Interfaces or Attributes for Tagging Classes?

Question

I have a couple of classes that I wish to tag with a particular attribute. I have two approaches in mind. One involves using an Attribute-extending class. The other uses an empty interface:

Attributes

public class FoodAttribute : Attribute { }

[Food]
public class Pizza { /* ... */ }

[Food]
public class Pancake { /* ... */ }

if (obj.IsDefined(typeof(FoodAttribute), false)) { /* ... */ }

Interface

public interface IFoodTag { }

public class Pizza : IFoodTag { /* ... */ }
public class Pancake : IFoodTag { /* ... */ }

if (obj is IFoodTag) { /* ... */ }

I'm hesitant to use the attributes due to its usage of Reflection. At the same time, however, I'm hesitant on creating an empty interface that is only really serving as a tag. I've stress-tested both and the time difference between the two is only about three milliseconds, so performance is not at stake here.

Answer 1

Well, with attributes, you can always create the attribute in such a way that its function doesn't propagate to descendant types automatically.

With interfaces, that's not possible.

I would go with attributes.

Answer 2

You probably have answered on you question by your own. Attributes is more logical here, reflection is not a BIG MONSTER WITH RED EYES =)

btw, can you show calling code, where you determine marked with interface types? Aren't you using reflection there?

Answer 3

In this case, as you say, you're not using an interface correctly.

What's wrong with using reflection the get you attributes? The usual answer is performance but that is generally not a real issue in nearly all cases.

Answer 4

I'll have to say otherwise. I think that, for your example, a marker interface makes more sense.

That's because it seems very likely that you might one day add some members to IFood.

Your design starts like this:

interface IFood {}

But then you decide to add something there:

interface IFood {
  int Calories { get; }
}

There are also other ways to extend interfaces:

static class FoodExtensions {
  public static void Lighten(this IFood self) { 
    self.Calories /= 2;
  } 
}