how to generate qr code in asp.net using c# Figure 4-8. Architecture of an AppDomain in VB.NET

Generate QR Code JIS X 0510 in VB.NET Figure 4-8. Architecture of an AppDomain

Figure 4-8. Architecture of an AppDomain
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CHAPTER 4 APPLICATION ARCHITECTURE
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For the purposes of illustration, both AppDomains A and B reside in the same Win32 process. In an implementation, the AppDomains could reside in different processes or on different computers. The assemblies that represent the dispatcher and client server are hosted in AppDomain A. The server assemblies are hosted in AppDomain B. To reload the server assembly, AppDomain B is unloaded and the assemblies are reloaded. As a safety measure, each instance of the server s objects could be hosted in their own AppDomain. It s safer coding because if one server instance messes up its state, the other server instances aren t affected. Now that you have an understanding of static and dynamic dispatchers, it s time to examine the Client-Dispatcher-Server pattern architecture.
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Architecting the Client-Dispatcher-Server Pattern
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The Client-Dispatcher-Server pattern is implemented using three architectural blocks. The first block is the base architecture, which is illustrated in Figure 4-9.
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Figure 4-9. UML architecture of the Client-Dispatcher-Server pattern The Client type uses the Dispatcher type, which has two methods: CanCreate and CreateInstance. The Dispatcher is a controller class that references a number of IResolver interface implementations. The purpose of the IResolver interface is to provide a mechanism whereby a type identifier can be converted into an object instance. The Dispatcher manages a collection of IResolver implementations, allowing servers to be registered and defined using multiple techniques. Both IResolver and Dispatcher use the type Identifier, which is a black box that makes type definition as flexible as possible.
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Architecting the Resolver Type
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In implementation terms, the IResolver type is defined as follows: public interface IResolver<Identifier> { void Load(); void Unload(); bool CanCreate( Identifier identifier ); ObjectType CreateInstance<ObjectType>( Identifier identifier ); }
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CHAPTER 4 APPLICATION ARCHITECTURE
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The IResolver interface is a Generics-based interface that has two Generic parameters: Identifier and ObjectType. The Generic parameter Identifier refers to the type used to identify an assembly and type. The Identifier could be the .NET framework Type or a custom identifier. The Generic type ObjectType associated with the method CreateInstance, known as a methodbased Generic type, represents the type that is being cast to. Usually, this will be an abstract base class or an interface. Using a Generic method saves a typecast, as an implementation of the CreateInstance method will perform the typecast. The methods Load and Unload load and unload an IResolver implementation. The idea behind different IResolver implementations is to let the implementation determine how to instantiate assemblies, which will include the dynamic loading and unloading of assemblies, but these processes are instigated using the methods Load and Unload. This makes it possible for an IResolver implementation to reference multiple AppDomains, remoting-based objects, or even web services objects.
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Architecting the Dispatcher Type
The Dispatcher type is defined as follows: public abstract class Dispatcher<Identifier> { protected LinkedList<IResolver<Identifier>> _resolvers = new LinkedList<IResolver<Identifier>>(); public abstract void Initialize(); public abstract void Destroy(); public void Load() { Initialize(); foreach( IResolver resolver in _resolvers ) { resolver.Load(); } } public void Unload() { foreach( IResolver resolver in _resolvers ) { resolver.Unload(); } Destroy(); } public ObjectType CreateInstance<ObjectType>( Identifier identifier ) { foreach( IResolver resolver in _resolvers ) { if( resolver.CanCreate( identifier ) ) { return resolver.CreateInstance<ObjectType>( identifier ); } } return default( ObjectType ); } }
CHAPTER 4 APPLICATION ARCHITECTURE
The implementation of the Dispatcher type is simple and follows the functionality defined previously in this chapter. The methods Load and Unload load and unload the IResolver implementations that most likely will load and unload AppDomains. The CreateInstance method instantiates a type. What is different is that no Add and Remove methods add or remove IResolver implementations. The different IResolver implementations are managed by the linked list variable _resolvers, but there is no way to add or remove IResolver implementations. No Add or Remove methods are included because they aren t needed. Dispatcher is an abstract base class, and to instantiate the class, another class must subclass Dispatcher. The responsibility of the subclassed class is to add and remove IResolver implementations. A very simple example implementation could be as follows: public class DispatcherImpl : Dispatcher<Identifier> { public DispatcherImpl( string applicationName ) { _resolvers.AddLast( new ResolverStaticAssemblies( applicationName ) ); } public override void Initialize() { } public override void Destroy() { } } At first glance, it would appear the type DispatcherImpl is breaking multiple pattern rules by not using a factory to instantiate the type ResolverStaticAssemblies. Additionally, the ResolverStaticAssemblies instance is added to the _resolvers variables using hard-coded method calls. The reason the rules are violated is because the DispatcherImpl is deliberately a tightly knit implementation. In 3, I mentioned that sometimes overusing Generics makes code more complicated than less so. And often you are left with no choice but to write very specific code that uses specific instances. The type DispatcherImpl is one of those cases. It isn t possible to define a dispatcher without having to fix the scope of the implementation. Think of it this way: the dispatcher executes in the context of a single AppDomain. This means whenever an IResolver implementation is loaded into the current AppDomain, it isn t possible to unload the IResolver implementation. When using a Dispatcher implementation, which is defined by Generic parameters, the subclassed type must specify the Generic parameters. This means the subclassed type has to have domain-specific knowledge about the IResolver implementations. Remember, to instantiate a resolver that uses directories, the directories have to be specified when the IResolver implementation is being instantiated. Even if server callbacks are used, some code somewhere has to start the ball rolling and establish the basic rules. Therefore, a Dispatcher-derived type has to know things, and it makes no sense trying to be general. Of course, an argument can be made that the DispatcherImpl and IResolver implementations all execute in their own AppDomains so that a dynamic reload is possible. That is a solution, but it s also a complication, since the IResolver implementation will not be loaded very often. After all, the Client-Dispatcher-Server pattern is creating an infrastructure for dynamic reloading of objects. By adding the complication of more AppDomains and more dynamic capabilities, the Client-Dispatcher-Server pattern is being recursively embedded in itself a solution that is asking for trouble. Having said all this and made the case for breaking several pattern rules, I ll add you should break rules as infrequently as possible and only when necessary. And when you do break the rules, break them all in one spot so that it s easy to diagnose and fix problems. Also,
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