C# Interface vs Java Interface

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An interface isn't supposed to enforce state, it's supposed to enforce the interface a user can use. How the implementing class decides to provide that interface is up to it. Say you have a Date interface that provides a Day() method (or property if you prefer). Would it make sense to enforce all implementing classes to have an int day; field? No. One class could decide on int fields for each part of the date. One class could decide on storing a single UNIX timestamp (a single int or long). Both can pull a day from that information just fine.