Are tag (or “marker”) interfaces obsolete?

Question

I'm trying to help a coworker come to terms with OO, and I'm finding that for some cases, it's hard to find solid real-world examples for the concept of a tag (or marker) interface. (An interface that contains no methods; it is used as a tag or marker or label only). While it really shouldn't matter for the sakes of our discussions, we're using PHP as the platform behind the discussions (because it's a common language between us). I'm probably not the best person to teach OO since most of my background is highly theoretical and about 15 years old, but I'm what he's got.

In any case, the dearth of discussions I've found regarding tag interfaces leads me to believe it's not even being used enough to warrant discussion. Am I wrong here?

Answer 1

The .NET Style guide says to use Attributes rather than tag/marker interfaces.

alt text http://www.freeimagehosting.net/uploads/th.4528577db5.jpg
Click for Full image
source: http://www.informit.com/articles/article.aspx?p=423349&seqNum=6
or any number of other exposure points of Cwalina's recommendations, like the book.