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. [Example: The example defines an attribute class named HelpAttribute that has one positional parameter (string url) and one named parameter (string 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: end example]