C# 3.0 Extension Methods - Attributes
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You’re already familiar with the notion of attributing code elements of a program with modifiers, such as virtual or ref. These constructs are built into the language. Attributes are an extensible mechanism for adding custom information to code elements (assemblies, types, members, return values, and parameters). This extensibility is useful for services that integrate deeply into the type system, without requiring special keywords or constructs in the C# language.
A good scenario for attributes is serialization—the process of converting arbitrary objects to and from a particular format. In this scenario, an attribute on a field can specify the translation between C#'s representation of the field and the format’s representation of the field.
Attribute Classes
An attribute is defined by a class that inherits (directly or indirectly) from the abstract class System.Attribute. To attach an attribute to a code element, specify the attribute’s type name in square brackets, before the code element. For example, the following attaches the ObsoleteAttribute to the Foo class:
[ObsoleteAttribute]
public class Foo {...}
This attribute is recognized by the compiler and will cause compiler warnings if a type or member marked obsolete is referenced. By convention, all attribute types end in the word “Attribute”. C# recognizes this and allows you to omit the suffix when attaching an attribute:
[Obsolete]
public class Foo {...}
ObsoleteAttributeis a type declared in theSystemnamespace as follows (simplified for brevity):
public sealed class SerializableAttribute : Attribute {...}
The C# language and the .NET Framework include a number of predefined attributes. We describe how to write your own attributes in Chapter 17.
Named and Positional Parameters
Attributes may have parameters. In the following example, we apply the XmlElement attribute to a class. The XmlElement attribute tells the System.Xml.Linq model how an object is represented in XML. The XmlElement attribute accepts several attribute parameters. The following attribute maps the CustomerEntityclass to an XML element namedCustomer, belonging to the http://oreilly.com namespace:
[XmlElement ("Customer", Namespace=http://blah)]
public class CustomerEntity { ... }
Attribute parameters fall into one of two categories: positional and named. In the preceding example, the first argument is a positional parameter; the second is a named parameter. Positional parameters correspond to parameters of the attribute type’s public constructors. Named parameters correspond to public fields or public properties on the attribute type.
When specifying an attribute, you must include positional parameters that correspond to one of the attribute’s constructors. Named parameters are optional.
In Chapter 17, we describe the valid parameter types and rules for their evaluation.
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter four of C# 3.0 in a Nutshell, Third Edition, A Desktop Quick Reference, written by Joseph Albahari and Ben Albahari (O'Reilly; ISBN: 0596527578). Check it out today at your favorite bookstore. Buy this book now.
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