• You can decorate properties, classes, interfaces, structs, enums, delegates, events, methods, constructors, fields, parameters, return values, assemblies, and type parameters.
• By default, an attribute applies to the element that it precedes.
• You can also explicitly identify. E.g., return attributes, appear before a method declaration but use the same type of syntax structure:

[return: Description("Returns true if the object is in a valid state.")]
public bool IsValid()
{
	throw new NotImplementedException();
}

• When applying an attribute to a construct, only literal values and types (such as typeof(int)) are allowed as arguments.
  • This is to enable their serialization into the resultant CIL.
  • Therefore, it is not possible to call a static method when applying an attribute.
  • Providing a constructor that takes arguments of type DateTime would be of little value, since there is no DateTime literal.

Common Attributes (C#)

Custom Attributes

• The characteristic that turns a general class into an attribute is that it derives from System.Attribute.

Reflection: Find Decorated Classes

public static IEnumerable<Type> GetDecorated<T>(this Assembly assembly, bool inherit = true)
    where T : Attribute 
    => assembly.GetTypes().Where(x => x.GetCustomAttributes(typeof(T), inherit).Any());

AttributeUsage Attribute

• If the attribute is used inappropriately it will cause a compile-time error.
• To avoid inappropriate use of an attribute, custom attributes can be decorated with System.AttributeUsage attribute.
• AttributeUsage attribute’s constructor takes an AttributesTargets flag.
• This enum provides a list of all the possible targets that the runtime allows an attribute to decorate.
• AllowMultiple parameter make a custom attribute single-use or multiuse.

// Restrict the attribute to properties and methods
[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property, AllowMultiple = true)]
public class CommandLineSwitchAliasAttribute : Attribute
{
	// ...
}

Attribute Static Methods

• You can check that a class has a specific attribute by calling the static IsDefined method on Attribute class.

var isDefined = Attribute.IsDefined(typeof(Person), typeof(SerializableAttribute));