SO Answer test for http://stackoverflow.com/a/11723399/215752

SO Answer

void Main()
{
    List<offer> OfferList = new List<offer>() {
	 new offer()  { id=1, f=new DateTime(2011,1,1), t=new DateTime(2011,1,31),status=true},
	 new offer()  { id=1, f=new DateTime(2011,2,1), t=new DateTime(2011,2,28), status=false},
	 new offer()  { id=2, f=new DateTime(2011,2,1), t=new DateTime(2011,2,28), status=false},
	 new offer()  { id=3, f=new DateTime(2011,1,1), t=new DateTime(2011,1,31), status=true },
	 new offer()  { id=3, f=new DateTime(2011,1,1), t=new DateTime(2011,1,26), status=true },
	 new offer()  { id=4, f=new DateTime(2011,1,1), t=new DateTime(2011,1,31), status=true },
	 new offer()  { id=1, f=new DateTime(2011,3,1), t=new DateTime(2011,3,31), status=false},
	};
	
	var r = (OfferList.GroupBy(offer => offer.id)
				 .Select(group =>
					 new { offerid = group.Key,
						   offers = group.OrderBy(o => o.f),
						   count = group.Count() })
		    	 .OrderBy(g => g.offers.First().f) // list with the oldest
				 .ThenByDescending(g => g.count)).Dump()  // then by most
			.First().offers.First();   
		 
    r.Dump();
}

// Define other methods and classes here
public class offer
{
   public int id { get; set; }
   public DateTime f { get; set; }
   public DateTime t { get; set; }
   public bool status { get; set; }
}