Will – The most significant disadvantage of using interfaces is the inability to control the marker when you start inheriting from the marked class. Attributes lets you specify if the marker should be inherited by derived classes while interfaces doesn’t allow you to have that control.

Also, attributes have the advantage that you can mark other things than classes, such as methods, parameters, constructors, fields, properties etc.

And as Skup points out the correct way to check for marker attributes is to use Attribute.Isdefined() or classInstance.GetType().IsDefined() and as he said you might even make it easier by adding some sort of generic methods or something… So I did. You can see the result on my blog (http://blog.johannesh.dk).

However, it doesn’t state why marker interfaces should be avoided. Certainly, there is a convenience to using interfaces. However, the extension method approach for attributes is minimal. Still, as Pieter says, this is code to be maintained and tested.

Unless there is a significant performance concern using marker interfaces (which seems unlikely to be slower than reflection), then I probably would lean toward interfaces over attributes.

Are there other advantages that attributes provide? I suppose, you can include parameters on your attributes to drive behavior.

As already mentionned, if i need compile-time certainty, the marker interface is the only one that can help me… (Otherwise, an attribute is probably a better ‘marker’…)

Thus in order to find out whether an object is FooAssignable I need to write a helper method (very sexy piece of code I must add!!), which internally uses Reflection and is an extra piece code to be maintained and tested.

I am not against Reflection, I just think it should be used in the appropriate places, and not to replace existing functionality in the framework.

To make a long story short, why write your own code to replace something that can already be achieved through the framework?

public class Repository where T : IAggregateRoot

This gives you compile-time protection for certain scenarios, while attributes are only run-time.