1.
OrderByDescending(c => c.ModifyTime).Select(c => c.ModifyTime).FirstOrDefault()
Select(c => c.ModifyTime).OrderByDescending(c => c.HasValue).FirstOrDefault()
1.
OrderByDescending(c => c.ModifyTime).Select(c => c.ModifyTime).FirstOrDefault()
Select(c => c.ModifyTime).OrderByDescending(c => c.HasValue).FirstOrDefault()