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This behavior is justified in that Parallels can support several operating systems at once, each in its own virtual machine. In contrast, Word, for example, can have many documents open, but they are all attended to by the same instance of the Word application.
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The concept of a Dock does not exist in Windows Vista itself, but there are third-party applications that mimic it.*
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The Singleton pattern adds functionality by modifying an existing class. The modifications required are: Make the constructor private and add a private static constructor as well. Add a private static read-only object that is internally instantiated using the private constructor. Add a public static property that accesses the private object. It is the public property (3) that is now visible to the world. All constructor requests for the class go to the property. The property accesses the private static object and will instantiate it if it does not already exist. The UML diagram in Figure 5-5 sums all this up.
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Singleton static uniqueInstance : Singleton = new Singleton( ) Singleton( ) static readonly Singleton( ) +static Instance( ) : Singleton
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returns the uniqueInstance
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The visible players in the Singleton pattern are:
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The class containing the mechanism for a unique instance of itself
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Instance
A property for making and accessing an instance of the Singleton class
* See http://www.stardock.com/products/objectdock/.
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5: Creational Patterns: Prototype, Factory Method, and Singleton
QUIZ
Match the Singleton Pattern Players with the Dock Illustration
To test whether you understand the Singleton pattern, cover the lefthand column of the table below and see if you can identify its players among the items from the illustrative example (Figure 5-4), as shown in the righthand column. Then check your answers against the lefthand column.
Singleton Instance
An application icon in the Dock An application
Implementation
A good implementation of the Singleton pattern in C# relies on the precise interpretation of language rules regarding construction.* Given the plan outlined in the previous section, a correct and elegant implementation is presented in Example 5-4.
1 public sealed class Singleton { 2 // Private Constructor 3 Singleton( ) { } 4 5 // Private object instantiated with private constructor 6 static readonly Singleton instance = new Singleton( ); 7 8 // Public static property to get the object 9 public static Singleton UniqueInstance { 10 get { return instance;} 11 } 12 }
The constructor in line 3 is private by default. To create an object, the client calls the UniqueInstance property defined in lines 9 11, as in:
Singleton s1 = Singleton.UniqueInstance;
As soon as a class is accessed by the first call on any of its methods, properties, or constructors (and in this case, only the property is callable), its fields are set up. Here, this involves initializing the field called uniqueInstance by invoking the constructor on line 3. For illustrative purposes, we can test that the following two instantiations will refer to the same object:
Singleton s1 = Singleton.Instance; Singleton s2 = Singleton.Instance;
* See Jon Skeets s article at http://www.csharphelp.com/archives4/archive670.html.
Singleton Pattern |
As with all static fields, the initialization is not repeated, and therefore the second call on the Instance property, for s2, returns the same object reference. The Singleton pattern would also like to offer lazy instantiation i.e., that the object is not initialized until its first invocation.
Be aware that in a multithreaded program, different threads could try to instantiate a class simultaneously. For this reason, a Singleton implementation that relies on an if statement to check whether the instance is null will not be thread-safe. Do not use code like that!
To do this, we put the instantiation of the Singleton object in a nested class, SingletonCreator, as in Example 5-5 in lines 6 11. The static constructor then falls away.
1 public class Singleton { 2 // Private constructor 3 Singleton ( ) { } 4 5 // Nested class for lazy instantiation 6 class SingletonCreator { 7 static SingletonCreator ( ) {} 8 // Private object instantiated with private constructor 9 internal static readonly 10 Singleton uniqueInstance = new Singleton( ); 11 } 12 13 // Public static property to get the object 14 public static Singleton UniqueInstance { 15 get {return SingletonCreator.uniqueInstance;} 16 } 17 }
Instantiation of the object will be triggered by the first reference to the static member of the nested class, which occurs in the UniqueInstance property (lines 14 16). This implementation is fully lazy but has all the performance benefits of the previous one. Note that although nested classes have access to the enclosing class s private members, the reverse is not true (hence the need for Instance to be internal here). That doesn t raise any other problems, though, as the class itself is private by default. An important part of the Singleton pattern is the initializing of resources in the
Singleton constructor. In fact, if no resources are being initialized in the constructor,
it usually means that a normal static class should be used rather than a Singleton. An example of such initialization is shown in the next section.
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