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In an attribute class, you will define the members that support the attribute Often attribute classes are quite simple, containing just a small number of fields or properties For example, an attribute might define a remark that describes the item to which the attribute is being attached Such an attribute might look like this:

= 5 We see that the required limit ( ) exists, and that it equals 5 Thus the function f ( x) = x 2 + x is differentiable at x = 2, and the value of the derivative is 5

[AttributeUsage(AttributeTargetsAll)] public class RemarkAttribute : Attribute { string pri_remark; // underlies Remark property public RemarkAttribute(string comment) { pri_remark = comment; } public string Remark { get { return pri_remark; } } }

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Let s look at this class, line by line The name of this attribute is RemarkAttribute Its declaration is preceded by the AttributeUsage attribute, which specifies that RemarkAttribute can be applied to all types of items Using AttributeUsage, it is possible to narrow the list of items to which an attribute can be attached, and we will examine its capabilities later in this chapter Next, RemarkAttribute is declared and it inherits Attribute Inside RemarkAttribute there is one private field, pri_remark, which supports one public, read-only property: Remark This property holds the description that will be associated with the attribute (Remark could also have been declared as an auto-implemented property with a private set accessor, but a read-only property is used for the purposes of illustration) There is one public constructor that takes a string argument and assigns it to Remark At this point, no other steps are needed, and RemarkAttribute is ready for use

Once you have defined an attribute class, you can attach the attribute to an item An attribute precedes the item to which it is attached and is specified by enclosing its constructor inside square brackets For example, here is how RemarkAttribute can be associated with a class:

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[RemarkAttribute("This class uses an attribute")] class UseAttrib { // }

This constructs a RemarkAttribute that contains the comment, This class uses an attribute This attribute is then associated with UseAttrib

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When attaching an attribute, it is not actually necessary to specify the Attribute suffix For example, the preceding class could be declared this way:

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[Remark("This class uses an attribute")] class UseAttrib { // }

Here, only the name Remark is used Although the short form is correct, it is usually safer to use the full name when attaching attributes, because it avoids possible confusion and ambiguity