public void UpdateTimestampForExistingUser() { var ui = new UserIndex() .UpdateTimestamp("user1", UnixMillisecondTime.OfMillis(1000)) .UpdateTimestamp("user2", UnixMillisecondTime.OfMillis(2000)); ui = ui.UpdateTimestamp("user1", UnixMillisecondTime.OfMillis(2001)); Assert.Collection(ui.Data, AssertEntry("user2", 2000), AssertEntry("user1", 2001)); }