Chapter 8. Attributes

Attributes are declarative tags that can be used to annotate types or class members , thereby modifying their meaning or customizing their behavior. This descriptive information provided by the attribute is stored as metadata in a .NET assembly and can be extracted either at design time or at runtime using reflection.

To see how attributes might be used, consider the <WebMethod> attribute, which might appear in code as follows :

 <WebMethod(Description:="Indicates the number of visitors to a page")> _            Public Function PageHitCount(strULR As String) As Integer 

Ordinarily, public methods of a class can be invoked locally from an instance of that class; they are not treated as members of a web service. In contrast, the <WebMethod> attribute marks a method as a function callable over the Internet as part of a web service. This <WebMethod> attribute also includes a single property, Description, which provides the text that will appear in the page describing the web service.

You may wonder why attributes are used on the .NET platform and why they are not simply implemented as language elements. The answer comes from the fact that attributes are stored as metadata in an assembly, rather than as part of its executable code. As an item of metadata, the attribute describes the program element to which it applies and is available through reflection both at design time (if a graphical environment such as Visual Studio .NET is used), at compile time (when the compiler can use it to modify, customize, or extend the compiler's basic operation), and at runtime (when it can be used by the Common Language Runtime or by other executable code to modify the code's ordinary runtime behavior).

The behavior of interface objects (i.e., controls) in Visual Studio .NET illustrates the importance of attributes. Since Visual Studio offers drag-and-drop placement of controls on forms or web pages, it is necessary for controls to have a design time behavior in addition to their runtime behavior. For instance, when you double click on a control in a designer, you ordinarily want the code or the code template for its default event handler to be displayed. Note that the question posed here is not how the control should respond to a double-click event, since the DoubleClick event occurs at runtime and, if an event handler is present, causes that event handler's executable code to be executed. Because we're concerned with the standard behavior of a control in its design time environment, an attribute provides an excellent solution. Indeed, the .NET Framework provides the <DefaultEvent> attribute, which allows you to define a control's default event. Since information on the attribute is stored in the assembly's metadata, Visual Studio can simply look to see whether a <DefaultEvent> attribute is attached to a particular control when it is double-clicked in a designer window.

The attribute-based system of programming implemented in .NET is extensible. In addition to the attributes predefined by Visual Basic or by the .NET Framework, you can define custom attributes that you apply to program elements. For an attribute to be meaningful, there must also be code that attempts to detect the presence of the attribute at design time, at compile time, or at runtime, and accordingly that performs an action dictated by the attribute's presence.

This chapter discusses the syntax and use of attributes, and then shows how to define and use custom attributes.