I had trouble finding information or examples showing how to use RealtimeDictionary, so I put together this doc that details the things I learned as I figured it out.
RealtimeDictionary isn't a generic C# dictionary that syncs across the network. It specifically maps from uint
s (unsigned ints, or integers from 0 to ~4 billion) to RealtimeModels. There are other guides which cover RealtimeModels, but they're essentially network-synced structs that can hold whatever data you like.
It's a little complicated if you were just looking to map between two primitive data types, but creating a small RealtimeModel that wraps a single attribute is pretty quick and easy.
Also note that your keys must be unsigned ints. RealtimeDictionary does not support keys of other types.