#472 – Attributes Can Be Attached to a Variety of Elements

Custom attributes can be attached not only to class methods, but to a number of different types of elements.

You can attach an attribute to any of the following elements:

• An assembly
• A class
• A constructor
• A delegate
• An enumerated type
• An event
• A field
• A generic parameter
• An interface
• A method
• A module (.dll or .exe)
• A parameter
• A property
• A return value
• A struct

Below is an example of attaching custom attributes to parameters.

    public sealed class PopCultureAttribute : Attribute
    {
        public string Trivia;
        public object SuggestedValue;

        public PopCultureAttribute(string trivia, object suggestedValue)
        {
            Trivia = trivia;
            SuggestedValue = suggestedValue;
        }
    }

        public void FeedCowInBarn(
            [PopCulture("Cows stomachs can bloat when feeding them rich grains", "Silage")] string feedType,
            [PopCulture("Cows spend 6 hrs/day eating", 60)] int numMinutes)
        {
            Console.WriteLine("Cow eats slop in dim confines of barn.  {0}, {1}", feedType, numMinutes);
        }

#470 – Defining Your Own Custom Attribute

You can use predefined attributes to attach metadata to type members.

You can also define a custom attribute by creating a new class inheriting from System.Attribute.  The class name must end in “Attribute”.  You typically define a constructor that takes arguments that consist of the metadata that you want to attach to the type member.

    /// <summary>
    /// Attach to a class method to indicate kg of methane that is
    /// output when calling the method.
    /// </summary>
    public class MethaneFootprintAttribute : Attribute
    {
        public double kgMethane;

        public MethaneFootprintAttribute(int kg)
        {
            kgMethane = kg;
        }
    }

You can use the new attribute to attach metadata to individual type members.  You use the name of the new class, without the trailing “Attribute”.

        [MethaneFootprint(45)]
        public void FeedCowInBarn()
        {
            Console.WriteLine("Cow eats slop in dim confines of barn");
        }

        [MethaneFootprint(29)]
        public void LetGrazeOutside()
        {
            Console.WriteLine("Cow enjoys grazing and ends up healthier");
        }