 <p>CLR does not allow multiple inheritance, which is precisely what you're trying to express. You want <code>T</code> to be <code>Address</code>, <code>Email</code> an <code>Phone</code> at the same time (I assume those are class names). Thus is impossible. What's event more, this whole method makes no sense. You'll either have to introduce a base interface for all three classes or use three overloads of an <code>Add</code> method.</p>