c# - 我如何从通用日期列表中选择不同的(按唯一的月份/年份)？

Question

因此不同是基于唯一的月/年，而不仅仅是一个不同的月份(所以我希望 2011 年 1 月和 2012 年 1 月是不同的)

 // Test set of data
        List<DateTime> CompleteListOfDates = new List<DateTime>();
        CompleteListOfDates.Add(new DateTime(2011, 1, 1));
        CompleteListOfDates.Add(new DateTime(2011, 1, 5));
        CompleteListOfDates.Add(new DateTime(2011, 3, 1));
        CompleteListOfDates.Add(new DateTime(2011, 5, 1));
        CompleteListOfDates.Add(new DateTime(2011, 5, 1));
        CompleteListOfDates.Add(new DateTime(2012, 1, 1));
        CompleteListOfDates.Add(new DateTime(2012, 2, 1));

        List<DateTime> UniqueMonthYears = new List<DateTime>();

        /* need distinct list of DateTimes that are distinct by Month and Year and should be in UniqueMonthYears
         For example:
        new DateTime(2011, 1, 1)
        new DateTime(2011, 3, 1)
        new DateTime(2011, 5, 1)
        new DateTime(2012, 1, 1)
        new DateTime(2012, 2, 1)
        */

Answer 1

List<DateTime> result = source
  .Select(d => new DateTime(d.Year, d.Month, 1))
  .Distinct()
  .ToList();

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也很有用:

ILookup<DateTime, Order> ordersByMonth = ordersSource
  .ToLookup(order => new DateTime(order.OrderDate.Year, order.OrderDate.Month, 1));