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Creator PDF417 in Visual C#.NET Attribute Application Keywords

Table 22-1. Attribute Application Keywords
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The attribute is on the assembly. The attribute is on the module. The attribute is on a class or struct.
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CHAPTER 22 ATTRIBUTES
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method property event field param return
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The attribute is on a method. The attribute is on a property. The attribute is on an event. The attribute is on a field. The attribute is on a parameter. The attribute is on the return value.
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Attributes that are applied to assemblies or modules must occur after any using clauses and before any code: using System; [assembly:CLSCompliant(true)] class Test { Test() {} } This example applies the ClsCompliant attribute to the entire assembly. All assembly-level attributes declared in any file that s in the assembly are grouped together and attached to the assembly. To use a predefined attribute, start by finding the constructor that best matches the information to be conveyed. Next, write the attribute, passing parameters to the constructor. Finally, use the named parameter syntax to pass additional information that wasn t part of the constructor parameters. For more examples of attribute use, refer to 33.
An Attribute of Your Own
To define attribute classes and reflect on them at runtime, you have to consider a few more issues. The following sections will discuss some things to consider when designing an attribute. You have two major things to determine when writing an attribute. The first is the program elements that the attribute may be applied to, and the second is the information that will be stored by the attribute.
Attribute Usage
Placing the AttributeUsage attribute on an attribute class controls where the attribute can be used. The possible values for the attribute are listed in the AttributeTargets enumerator (see Table 22-2). As part of the AttributeUsage attribute, one of these can be specified or a list of them can be ORed together.
CHAPTER 22 ATTRIBUTES
Table 22-2. AttributeTargets Values
Return Value
Assembly Module Class Struct Enum Constructor Method Property Field Event Interface Parameter ReturnValue Delegate All ClassMembers
Meaning
The program assembly The current program file A class A struct An enumerator A constructor A method (member function) A property A field An event An interface A method parameter The method return value A delegate Anywhere Class, struct, enum, constructor, method, property, field, event, delegate, interface
You can also use the AttributeUsage attribute to specify whether an attribute is single-use or multiuse. You do this with the named parameter AllowMultiple. Such an attribute looks like this: [AttributeUsage(AttributeTargets.Method | AttributeTargets.Event, AllowMultiple = true)]
Attribute Parameters
The information the attribute will store should be divided into two groups: the information that s required for every use and the optional items. The information that s required for every use should be obtained via the constructor for the attribute class. This forces the user to specify all the parameters when they use the attribute. Optional items should be implemented as named parameters, which allows the user to specify whichever optional items are appropriate. If an attribute has several different ways in which it can be created, with different required information, separate constructors can be declared for each usage. Don t use separate constructors as an alternative to optional items.
CHAPTER 22 ATTRIBUTES
Attribute Parameter Types
The attribute pickling format supports only a subset of all the .NET runtime types, and therefore, you can use only some types as attribute parameters. The types allowed are the following: bool, byte, char, double, float, int, long, short, string object System.Type An enum that has public accessibility (not nested inside something nonpublic) A one-dimensional array of one of the previous types
Reflecting on Attributes
Once attributes are defined on some code, it s useful to be able to find the attribute values. You can do this through reflection. The following code shows an attribute class, the application of the attribute to a class, and the reflection on the class to retrieve the attribute: using System; using System.Reflection; [AttributeUsage(AttributeTargets.Class, AllowMultiple=true)] public class CodeReviewAttribute: System.Attribute { public CodeReviewAttribute(string reviewer, string date) { this.reviewer = reviewer; this.date = date; } public string Comment { get { return(comment); } set { comment = value; } }
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