How do I resolve C# dependencies automatically?

I've been reading about Unity's dependency injection and I understand it's a thing and that it allows you to type a class to an interface. What I'm curious about is, do I HAVE to? In the below scenario there's a TerrainGenerator and TileCreator in the same space. How can I get the TileCreator within the generator as a dependency?

http://geekswithblogs.net/danielggarcia/archive/2014/01/23/introduction-to-dependency-injection-with-unity.aspx walks me through registering a type, but I read somewhere that as long as the class is visible in the Unity Assets section it'll be able to auto inject it, I just can't figure out the syntax (if it's possible).

I put all the classes in a single file... with a large system that could be pretty annoying. In the meantime it's an approach I'll try - better than having it not work at all.

Seems like Unity should be able to look at a class' constructor and perform these resolutions automatically and inject them in my class' constructor. Is that possible?