My interviewer was a very nice person, technically deep as well, contrary to most of the interview experiences here on Glassdoor. He was helpful throughout the interview process, gave hints, and helped fix the code when I made minor errors. He told me at the end that he needs someone with more in-depth C# implementation experience on a daily basis, so I know I won't be getting an offer.

Implement a MultiMap where k is a key and V can be a list of values.

They had an interface already created on HackerRank. The methods you were supposed to implement were listed. The odd one out is Flatten().

Also, there were some extension methods they wanted me to implement, but I didn't have enough time.