What kind API (Application Interface) does Autocad use that you will be "programming for"?
What kind of samples have you found related to what you want to do?
Have you "studied" these samples to determine what knowledge set they require?
You need a learning plan related to "your" "5 year plan".
If your plan requires learning C#, from scratch, you can start here:
Learn C# for Beginners – Microsoft Virtual Academy[
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