public class job
        {
            public DateTime strt_time { get; set; }
            public DateTime fnl_date { get; set; }

   // еще  10-15 свойств, не важно каких 
        }

    public class pereriv
        {
            public DateTime strt_time { get; set; }
            public DateTime fnl_date { get; set; }
        }

public abstract class BaseClass
    {
        public DateTime strt_time { get; set; }
        public DateTime fnl_date { get; set; }

    }

    public class inheritedjob: BaseClass
    {
        public string MyProperty1 { get; set; }
        public string MyProperty2 { get; set; }
        public string MyProperty3 { get; set; }
        public string MyProperty4 { get; set; }
        public string MyProperty5 { get; set; } 
        public bool InWork { get; set; }

    }

    public class inheritedpereriv: BaseClass
    {

    }

   static void Main(string[] args)
        {

            HashSet<BaseClass> abstractHash = new HashSet<BaseClass>();

            abstractHash.Add(new inheritedjob() {  InWork =true, strt_time = DateTime.Now });
            abstractHash.Add(new inheritedjob() {  strt_time =DateTime.Today});
            abstractHash.Add(new inheritedjob());
            abstractHash.Add(new inheritedjob());
            abstractHash.Add(new inheritedjob() { InWork = true });

            abstractHash.Add(new inheritedpereriv());
            abstractHash.Add(new inheritedpereriv() );
            abstractHash.Add(new inheritedpereriv());
            abstractHash.Add(new inheritedpereriv());

            var d2 = abstractHash.OfType<inheritedjob>().
                Where(p=>p.InWork);

        }