How to extract the most common YEAR from an array of DateTime objects using LINQ

Question

Hi, im trying to extract the most common YEAR from an array of DateTime objects using LINQ.

can someone help me? With the following its telling me i need to implement IComparable..

            DateTime modeDate = (from c in dates 
                                 group c by c.Year into g 
                                 select g).Max().First();

Answer 1

I think you're pretty close.

var dateUsedMostOften = (from d in dates
                         group d by d.Year into y
                         orderby y.Count() descending
                         select y).First().Key;

Untested. See if that works.

Note: If there are no items in the list this'll probably fail. You could get around it by calling FirstOrDefault() and then testing if that has returned null. Only look for .Key if you know it's non-null.

Answer 2

This part of your query

from c in dates 
    group c by c.Year into g 
    select g

returns an IGrouping<int, DateTime> and that does not implement ICompareable

other people have provided good answers for the query you want, but I thought you might want to know where the original error came from.

Answer 3

int commonYear = dates.GroupBy(date => date.Year)
                      .OrderByDescending(g => g.Count())
                      .First().Key;

Answer 4

This should do the job.

        List<DateTime> list = new List<DateTime> ();
        list.Add (DateTime.Parse ("2002 Jan 01"));
        list.Add (DateTime.Parse ("2003 Jan 01"));
        list.Add (DateTime.Parse ("2004 Jan 01"));
        list.Add (DateTime.Parse ("2005 Jan 01"));
        list.Add (DateTime.Parse ("2004 Jan 01"));
        list.Add (DateTime.Parse ("2004 Jan 01"));
        list.Add (DateTime.Parse ("2007 Jan 01"));

        int year = list.Select (d => d.Year)
            .GroupBy (y => y)
            .OrderBy (g => g.Count ())
            .Last ()
            .Key;