Most of the written information are from before C# was popular here, so a lot of the stuff for example by @

is C++.
He also wrote "

" which explains how the silkroad network protocol works. The code examples are, you guessed it, in C++, but to understand the concept you don't really need to fully know the language. If you don't want to bother with writing an implementation of the protocol then you can use:
*

he released.
*

*

and learn from it how to use it.
If you're really interested you should really consume

because it is very insightful.
Next I have to mention florian0 and his

for the SRO_Client that is very popular when creating features for privat servers. The server equivalent to this being

.
You'll eventually realized, that it is beneficial to learn reverse engineering with IDA or Ghidra (free) to a certain degree. It helps you figuring out how things in this game work from

to certain hardcoded details like potion cooldowns.
If you just want to edit some files there is a

by JellyBitz.
Lastly I'll have to shamelessly plug myself and the

that is more or less accurate but somewhat incomplete, due to my laziness when it comes to translating my personal packet documentation to beautiful markdown. The file format pages are up to date tho.
Everything else needs to be
learned by yourself from reading through released projects, experimenting with your own projects or talking to people when you're stuck.

is probably the best place to ask silkroad related questions aside from this forum section.