How to: Provide Metadata Descriptions About Your Component

You can provide descriptive metadata about your component through attributes. Attributes are specialized classes that are applied to code elements. At compile time, attributes are emitted into metadata that is available to the common language runtime or to custom tools and applications through the System.Reflection namespace.

Attributes are attached to a component by preceding the component with a reference to the attribute and providing any relevant parameters or flags. This call to the constructor is placed within angle brackets <> in Visual Basic and regular brackets [] in C#.

By convention, all attribute classes end in "Attribute." For example, there are DescriptionAttribute, ObsoleteAttribute, and BrowsableAttribute classes. However, several languages that target the common language runtime, including Visual Basic and C#, do not require the full name of the attribute. For example, the ObsoleteAttribute may be referred to in code as Obsolete.

To attach an existing attribute to your component

1. Determine which attributes are required for your component.

2. Attach the attributes to your component. Note that you must either use the fully qualified name of the attribute or add the appropriate Imports (using) statement. The following example shows how to attach the DescriptionAttribute attribute:

   Imports System.ComponentModel
   <Description("This is a description string")> Public Class TheClass
   End Class
   

   using System.ComponentModel;
   [Description("This is a description string")]
   public class TheClass
   {
   }
   

Custom Attributes

You can also create your own attributes for use with your own custom tools or applications by inheriting from Attribute. You can add to this base class any custom properties or methods needed by your application.

To create and apply a custom attribute

1. Create a class that inherits from Attribute.

   Public Class WidgetAttribute
      Inherits System.Attribute
   End Class
   

   public class WidgetAttribute: System.Attribute
   {
   }
   

2. Determine which properties and methods your attribute needs and write the code for them. The following example shows how to create a WidgetType property that is set in the constructor of the WidgetAttribute class. TheAttributeUsageAttribute attribute sets what code members the attribute can target.

   <AttributeUsage(System.AttributeTargets.Class)> Public Class _
      WidgetAttribute
      Inherits System.Attribute
      Private mWidgetType as WidgetTypeEnum
      ' Creates a readonly property for the WidgetAttribute class.
      Public ReadOnly Property WidgetType as WidgetTypeEnum
         Get
            Return mWidgetType
         End Get
      End Property
      ' Creates a constructor that accepts a parameter and assigns the 
      ' value of that parameter to the WidgetType property.
      Public Sub New(type as WidgetTypeEnum)
         MyBase.New()
         mWidgetType = type
      End Sub
   End Class
   

   [AttributeUsage(System.AttributeTargets.Class)]
   public class WidgetAttribute: System.Attribute
   {
      private WidgetTypeEnum widgetType;
   
      // Creates a readonly property for the WidgetAttribute class.
      public WidgetTypeEnum WidgetType
         {
            get {return widgetType;}
         }
   
      public WidgetAttribute(WidgetTypeEnum type): base()
         {
            widgetType = type;
         }
   }
   

3. Apply this attribute as you would any other attribute, being certain to apply any needed parameters.

   <WidgetAttribute(WidgetTypeEnum.VerticalWidget)> _
      Public Class WidgetFortyFive
   End Class
   

   [WidgetAttribute(WidgetTypeEnum.VerticalWidget)]
   public class WidgetFortyFive
   {
   }
   

See Also

Tasks

How to: Provide Metadata for Component Properties, Methods, and Events

Concepts

Accessing Custom Attributes

Retrieving Information Stored in Attributes

Reference

Attribute

Other Resources

User Assistance for Components