No.  You can't have conditional inheritance.

However, it's a great example of why composition is often better than  
inheritance anyway.  It'd be nice if C# had a more convenient way of  
exposing composited interfaces than manually writing stubs to wrap an  
internally stored class implementing the interface.  But other than that  
inconvenience (something shared by other popular OOP languages anyway),  
it's not bad.

So, just make your collection class expose whatever interface(s) you need,  
and then use the appropriate collection class internally to store the data.

I assume that your desire to optionally use a Dictionary<> instead of  
SortedDictionary<> is a performance issue?  Because otherwise, I'd think  
you'd just use a SortedDictionary<> all the time.

Pete