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A list of actions that can be done on the pages Actual pages belonging to one person Actually changing the pages Frontend to the actual pages Performs some action before routing the Request onward A user visiting her pages
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There are several kinds of proxies, the most common of which are: Virtual proxies Hands the creation of an object over to another object (useful if the creation process might be slow or might prove unnecessary) Authentication proxies Checks that the access permissions for a request are correct Remote proxies Encodes requests and send them across a network Smart proxies Adds to or change requests before sending them on Within the scope of the social networking system mentioned earlier, these map as follows: Delaying the creation of a rich environment (virtual proxy) Logging in users (authentication proxy) Sending requests across the network (remote proxy) Performing actions on friends books (smart proxy
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A major aim of this book is to introduce interesting features of C# and show how they can make writing patterns easier, clearer, and more secure. As each new feature finds a place in a pattern, I will introduce it via a sidebar. This example illustrates the use of some of C# s access modifiers the focus of our first sidebar.
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2: Structural Patterns: Decorator, Proxy, and Bridge
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Table 2-1. Access modifiers Modifier
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private internal protected protected internal public
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Effect Accessible within the method or type in which it is declared Accessible within the assembly Accessible within a class and its derived classes Accessible within a class, its derived classes, or an assembly Accessible everywhere
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Default for Members of classes and structs; nested types Non-nested types Members of enums and interfaces
Now, let s look at a small theoretical example of the pattern that aims to show: How the division between the Client on the one hand and the proxies and the Subject on the other can be implemented with the strictest access modifiers possible How virtual and protection proxies perform their work and route requests onward The nub of the virtual proxy code is this:
public class Proxy : ISubject { Subject subject; public string Request( ) { // A virtual proxy creates the object only on its first method call if (subject == null) subject = new Subject( ); return subject.Request(); } }
The Client can create the Proxy at any time using a normal instantiation. Because a default constructor is used, the reference to a Subject is not made at this time. It is only when the Request method is called that the Subject reference is checked and, if null, assigned to an instantiated object. The full code for the theory example is shown in Example 2-3. The example emphasizes that all instances in the client are declared of the interface type, ISubject. This means that access to additional operations in proxies, such as Authenticate, will require the ISubject object to be suitably type-cast, as happens on lines 63 and 64. The example includes the second proxy in the ProtectionProxy class (lines 31 50). This proxy is independent of the other one. If a ProtectionProxy object is declared, as on line 61, all first requests will be met by an instruction to authenticate. In this example, Authenticate is a separate method that must be called, as in lines 63 and 64. In the next example, registrations and authentication will start up automatically when Request is called.
Proxy Pattern
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C# Feature Access Modi ers
At the physical level, a C# program consists of one or more compilation units, each contained in a separate source file. When a C# program is compiled, all of the compilation units are processed together. Thus, compilation units can depend on each other, possibly in a circular fashion. An assembly is a physical container for types and the resources associated with them. At the logical level, C# programs are organized using namespaces. Namespaces are used as a way of presenting program elements that are exposed to other programs in the same or different assemblies. Access modifiers are a way of specifying which members of a namespace will be exposed and to what degree. In C#, an access modifier alters the default accessibility of a type or its members. The available modifiers and the defaults that are applied are shown in Table 2-1. In this table, types include classes, structs, interfaces, enums, and delegates. The interpretation of this table for classes is as follows: A non-nested class is by default visible to other classes in the same assembly. A nested class is by default private and can have no more accessibility than its enclosing class. A member of a class (e.g., field, method, property) is by default private. An inherited class can have special access to a member in its superclass that has been tagged with the protected modifier. Unless you are going to do cross-assembly development, the public modifier is not strictly necessary; however, it has a currency in programming that the internal modifier does not. It is therefore acceptable to use public when tagging members for access outside of classes. The real need for the more restrictive internal modifier is illustrated in the Example section. There are some restrictions as to where and how access modifiers can be placed. Interface members are always public, and no modifiers are allowed. Namespaces cannot have private or protected members. Modifiers can only be added to types and members to make them as accessible as their enclosing methods or classes. cf. C# Language Specification Version 3.0, September 2007, Section 10.3.5
1 2 3 4 5 6 7 8 9 10 using System; // Proxy Pattern Judith Bishop // Shows virtual and protection proxies class SubjectAccessor { public interface ISubject { string Request ( ); } Dec 2006
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