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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. // results in compiler error CS0641: Attribute 'AttributeUsage' is // only valid on classes derived from System.Attribute [AttributeUsage(AttributeTargets.Class)] internal class ToolDeclarationAttribute { internal ToolDeclarationAttribute() { } } My question is how does the compiler know the [AttributeUsage] attribute can only be applied to a subclass of System.Attribute ? Using .NET Reflector I don't see anything special on the AttributeUsageAttribute class declaration itself. Unfortunately this might just be a special case generated by the compiler itself. [Serializable, ComVisible(true), AttributeUsage(AttributeTargets.Class, Inherited=true)] public sealed class AttributeUsageAttribute : Attribute { ... 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?

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