Chapter 1. Threading Basics

In this chapter, we will cover the basic tasks to work with threads in C#. You will learn the following recipes:

Creating a thread in C#
Pausing a thread
Making a thread wait
Aborting a thread
Determining a thread state
Thread priority
Foreground and background threads
Passing parameters to a thread
Locking with a C# lock keyword
Locking with a Monitor construct
Handling exceptions

Introduction

At some point of time in the past, the common computer had only one computing unit and could not execute several computing tasks simultaneously. However, operating systems could already work with multiple programs simultaneously, implementing the concept of multitasking. To prevent the possibility of one program taking control of the CPU forever, causing other applications and the operating system itself to hang, the operating systems had to split a physical computing unit across a few virtualized processors in some way and give a certain amount of computing power to each executing program. Moreover, an operating system must always have priority access to the CPU and should be able to prioritize CPU access to different programs. A thread is an implementation of this concept. It could be considered as a virtual processor that is given to the one specific program and runs it independently.

Note

Remember that a thread consumes a significant amount of operating system resources. Trying to share one physical processor across many threads will lead to a situation where an operating system is busy just managing threads instead of running programs.

Therefore, while it was possible to enhance computer processors, making them execute more and more commands per second, working with threads was usually an operating system task. There was no sense in trying to compute some tasks in parallel on a single-core CPU because it would take more time than running those computations sequentially. However, when processors started to have more computing cores, older programs could not take advantage of this because they just used one processor core.

To use a modern processor's computing power effectively, it is very important to be able to compose a program in a way that it can use more than one computing core, which leads to organizing it as several threads that communicate and synchronize with each other.

The recipes in this chapter focus on performing some very basic operations with threads in the C# language. We will cover a thread's life cycle, which includes creating, suspending, making a thread wait, and aborting a thread, and then, we will go through the basic synchronization techniques.

Creating a thread in C#

Throughout the following recipes, we will use Visual Studio 2015 as the main tool to write multithreaded programs in C#. This recipe will show you how to create a new C# program and use threads in it.

Note

A free Visual Studio Community 2015 IDE can be downloaded from the Microsoft website and used to run the code samples.

Getting ready

To work through this recipe, you will need Visual Studio 2015. There are no other prerequisites. The source code for this recipe can be found in the BookSamples\Chapter1\Recipe1 directory.

Tip

Downloading the example code

You can download the example code files for this book from your account at http://www.packtpub.com. If you purchased this book elsewhere, you can visit http://www.packtpub.com/support and register to have the files e-mailed directly to you.

You can download the code files by following these steps:

Log in or register to our website using your e-mail address and password.
Hover the mouse pointer on the SUPPORT tab at the top.
Click on Code Downloads & Errata.
Enter the name of the book in the Search box.
Select the book for which you're looking to download the code files.
Choose from the drop-down menu where you purchased this book from.
Click on Code Download.

Once the file is downloaded, please make sure that you unzip or extract the folder using the latest version of:

WinRAR/7-Zip for Windows
Zipeg/iZip / UnRarX for Mac
7-Zip/PeaZip for Linux

How to do it...

To understand how to create a new C# program and use threads in it, perform the following steps:

Start Visual Studio 2015. Create a new C# console application project.
Make sure that the project uses .NET Framework 4.6 or higher; however, the code in this chapter will work with previous versions.

In the Program.cs file, add the following using directives:

using System;
using System.Threading;
using static System.Console;

Add the following code snippet below the Main method:

static void PrintNumbers()
{
  WriteLine("Starting...");
  for (int i = 1; i < 10; i++)
  {
    WriteLine(i);
  }
}

Add the following code snippet inside the Main method:

Thread t = new Thread(PrintNumbers);
t.Start();
PrintNumbers();

Run the program. The output will be something like the following screenshot:

How it works...

In step 1 and 2, we created a simple console application in C# using .Net Framework version 4.0. Then, in step 3, we included the System.Threading namespace, which contains all the types needed for the program. Then, we used the using static feature from C# 6.0, which allows us to use the System.Console type's static methods without specifying the type name.

Note

An instance of a program that is being executed can be referred to as a process. A process consists of one or more threads. This means that when we run a program, we always have one main thread that executes the program code.

In step 4, we defined the PrintNumbers method, which will be used in both the main and newly created threads. Then, in step 5, we created a thread that runs PrintNumbers. When we construct a thread, an instance of the ThreadStart or ParameterizedThreadStart delegate is passed to the constructor. The C# compiler creates this object behind the scenes when we just type the name of the method we want to run in a different thread. Then, we start a thread and run PrintNumbers in the usual manner on the main thread.

As a result, there will be two ranges of numbers from 1 to 10 randomly crossing each other. This illustrates that the PrintNumbers method runs simultaneously on the main thread and on the other thread.

Pausing a thread

This recipe will show you how to make a thread wait for some time without wasting operating system resources.

Getting ready

To work through this recipe, you will need Visual Studio 2015. There are no other prerequisites. The source code for this recipe can be found at BookSamples\Chapter1\Recipe2.

How to do it...

To understand how to make a thread wait without wasting operating system resources, perform the following steps:

Start Visual Studio 2015. Create a new C# console application project.

In the Program.cs file, add the following using directives:

using System;
using System.Threading;
using static System.Console;
using static System.Threading.Thread;

Add the following code snippet below the Main method:

static void PrintNumbers()
{
  WriteLine("Starting...");
  for (int i = 1; i < 10; i++)
  {
    WriteLine(i);
  }
}
static void PrintNumbersWithDelay()
{
  WriteLine("Starting...");
  for (int i = 1; i < 10; i++)
  {
    Sleep(TimeSpan.FromSeconds(2));
    WriteLine(i);
  }
}

Add the following code snippet inside the Main method:

Thread t = new Thread(PrintNumbersWithDelay);
t.Start();
PrintNumbers();

Run the program.

How it works...

When the program is run, it creates a thread that will execute a code in the PrintNumbersWithDelay method. Immediately after that, it runs the PrintNumbers method. The key feature here is adding the Thread.Sleep method call to a PrintNumbersWithDelay method. It causes the thread executing this code to wait a specified amount of time (2 seconds in our case) before printing each number. While a thread sleeps, it uses as little CPU time as possible. As a result, we will see that the code in the PrintNumbers method, which usually runs later, will be executed before the code in the PrintNumbersWithDelay method in a separate thread.

Making a thread wait

This recipe will show you how a program can wait for some computation in another thread to complete to use its result later in the code. It is not enough to use the Thread.Sleep method because we don't know the exact time the computation will take.

Getting ready

To work through this recipe, you will need Visual Studio 2015. There are no other prerequisites. The source code for this recipe can be found at BookSamples\Chapter1\Recipe3.

How to do it...

To understand how a program waits for some computation in another thread to complete in order to use its result later, perform the following steps:

Start Visual Studio 2015. Create a new C# console application project.

In the Program.cs file, add the following using directives:

using System;
using System.Threading;
using static System.Console;
using static System.Threading.Thread;

Add the following code snippet below the Main method:

static void PrintNumbersWithDelay()
{
  WriteLine("Starting...");
  for (int i = 1; i < 10; i++)
  {
    Sleep(TimeSpan.FromSeconds(2));
    WriteLine(i);
  }
}

Add the following code snippet inside the Main method:

WriteLine("Starting...");
Thread t = new Thread(PrintNumbersWithDelay);
t.Start();
t.Join();
WriteLine("Thread completed");

Run the program.

How it works...

When the program is run, it runs a long-running thread that prints out numbers and waits two seconds before printing each number. But, in the main program, we called the t.Join method, which allows us to wait for the thread t to complete working. When it is complete, the main program continues to run. With the help of this technique, it is possible to synchronize execution steps between two threads. The first one waits until another one is complete and then continues to work. While the first thread waits, it is in a blocked state (as it is in the previous recipe when you call Thread.Sleep).

Aborting a thread

In this recipe, we will describe how to abort another thread's execution.

Getting ready

To work through this recipe, you will need Visual Studio 2015. There are no other prerequisites. The source code for this recipe can be found at BookSamples\Chapter1\Recipe4.

How to do it...

To understand how to abort another thread's execution, perform the following steps:

Start Visual Studio 2015. Create a new C# console application project.

In the Program.cs file, add the following using directives:

using System;
using System.Threading;
using static System.Console;

Using the static System.Threading.Thread, add the following code snippet below the Main method:

static void PrintNumbersWithDelay()
{
  WriteLine("Starting...");
  for (int i = 1; i < 10; i++)
  {
    Sleep(TimeSpan.FromSeconds(2));
    WriteLine(i);
  }
}

Add the following code snippet inside the Main method:

WriteLine("Starting program...");
Thread t = new Thread(PrintNumbersWithDelay);
t.Start();
Thread.Sleep(TimeSpan.FromSeconds(6));
t.Abort();
WriteLine("A thread has been aborted");
Thread t = new Thread(PrintNumbers);
t.Start();
PrintNumbers();

Run the program.

How it works...

When the main program and a separate number-printing thread run, we wait for six seconds and then call a t.Abort method on a thread. This injects a ThreadAbortException method into a thread, causing it to terminate. It is very dangerous, generally because this exception can happen at any point and may totally destroy the application. In addition, it is not always possible to terminate a thread with this technique. The target thread may refuse to abort by handling this exception by calling the Thread.ResetAbort method. Thus, it is not recommended that you use the Abort method to close a thread. There are different methods that are preferred, such as providing a CancellationToken object to cancel a thread execution. This approach will be described in Chapter 3, Using a Thread Pool.