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

Normally it's similar to attributes. Using attributes is a preferred to empty interfaces (at least as much as FxCop is aware). However .NET itself uses some of these interfaces like IRequiresSessionState and IReadOnlySessionState. I think there is performance loss in metadata lookup when you use attributes that made them use interfaces instead.

Answer 2

Empty interfaces are used to mark the class, at run time type check can be performed using the interfaces.

For example An application of marker interfaces from the Java programming language is the Serializable interface. A class implements this interface to indicate that its non-transient data members can be written to an ObjectOutputStream. The ObjectOutputStream private method writeObject() contains a series of instanceof tests to determine writeability, one of which looks for the Serializable interface. If any of these tests fails, the method throws a NotSerializableException.

Answer 3

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.

Answer 4

In java Serializable is the perfect example for this. It defines no methods but every class that "implements" it has to make sure, that it is really serializable and holds no reference to things that cannot be serialized, like database connections, open files etc.

Answer 5

In Java, empty interfaces were usually used for "tagging" classes - these days annotations would normally be used.

It's just a way of adding a bit of metadata to a class saying, "This class is suitable for <this> kind of use" even when no common members will be involved.

Answer 6

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.