class ImplicitFieldCopy
{
    private readonly YearMonthDay readOnlyField =
        new YearMonthDay(2018, 3, 1);
    private YearMonthDay readWriteField =
        new YearMonthDay(2018, 3, 1);

    public void CheckYear()
    {
        int readOnlyFieldYear = readOnlyField.Year;
        int readWriteFieldYear = readWriteField.Year;
    }
}