10 iec t1_s1_oo_ps_session_14

Object-Oriented Programming Using C#
Objectives

In this session, you will learn to:
Handle exceptions
Implement the user-defined exceptions
Implement threads
Define the life cycle of a thread

Ver. 1.0 Session 14 Slide 1 of 30

Handling Exceptions

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 handlers.
The exception handlers follow a control structure and a
uniform way of handling the system level and application
level errors.
The blocks for exception-handling can be implemented
using the following keywords:
try
catch
finally

Let us look at each of these keywords in detail.


Handling Exceptions (Contd.)

The try block:
The try block guards statements that may throw an exception.
Following is the syntax of try block statement:
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.
A try block must have at least one catch block.



The catch block:
The catch statement of the catch block takes an object of the
exception class as a parameter, which refers to the raised
exception.
You can associate an exception-handler with the try block by
providing one or more catch handlers, immediately after the try
block:
try
{
}
catch (…)
{
//error handling code
}



The finally block:
The finally block is used to execute a given set of statements,
whether an exception is thrown or not thrown:
try
{
}
catch (…)
{
//error handling code
}
finally
{
//statements to be executed
}

Demo: Handling Exception for Arrays Beyond Limit

Problem Statement:
David is working on a project where he is calculating the sum
of values in an integer array. David needs to handle the
exceptions, which can occur while he is working with the
arrays. If any exceptional condition is reached when David is
executing the application, the application needs to display an
exception message.
Help David to handle the exceptions.


Demo: Handling Exception for Arrays Beyond Limit (Contd.)

Solution:
To develop a console-based application, David need to
perform the following tasks:
1. Create a console-based application.
2. Build and execute an application.


Implementing the User-Defined Exceptions

In C#, you can create your own exception class. Such kinds
of exceptions are known as user-defined exceptions.
The Exception class is the base class for all the
exceptions in C#.
The user-defined exception classes must follow the
hierarchy of either the exception class or of one of the
standard inherited classes.


Implementing the User-Defined Exceptions (Contd.)

User-defined exception classes are derived from the
ApplicationException class.
To implement user-defined exceptions, you need to:
– Raise your exception: You can use the throw statement to
raise your own exceptions.
– Throw an object: You can throw an object if the object is
either directly or indirectly derived from System.Exception.
You can use a throw statement in the catch block to throw the
present object, as shown in the following code:
catch(Exception caught)
{
. . .
throw caught
}


Implementing Threads

A thread is defined as the execution path of a program.
You can define a unique flow of a control in a program,
using a thread.
Threads are used to run applications that perform large and
complex computations.
A process that is executed using one thread is known as a
single-threaded process, where the process is a running
instance of a program.
Single-threaded application can perform only one task at a
time. You have to wait for one task to complete before
another task can start.


Implementing Threads (Contd.)

The following figure shows a single-threaded process.

• To execute more than one task at a time, you can create
multiple threads in a program.
• A process that creates two or more threads is called a
multithreaded process.


Implementing Threads (Contd.)

The following figure shows a multithreaded process.


The Thread Model in C#

In single-threaded systems, an approach called event loop
with polling is used.
Polling is the process in which a single event is executed at
a time.
In the event loop with polling approach, a single thread runs
in an infinite loop till its operation is completed.
In a single-threaded application if the thread is suspended
from execution because it is waiting for a system resource,
the entire program stops executing.
In multithreading, the time for which a thread waits for the
CPU time can be utilized to perform another task.


The Thread Model in C#(Contd.)

• In C#, you will use the Thread class to work with threads.
• The System.Threading.Thread class is used to
construct and access individual threads in a multithreaded
application.


The Main Thread

• The main thread is created automatically on the start up of a
C# program execution.
• The threads which are created exclusively using the
Thread class are called as child threads, where the main
thread is called a parent thread or a primary thread.
• You can access a thread using the CurrentThread
property of the Thread class.


Working with Threads

• In C#, you create a thread by creating an object of type
Thread, giving its constructor a ThreadStart reference,
and calling the new thread’s Start() method.
• The new thread starts executing asynchronously with an
invocation of the thread’s method.
• There are various methods available with the Thread class.
Using these methods, you can control the execution of
threads. Few of these methods are:
– Start(): Starts a thread.
– Sleep(): Makes the thread to pause for a period of time.
– Abort(): Terminates the thread.
– Suspend(): Suspends a thread. If the thread is already
suspended it has no effect.
– Resume(): Resumes the suspended thread.


Creating Threads

• You can create threads by extending the Thread class.
• The extended thread class calls the Start() method to
begin the child thread execution. Following is an example of
creating threads:
ThreadStart ChildRef = new
ThreadStart(ChildThreadCall);
Thread ChildThread = new Thread(ChildRef);
ChildThread.Start();


Managing Threads

• There are many tasks you might need to perform to manage
the activity or life of a thread.
• You can manage all these tasks by using the various thread
methods available with the Thread class.
• The static Thread.Sleep() method calls the static
CurrentThread method, which then pauses that thread
for the specified amount of time.


Destroying Threads

• If the thread is required to be destroyed, the
Thread.Abort() method will allow you to accomplish the
task.
• The runtime aborts the thread by throwing a
ThreadAbortException. This exception cannot be
caught.
• If the finally block is present in the method, the runtime
will send the control to it.


Thread Life Cycle

• The lifecycle of a thread starts when an object of the
System.Threading.Thread class is created. The life
cycle of the thread ends with task execution.
• There are various states in the life cycle of a thread. These
states are:
The Unstarted state
The Runnable state
The Not Runnable state
The Dead state

Let us understand the life cycle of the thread with the help
of the following figure.



Start() Started

Work Completed Stopped



Start() Started Suspend() Suspended

Resume()



Start() Started

Sleep() Wait/Join
Sleep

Interrupt()

Time Expires



Start() Started

Thread Responds to
Abort() Stop Request Stop Request

Stopped


The Unstarted State

• When an instance of the Thread class is created, the
thread enters the unstarted state.
• A new thread is an empty object of the Thread class, and
no system resources such as memory are allocated to it.


The Runnable State

• The thread remains in the unstarted state until the program
calls the Start() method of the Thread class, which
places the thread in the runnable state and immediately
returns control to the calling thread.
• This state is also called as the ready or started state.
• The newly started thread and any other threads in the
program execute concurrently.


The Not Runnable State

A thread is not in the runnable state if it is:
Sleeping
Waiting
Blocked


The Dead State

• A running thread enters the dead state when the statements
of the threads method are complete. This state is also called
the terminated state.
• A program can force a thread into the dead state by calling
the Abort() method of the Thread class on the
appropriate thread object.


Summary

In this session, you learned that:
Exception handling is implemented using the following
keywords:
• try
• catch
• finally
– Exception-handling provides a structured and uniform way of
handling system-level and application-level errors.
– Exception handling is the process of providing an alternative
path to be executed when an application is not able to execute
in the desired.
– In addition to handling pre-defined exceptions, users can
create their own exceptions by deriving an exception class
from the ApplicationException class.


Summary (Contd.)

– You can only throw an object if the types of objects either
directly or indirectly derives from System.Exception.
– You can use the throw statement to raise your own
exceptions.
– A thread is defined as the path of execution of a program. It is
a sequence of instructions that is executed to define a unique
flow of control.
– A program that creates two or more threads is called a
multithreaded program.
– The System.Threading class is used to construct and
access individual threads in a multithreaded application.
– The various states in the life cycle of a thread are:
Unstarted state
Runnable state
Not Runnable state
Dead state


10 iec t1_s1_oo_ps_session_14

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