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17.Attributes


Much 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).

17.1Attribute classes


A class that derives from the abstract class System.Attribute, whether directly or indirectly, is an attribute class. The declaration of an attribute class defines a new kind of attribute that can be placed on a declaration. By convention, attribute classes are named with a suffix of Attribute. Uses of an attribute may either include or omit this suffix.

17.1.1Attribute usage


The attribute AttributeUsage (§17.4.1) is used to describe how an attribute class can be used.

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 class AuthorAttribute: Attribute
{
private string name;

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")]
class Class1
{
...
}

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(
AttributeTargets.All,
AllowMultiple = false,
Inherited = true)
]
class X: Attribute {...}

17.1.2Positional and named parameters


Attribute classes can have positional parameters and named parameters. Each public instance constructor for an attribute class defines a valid sequence of positional parameters for that attribute class. Each non-static public read-write field and property for an attribute class defines a named parameter for the attribute class.

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 types


The types of positional and named parameters for an attribute class are limited to the attribute parameter types, which are:

  • One of the following types: bool, byte, char, double, float, int, long, sbyte, short, string, uint, ulong, ushort.

  • The type object.

  • The type System.Type.

  • An enum type, provided it has public accessibility and the types in which it is nested (if any) also have public accessibility (§17.2).

  • Single-dimensional arrays of the above types.

    A constructor argument or public field which does not have one of these types, cannot be used as a positional or named parameter in an attribute specification.




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