Brian Ensink Brian Ensink - 11 days ago 7
C# Question

Specify required base class for .NET attribute targets

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?

Answer

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; }
}