programmatically, you can create user-defined exceptions. For example, while creating File Transfer Protocol (FTP) component to perform FTP tasks, you may want to represent the FTP-related errors as specific exception types. The application that uses the FTP component can differentiate the FTP-related exception from the other exceptions and perform possible remedial tasks accordingly. In such cases, you can define custom exception classes to represent the possible type of exceptions that may arise.

For example,