AttributeUsage.Inherited flag

I found the documentation for AttribuetUsageAttribute to be very ambiguous, particularly regarding the Inherited property. Here’s a quick test on the behavior of the AttributeUsage.Inherited flag.  This affects how the attribute is queried via GetCustomerAttributes(). Here’s how …

There are 4 pivots:

1. In the usage case, is there an attribute on the derived class of the same type as the base class?

     [Test(Value="Base")]
    class Base
    {
    }

    [Test(Value = "Derived")]
    class Derived : Base
    {
    }

2. On AttributeUsage, setting AllowMultiple

3. On AttributeUsage, setting Inherited

     [AttributeUsage(AttributeTargets.Class, AllowMultiple=false, Inherited=false)]
    class TestAttribute : Attribute
    {
        public string Value { get; set; }

        public override string ToString()
        {
            return Value;
        }
    }

4. In the query case, is inherit true or false?

     var attrs = typeof(Derived).GetCustomAttributes(inherit: true);

Outputs for each combination

That’s 2^4 = 16 cases.  Here are the outputs for the following code snippet. Blank cells means false. Results is

             foreach (var attr in attrs)
            {
                Console.WriteLine(attr);
            }

Other comments:

I found case #1 vs. case #5 to be particularly interesting.

Of course, you can always manually walk the inheritance chain. This has the added perk of telling you at which level the attribute occurs :

             for(var t = typeof(Derived); t != null; t = t.BaseType)            
            {
                foreach(var attr in t.GetCustomAttributes(inherit:false))
                {
                    Console.WriteLine("type={0},attr={1}", t.Name, attr);
                }
            }

Assuming the derived class has an attr, this will print:

 type=Derived,attr=Derived
type=Base,attr=Base
type=Object,attr=System.Runtime.InteropServices.ComVisibleAttribute
type=Object,attr=System.Runtime.InteropServices.ClassInterfaceAttribute
type=Object,attr=__DynamicallyInvokableAttribute
type=Object,attr=System.SerializableAttribute

Note the attributes on System.Object. These have inherited=false, so don’t show up until we explicitly query.