What is an empty interface used for

Question

I am looking at nServiceBus and came over this interface

namespace NServiceBus
{
    public interface IMessage
    {
    }
}

What is the us

Answer 1

They're called "Mark Interfaces" and are meant to signal instances of the marked classes.

For example... in C++ is a common practice to mark as "ICollectible" objects so they can be stored in generic non typed collections.

So like someone over says, they're to signal some object supported behavior, like ability to be collected, serialized, etc.