Do custom attribute classes use the Inherited AttributeUsage flag?

Question

Consider the following scenario:

• The base class of the BaseAttribute attribute has an AttributeUsageAttribute indicating that it is not inherited ( Inherited = False ).
• The DerivedAttribute derived attribute DerivedAttribute inherits from this base attribute class.
• The base class of the Base domain applies a derived attribute.
• The Derived domain class, which inherits from the base domain, is requested for its own attributes, including inherited attributes ( inherit: true ).

Here is the relevant code:

 using System; using System.Linq; namespace ConsoleApplication26 { class Program { static void Main () { var attributes = typeof (Derived).GetCustomAttributes (true); foreach (var attribute in attributes) { Console.WriteLine ( "{0}: Inherited = {1}", attribute.GetType().Name, attribute.GetType().GetCustomAttributes (typeof (AttributeUsageAttribute), true).Cast<AttributeUsageAttribute>().Single().Inherited); } } } [AttributeUsage (AttributeTargets.All, Inherited = false)] public class BaseAttribute : Attribute { } public class DerivedAttribute : BaseAttribute { } [Derived] public class Base { } public class Derived : Base { } } 

In this case, the GetCustomAttributes API returns an instance of the DerivedAttribute class. I would expect it to not return this instance, because http://msdn.microsoft.com/en-us/library/system.attributeusageattribute.aspx says that AttributeUsageAttribute itself is inherited.

Now, is this a mistake, or can it be expected / documented somewhere?

Note (2013-02-20): Experiments show that the AttributeTargets part of the BaseAttribute class BaseAttribute indeed inherited by the DerivedAttribute class. For example, when I change valid targets to BaseAttribute to AttributeTargets.Method , the C # compiler will not let me apply DerivedAttribute to the class. Therefore, it does not make sense that the Inherited = False not inherited by DerivedAttribute , and therefore I tend to think about an error in the implementation of GetCustomAttributes .

Answer 1

According to metadata in .NET 4.0, the AttributeUsageAttribute class is marked [AttributeUsage(AttributeTargets.Class, Inherited = true)] . So, if your attribute class is ( BaseAttribute , in your example), AttributeUsageAttribute applies to it (like all Attribute classes, but don’t break anything if they don’t see below), then any classes extracted from BaseAttribute should inherit AttributeUsage AttributeUsage . applied to it.

Your Derived class inherits DerivedAttribute from Base because DerivedAttribute does not have its own AttributeUsageAttribute applied to it, so the reflection API relies on the BaseAttribute attribute. Now release the AttributeUsageAttribute your BaseAttribute class and you will get the same result, because the base class System.Attribute marked [AttributeUsage(AttributeTargets.All, Inherited = true, AllowMultiple = false)] . Thus, any attribute class inherits this attribute, unless you specify another.

Wow, these are complicated paragraphs. Attribute Attributes do some heavy reading: P