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- 17.1Attribute classes
- 17.1.1Attribute usage
- 17.1.2Positional and named parameters
- 17.1.3Attribute parameter types
17.AttributesMuch of the C# language enables the programmer to specify declarative information about the entities defined in the program. For example, the accessibility of a method in a class is specified by decorating it with the method-modifiers public, protected, internal, and private. C# enables programmers to invent new kinds of declarative information, called attributes. Programmers can then attach attributes to various program entities, and retrieve attribute information in a run-time environment. For instance, a framework might define a HelpAttribute attribute that can be placed on certain program elements (such as classes and methods) to provide a mapping from those program elements to their documentation. Attributes are defined through the declaration of attribute classes (§17.1), which may have positional and named parameters (§17.1.2). Attributes are attached to entities in a C# program using attribute specifications (§17.2), and can be retrieved at run-time as attribute instances (§17.3).
AttributeUsage has a positional parameter (§17.1.2) that enables an attribute class to specify the kinds of declarations on which it can be used. The example using System; [AttributeUsage(AttributeTargets.Class | AttributeTargets.Interface)] public class SimpleAttribute: Attribute { ... } defines an attribute class named SimpleAttribute that can be placed on class-declarations and interface-declarations only. The example [Simple] class Class1 {...} [Simple] interface Interface1 {...} shows several uses of the Simple attribute. Although this attribute is defined with the name SimpleAttribute, when this attribute is used, the Attribute suffix may be omitted, resulting in the short name Simple. Thus, the example above is semantically equivalent to the following: [SimpleAttribute] class Class1 {...} [SimpleAttribute] interface Interface1 {...} AttributeUsage has a named parameter (§17.1.2) called AllowMultiple, which indicates whether the attribute can be specified more than once for a given entity. If AllowMultiple for an attribute class is true, then that attribute class is a multi-use attribute class, and can be specified more than once on an entity. If AllowMultiple for an attribute class is false or it is unspecified, then that attribute class is a single-use attribute class, and can be specified at most once on an entity. The example using System; [AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public AuthorAttribute(string name) { this.name = name; } public string Name { get { return name; } } } defines a multi-use attribute class named AuthorAttribute. The example [Author("Brian Kernighan"), Author("Dennis Ritchie")]
shows a class declaration with two uses of the Author attribute. AttributeUsage has another named parameter called Inherited, which indicates whether the attribute, when specified on a base class, is also inherited by classes that derive from that base class. If Inherited for an attribute class is true, then that attribute is inherited. If Inherited for an attribute class is false then that attribute is not inherited. If it is unspecified, its default value is true. An attribute class X not having an AttributeUsage attribute attached to it, as in using System; class X: Attribute {...} is equivalent to the following: using System; [AttributeUsage(
The example using System; [AttributeUsage(AttributeTargets.Class)] public class HelpAttribute: Attribute { public HelpAttribute(string url) { // Positional parameter ... } public string Topic { // Named parameter get {...} set {...} } public string Url { get {...} } } defines an attribute class named HelpAttribute that has one positional parameter, url, and one named parameter, Topic. Although it is non-static and public, the property Url does not define a named parameter, since it is not read-write. This attribute class might be used as follows: [Help("http://www.mycompany.com/.../Class1.htm")] class Class1 { ... } [Help("http://www.mycompany.com/.../Misc.htm", Topic = "Class2")] class Class2 { ... } 17.1.3Attribute parameter typesThe types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:
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