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Vaskaran Sarcar - Design Patterns in C#: A Hands-on Guide with Real-World Examples

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Book:
Design Patterns in C#: A Hands-on Guide with Real-World Examples
Author:
Vaskaran Sarcar
Publisher:
Apress
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Year:
2018
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Get hands-on experience with each Gang of Four design pattern using C#. For each of the patterns, youll see at least one real-world scenario, a coding example, and a complete implementation including output. In the first part of Design Patterns in C#, you will cover the 23 Gang of Four (GoF) design patterns, before moving onto some alternative design patterns, including the Simple Factory Pattern, the Null Object Pattern, and the MVC Pattern. The final part winds up with a conclusion and criticisms of design patterns with chapters on anti-patterns and memory leaks. By working through easy-to-follow examples, you will understand the concepts in depth and have a collection of programs to port over to your own projects. Along the way, the author discusses the different creational, structural, and behavioral patterns and why such classifications are useful. In each of these chapters, there is a Q&A session that clears up any doubts and covers the pros and cons of each of these patterns.He finishes the book with FAQs that will help you consolidate your knowledge. This book presents the topic of design patterns in C# in such a way that anyone can grasp the idea. What You Will Learn Work with each of the design patterns Implement the design patterns in real-world applications Select an alternative to these patterns by comparing their pros and cons Use Visual Studio Community Edition 2017 to write code and generate output Who This Book Is For Software developers, software testers, and software architects.

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Part I
Gang of Four Design Patterns

Vaskaran Sarcar 2018

Vaskaran Sarcar Design Patterns in C#

1. Singleton Pattern

Vaskaran Sarcar 1

(1)

Whitefield, Bangalore, Karnataka, India

This chapter covers the Singleton pattern.

GoF Definition

Ensure a class has only one instance, and provide a global point of access to it.

Concept

A particular class should have only one instance. You can use this instance whenever you need it and therefore avoid creating unnecessary objects.

Real-Life Example

Suppose you are a member of a sports team and your team is participating in a tournament. When your team plays against another team, as per the rules of the game, the captains of the two sides must have a coin toss. If your team does not have a captain, you need to elect someone to be the captain first. Your team must have one and only one captain.

Computer World Example

In some software systems , you may decide to maintain only one file system so that you can use it for the centralized management of resources.

Illustration

These are the key characteristics in the following implementation :

The constructor is private in this example. So, you cannot instantiate in a normal fashion (using new ).
Before you attempt to create an instance of a class, you check whether you already have an available copy. If you do not have any such copy, you create it; otherwise, you simply reuse the existing copy.

Class Diagram

Figure shows the class diagram for the illustration of the Singleton pattern.

Figure 1-1

Class diagram

Solution Explorer View

Figure shows the high-level structure of the parts of the program.

Figure 1-2

Solution Explorer View

Discussion

This simple example illustrates the concept of the Singleton pattern. This approach is known as static initialization .

Initially the C++ specification had some ambiguity about the initialization order of static variables (remember that the origin of C# is closely tied with C and C++), but the .NET Framework resolved these issues .

The following are the notable characteristics of this approach:

The Common Language Runtime (CLR) takes care of the variable initialization process.
You create an instance when any member of the class is referenced.
The public static member ensures a global point of access. It confirms that the instantiation process will not start until you invoke the Instance property of the class (in other words, it supports lazy instantiation). The sealed keyword prevents the further derivation of the class (so that its subclass cannot misuse it), and readonly ensures that the assignment process takes place during the static initialization.
The constructor is private. So, you cannot instantiate the Singleton class inside Main() . This will help you refer to the one instance that can exist in the system.

Implementation

Here is the implementation of the example:

using System;

namespace SingletonPatternEx

{

public sealed class Singleton

{

private static readonly Singleton instance=new Singleton();

private int numberOfInstances = 0;

//Private constructor is used to prevent

//creation of instances with 'new' keyword outside this class

private Singleton()

{

Console.WriteLine("Instantiating inside the private constructor.");

numberOfInstances++;

Console.WriteLine("Number of instances ={0}", numberOfInstances);

}

public static Singleton Instance

{

get

{

Console.WriteLine("We already have an instance now.Use it.");

return instance;

}

class Program

{

static void Main(string[] args)

{

Console.WriteLine("***Singleton Pattern Demo***\n");

//Console.WriteLine(Singleton.MyInt);

// Private Constructor.So,we cannot use 'new' keyword.

Console.WriteLine("Trying to create instance s1.");

Singleton s1 = Singleton.Instance;

Console.WriteLine("Trying to create instance s2.");

Singleton s2 = Singleton.Instance;

if (s1 == s2)

{

Console.WriteLine("Only one instance exists.");

}

else

{

Console.WriteLine("Different instances exist.");

}

Console.Read();

}

Output

Here is the output of the example:

***Singleton Pattern Demo***

Trying to create instance s1.

Instantiating inside the private constructor.

Number of instances =1

We already have an instance now.Use it.

Trying to create instance s2.

We already have an instance now.Use it.

Only one instance exists .

Challenges

Consider the following code. Suppose you have added one more line of code (shown in bold) in the Singleton class .

public sealed class Singleton

{

private static readonly Singleton instance = new Singleton();

private int numberOfInstances = 0;

//Private constructor is used to prevent

//creation of instances with 'new' keyword outside this class

private Singleton()

{

Console.WriteLine("Instantiating inside the private constructor.");

numberOfInstances++;

Console.WriteLine("Number of instances ={0}", numberOfInstances);

}

public static Singleton Instance

{

get

{

Console.WriteLine("We already have an instance now.Use it.");

return instance;

}

public static int MyInt = 25;

}

And suppose, your Main() method looks like this:

class Program

{

static void Main(string[] args)

{

Console.WriteLine("***Singleton Pattern Demo***\n");

Console.WriteLine(Singleton.MyInt);

Console.Read();

}

Now, if you execute the program , you will see following output:

Number of instances =1

***Singleton Pattern Demo***

This illustrates the downside of this approach. Specifically, inside the Main() method, you tried to use the static variable MyInt , but your application still created an instance of the Singleton class. In other words, with this approach, you have less control over the instantiation process, which starts whenever you refer to any static member of the class.

However, in most cases, you do not care about this drawback. You can tolerate it because you know that it is a one-time activity and the process will not be repeated, so this approach is widely used in .NET applications.

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