They must be inherited from the System.Attribute
Attributes are instantiated at the same time as instances of the class to which they are applied.
Attribute classes may be restricted to be applied only to application element types.
By default, a given attribute may be applied multiple times to the same application element.
All attribute types derive directly or indirectly from the Attribute class. Attributes can be applied to any target element; multiple attributes can be applied to the same target element; and attributes can be inherited by an element derived from a target element. Use the AttributeTargets class to specify the target element to which the attribute is applied. Useful resources, Attribute Class , Attribute