C#

  public abstract class Cost
  {
      private Int32 id;

      public Cost()
      {
      }
      public abstract override string ToString();

      public Task Task
      {
          get
          {
              return this.m_task;
          }
          set
          {
              this.m_task = value;
          }
      }
      public Int32 Id
      {
          get
          {
              return this.id;
          }
          set
          {
              this.id = value;
          }
      }
      public abstract double Value
      {
          get;
          set;
      }
  }//end Cost

  public class Materials : Cost
  {

      private string name;
      private string description;
      private double value;
      private DateTime orderDate;
      private DateTime realizationDate;
      private string orderNumber;

      public Materials()
          : base()
      {
      }

      public override double Value
      {
          get
          {
              return this.value;
          }
          set
          {
              this.value = value;
          }
      }


      public string OrderNumber
      {
          get
          {
              return this.orderNumber;
          }
          set
          {
              this.orderNumber = value;
          }
      }
  }//end Materials

public class PlannedOperation : Cost
  {

      private double hours;

      public PlannedOperation()
      {

      }

      public double Hours
      {
          get
          {
              return this.hours;
          }
          set
          {
              this.hours = value;
          }
      }

      public override double Value
      {
          get
          {
              return this.hours * this.m_OperationMachine.Operation.CC.SearchRate
                  (this.Task.AddDate.Year.ToString()).Value;
          }
          set
          {

          }
      }
  }//end PlannedOperation