Threads And Synchronization in C#

Software Development Technologies
Threads and Synchronization in C#
Muhammad Rizwan
rairizwanali@gmail.com

Contents
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What is a Thread?
Threads in C#
Types of Threads
Threads Priority in C#
Controlling Threads in C#

What is Thread ?
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In computer science, a Thread is the smallest unit of
processing that can be scheduled by an operating system.
It generally results from a fork of a computer program.
The implementation of threads and processes differs from one
operating system to another, but in most cases, a thread is
contained inside a process.
Multiple threads can exist within the same process and share
resources such as memory, while different processes do not
share these resources.
Consider the Example of a Word Processing Application i.e.,
Ms Word. Ms Word is a process and spell-checker within it is
a Thread. And the memory they share is Word document.

Threads in C#
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A thread is an independent stream of instructions
in a program.
All your C# programs up to this point have one
entry point — the Main() method.
Execution starts with the first statement in the
Main() method and continues until that method
returns.
This program structure is all very well for programs
in which there is one identifiable sequence of
tasks With the Thread class, you can create and
control threads.

Threads in C#
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The code here is a very simple example of creating
and starting a new thread.
The constructor of the Thread class is overloaded
to accept a delegate parameter of type ThreadStart
OR, a simple method without any return value and
input parameters.
The ThreadStart delegate defines a method with a
void return type and without arguments.
After the Thread object is created, you can start the
thread with the Start() method.

Threads in C#
using System;
using System.Threading;
namespace Wrox.ProCSharp.Threading
{
class Program
{
static void Main()
{
Thread t1 = new Thread(ThreadMain);
t1.Start();
Console.WriteLine("This is the main thread.");
}
static void ThreadMain()
{
Console.WriteLine("Running in a thread.");
}
}
}

Threads in C#
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When you run the application, you get the output of
the two threads:
This is the main thread.
Running in a thread.

Types of Threads
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There are two types of Threads
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Foreground Threads
Background Threads

The process of the application keeps running as
long as at least one foreground thread is running.
Even if Main() method ends, the process of the
application remains active until all foreground
threads finish their work.
A thread you create with the Thread class, by
default, is a foreground thread.

Threads in C#
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When you create a thread with the Thread class, you can
define whether it should be a foreground or background
thread by setting the property IsBackground.
Background threads are very useful for background tasks.
For example, when you close the Word application, it doesn’t
make sense for the spell checker to keep its process running.
The spell-checker thread can be killed when the application is
closed (Background Thread).
However, the thread organizing the Outlook message store
should remain active until it is finished, even if Outlook is
closed (Foreground Thread).

Threads Priority in C#
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The operating system schedules threads based on
a priority, and the thread with the highest priority is
scheduled to run in the CPU.
With the Thread class, you can influence the priority
of the thread by setting the Priority property.
The Priority property requires a value that is defined
by the ThreadPriority enumeration.
The levels defined are Highest , AboveNormal ,
Normal , BelowNormal , and Lowest .

Controlling Threads
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The thread is invoked by the Start() method of a Thread
object.
However, after invoking the Start() method, the new thread is
still not in the Running state, but in the Unstarted state.
The thread changes to the Running state as soon as the
operating system thread scheduler selects the thread to run.
You can read the current state of a thread by reading the
property Thread.ThreadState.
With the Thread.Sleep() method, a thread goes into the
WaitSleepJoin state.
And waits until it is woken up again after the time span defined
with the Sleep() method has elapsed.

Controlling Threads
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To stop another thread, you can invoke the method
Thread.Abort().
When this method is called, an exception of type
ThreadAbortException is thrown in the thread that
receives the abort.
With a handler to catch this exception, the thread
can do some clean-up before it ends.

Thread Synchronization
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A race condition can occur if two or more threads
access the shared data in the absence of
synchronization.
It is best to avoid synchronization issues by not
sharing data between threads. Of course, this is not
always possible.
If data sharing is necessary, you must use
synchronization techniques so that only one thread
at a time accesses and changes shared state.

public class myClass
{
public static int count;
public void A()
{
for (int i = 0; i <100; i++)
{
count++;
}
}
}

public class myProgram
{
Thread T1 = new Thread(A);
Thread T2 = new Thread(A);
T1.Start();
T2.Start();
Console.WriteLine(“ The count variable = {0}”,myClass.count);
}

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These two threads run as two independent threads.
If you run this program then you will discover that
the final value of count isn’t predictable because it
all depends on when the two threads get access to
count.
Every time you run this program the last value of
count variable would be different.

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C# has its own keyword for the synchronization of
multiple threads: the lock statement.
The lock statement is an easy way to hold for a lock
and release it.
public void A()
{
lock (this)
{
for (int i = 0; i <100; i++)
{
count++;
}
}
}

The End
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Thanks for listening
Questions would be appreciated.

Threads And Synchronization in C#

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