Question

It is valid (ie. it compiles and runs) to put an attribute on the generic parameter for a class or a method:

public class MyClass<[My] T>
{
    private void MyMethod<[My] T>()
    {}
}

public class MyAttribute : Attribute
{}

I’ve never seen this used, and am struggling to come up with a reason as to why you would want to.

Is it just a quirk/side-effect of the language specification, or is there a valid/useful reason to put an attribute in this position?

Answer 1

For the same reason attributes are useful on any construct; they supply meta-data that can be used by Reflection or other post-processors to do various things. For instance, you might have an AOP system that uses an attribute on a type argument to apply certain run-time constraints that otherwise could not be expressed. I’m not sure if there are any systems that actually use these attributes to do anything, but there’s no reason to disallow them as metadata.