对象引用未设置为对象的实例 - "Object includes a list property"

我只是创建了一个类的对象。这个类的属性之一是另一个类的对象列表。当我想从该列表中的第二个类中强制转换对象时，将给出错误"对象引用未设置为对象实例"。

这是第一类的代码：

public class RCSection<Bar>
    {
       private string RCSectionName;
       private int NumberOfBars;
       private double NumberOfInnerBars;
       private double NumberOfOuterBars;
       private double TransverseSpacing;

       private Steel LongitudinalSteel;
       private Steel TransevrseSteel;
       private Concrete Concrete;
       private List<Bar> LongitudinalBar;

       private Bar TransverseBar;

       private Section Section;


       public string rCSectionName
       {
           set { RCSectionName = value; }
           get { return RCSectionName; }
       }
       public int numberOfBars
       {
           set { NumberOfBars = value; }
           get { return NumberOfBars; }
       }
       public double transverseSpacing
       {
           set { TransverseSpacing = value; }
           get { return TransverseSpacing; }
       }
       public double numberOfInnerBars
       {
           set { NumberOfInnerBars = value; }
           get { return NumberOfInnerBars; }
       }
       public double numberOfOuterBars
       {
        set { NumberOfOuterBars = value; }
        get { return NumberOfOuterBars; }
       }

       public Steel longitudinalSteel
       {
           set { LongitudinalSteel = value; }
           get { return LongitudinalSteel; }
       }
       public Steel transverseSteel
       {
           set { TransevrseSteel = value; }
           get { return TransevrseSteel; }
       }
       public Concrete concrete
       {
           set { Concrete = value; }
           get { return Concrete; }
       }
       public List<Bar> longitudinalBar
       {
           set { LongitudinalBar = value; }
           get { return LongitudinalBar; }
       }
       public Bar transverseBar
       {
           set { TransverseBar = value; }
           get { return TransverseBar; }
       }

       public Section section
       {
           set { Section = value; }
           get { return Section; }
       }

}

一开始我想知道，我为列表创建属性的方式，对吗？！在那之后，下面的代码与这个类的对象的使用以及在其中投射一个对象有关

  for (int i = 0; i < myRCSection.numberOfBars; i++)
  {
       Bar mybar = new Bar(newFormRCSection.comboBoxSteelSize1.Text,"SI");
       myRCSection.longitudinalBar[i] = mybar;//Error will appear here :(
  }