c# barcode code 39 5: Creational Patterns: Prototype, Factory Method, and Singleton in C#.NET

Printer Code-39 in C#.NET 5: Creational Patterns: Prototype, Factory Method, and Singleton

5: Creational Patterns: Prototype, Factory Method, and Singleton
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Example: Singleton Fa ade
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There are several patterns that make the implicit assumption that only one instance of a class will be created. The Fa ade pattern is one of these there should be only one instance of each Fa ade. The Singleton pattern can enforce this requirement. To integrate the Singleton into an existing pattern, we again follow the steps outlined earlier. The revised constructor for the Fa ade is as follows:
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// start Singleton pattern // private constructors Fa ade( ) { a = new SubsystemA( ); b = new SubsystemB( ); c = new SubsystemC( ); } static Fa ade( ) {} // private object static readonly Fa ade uniqueInstance = new Fa ade( ); // public static property public static Fa ade Instance { get {return uniqueInstance;} } // end Singleton pattern
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Then, in the client, instead of:
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Fa ade fa ade = new Fa ade( );
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(which would not compile because the constructor is private now), we use:
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Fa ade fa ade = Fa ade.Instance;
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If we need to refer to the Fa ade again from another place in the system, we can declare a new variable and create a new Fa ade object, but it will still access the same instance.
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Many other patterns for example, the Builder, Prototype, and Abstract Factory patterns can make use of the Singleton pattern to ensure that only one copy of a class is created. However, the effectiveness of the Singleton pattern relies entirely on developers all using the same rules. Even though implementing a singleton as outlined in this chapter doesn t require much coding effort, it would be nice to be able to reuse the implementation. But if the standard rules are not followed, one developer may use an Instance property and another a Create method, for example, or one may use the nested class while the other doesn t. Even if two developers use the same nested class, they might give the class different names. The result could be a complete mess, even for a simple singleton! A solution to this issue is proposed in the Pattern Comparison section, later in this chapter.
Use the Singleton pattern when You need to ensure there is only one instance of a class. Controlled access to that instance is essential. You might need more than one instance at a later stage. The control should be localized in the instantiated class, not in some other mechanism.
Exercises
1. The community in the SpaceBook application (Example 2-4) was created as static. Rework SpaceBook as a Singleton class. 2. Consider an international airline with a number of servers situated all over the world that are used to process reservations. At different times of the 24-hour clock, some will be heavily loaded and some lightly loaded. Each server can apply to a load balancer to be added to or removed from the system temporarily as time goes by. Model this system with the load balancer as a singleton.
Pattern Comparison
There is a symbiosis between patterns and languages, and it is dynamic. As this book has shown, the new features of C# 3.0 have made implementing patterns easier. (A quick comparison of the code in this book with standard C# pattern code will confirm this statement.) Two of the patterns discussed in this chapter the Prototype and Singleton patterns raise some interesting points about pattern language features. Implementing the Prototype pattern was quite a challenge 10 years ago, when obtaining a deep copy of an arbitrary data structure meant creating a graph traversal algorithm from scratch. Now, it can all be done with one method call to Serialize, plus some associated setup of streams. This facility is available to all languages in .NET, and Java has a similar mechanism. Thus, the implementation of the pattern as it was originally envisaged has almost disappeared. Nevertheless, its intent remains, and managing prototypes is still very much part of the developer s task. Considering the Singleton pattern, is there any way in which the language might help to make it reusable * One solution for achieving reusability is to use C# generics, as shown in Example 5-6.
1 using System; 2 3 // Singleton Pattern Judith Bishop Nov 2007 4 // Generic version 5 6 public class Singleton <T> where T : class, new( ){
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