linq 日期时间不包含最大值或最小值

Question

I am getting this error that the datetime does not contain max or min method. I am trying to get the meeting max and min date and see if they are equal and then set MeetingDate to the min datetime. If they are not equal then I will concat both min StartDate and max StartDate together. below is what the output should look like and the error I am getting is datetime not contain max or min method.

class Program
    {
        static void Main(string[] args)
        {

            List<MeetingModel> Meeting = new List<MeetingModel>();

            Meeting.Add(new MeetingModel() { MeetingId = 1, startTime = new DateTime(2017, 1, 18) });
            Meeting.Add(new MeetingModel() { MeetingId = 1, startTime = new DateTime(2017, 1, 19) });
            Meeting.Add(new MeetingModel() { MeetingId = 2, startTime = new DateTime(2017, 1, 22) });
            Meeting.Add(new MeetingModel() { MeetingId = 2, startTime = new DateTime(2017, 1, 22) });
            Meeting.Add(new MeetingModel() { MeetingId = 3, startTime = new DateTime(2017, 1, 23) });
            Meeting.Add(new MeetingModel() { MeetingId = 3, startTime = new DateTime(2017, 1, 23) });
            Meeting.Add(new MeetingModel() { MeetingId = 4, startTime = new DateTime(2017, 1, 25) });
            Meeting.Add(new MeetingModel() { MeetingId = 4, startTime = new DateTime(2017, 1, 29) });


            var result =  Meeting
                          .Select (p=> new MeetingViewModel
                          {
                             MeetingId = p.MeetingId,
                             MeetingDate = p.startTime.Max() == p.startTime.Min() ? p.startTime.Min().ToString("MM/dd/yyyy") :
                                        string.Format("{0: MM/dd/yyyy} - {1: MM/dd/yyyy}", p.startTime.Min(), p.startTime.Max()
                          }).ToList();

            foreach (var meeting in result)
            {

                Console.WriteLine("{0}", meeting.MeetingId);
                Console.WriteLine("{0}", meeting.MeetingDate);
            }
        }


        class MeetingModel
        {
            public int MeetingId { get; set; }
            public DateTime startTime { get; set; }        
        }

        class MeetingViewModel
        {
            public int MeetingId { get; set; }
            public string MeetingDate { get; set; }
        }
    }

Error

Severity Code Description Project File Line Suppression State Suppression State Error CS1929 'DateTime' does not contain a definition for 'Max' and the best extension method overload 'Enumerable.Max(IEnumerable)' requires a receiver of type 'IEnumerable'

Output

Meeting 1 2017/1/18 - 2017/1/19
Meeting 2 2017/1/22
Meeting 3 2017/1/23
Meeting 4 2017/1/25 - 2017/1/29

EF

select new Model.Meeting
                       {
                           MeetingId = m.MeetingId,
                           GroupId = m.GroupId,
                           MeetingDate = m.Min(x => x.startTime) == m.Max(x => x.startTime) ?
                                         m.Max(x => x.startTime).ToString("MM/dd/yyyy") :
                                        string.Format("{0: MM/dd/yyyy} - {1: MM/dd/yyyy}", 
                                        md.Min(x => x.startTime), md.Max(x => x.startTime))
                       }).GroupBy(x => x.MeetingId).ToList();

Answer 1

You could group by MeetingId and use .Min and .Max linq methods to fetch dates:

var result = Meeting
    .GroupBy(x => x.MeetingId)
    .Select(p => new MeetingViewModel
    {
        MeetingId = p.Key,
        MeetingDate = p.Min(x => x.startTime) == p.Max(x => x.startTime) ?
            p.Max(x => x.startTime).ToString("MM/dd/yyyy") :
            string.Format("{0: MM/dd/yyyy} - {1: MM/dd/yyyy}", p.Min(x => x.startTime), p.Max(x => x.startTime))
    }).ToList();

Test in dotnetfiddle: https://dotnetfiddle.net/IJaehf

I hope you find this helpful.

Answer 2

Please try DateTime.MinValue or SqlDateTime.MinValue . These are implemented as static readonly fields and not methods.