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 acts simply as a placeholder for a data type no better specified in its interface behaviour.

In Java, the mechanism of the interface extension represents a good example of use. For example, let's say that we've the following

interface one {}
interface two {}

interface three extends one, two {}

Interface three will inherit the behaviour of 'one' and 'two', and so

class four implements three { ... }

has to specify the two methods, being of type 'three'.

As you can see, from the above example, empty interface can be seen also as a point of multiple inheritance (not allowed in Java).

Hoping this helps to clarify with a further viewpoint.