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

An empty interface can be used to classify classes under a specific purpose. (Marker Interface)

Example : Database Entities

public interface IEntity {

}

public class Question implements IEntity {
   // Implementation Goes Here
}

public class Answer implements IEntity {
   // Implementation Goes Here
}

For Instance, If you will be using Generic Repository(ex. IEntityRepository), using generic constraints, you can prevent the classes that do not implement the IEntity interface from being sent by the developers.