How to create c# library

Question

How to build a c# library  in which following user story is satisfied:

 

Here are the user stories:

1. A user should be able to add 3 types of fish to the tank ( A fish, B fish, C

fish) and name the fish

2.  A user should be able to see how much food to put in the tank with a Tank.Feed()

method.

 This should return the weight in grams of the total required fish food.

-- 0.1g for each A fish

 

-- 0.2g for each B fish

-- 0.3g for each C fish

3. A user should be able to save the data about the fish in the tank to an xml file

4. Ensure the design allows to add more types of fish in the future without having

to change the tank class.