Specify required base class for .NET attribute targets

Question

I tried to create a custom .NET attribute with the code below but accidentally left off the subclass. This generated an easily-fixed compiler error shown in the comment.

Answer 1

I would like to be able to specify that my custom attribute can only be placed on subclasses of a particular class (or interface). Is this possible?

Actually, there is a way to do this for subclasses (but not interfaces) using protected - see Restricting Attribute Usage. To reproduce the code (but not the discussion):

abstract class MyBase {
    [AttributeUsage(AttributeTargets.Property)]
    protected sealed class SpecialAttribute : Attribute {}
}
class ShouldBeValid : MyBase {
    [Special] // works fine
    public int Foo { get; set; }
}
class ShouldBeInvalid { // not a subclass of MyBase
    [Special] // type or namespace not found
    [MyBase.Special] // inaccessible due to protection level
    public int Bar{ get; set; }
}