C#中实体类中包含另一个实体类无法包不能初始化

[coding_dobby]

public partial class commodity
	{
		public commodity()
		{}
		#region Model
		private int _number;
		private string _commoditynumber;
		private string _commodityname;
		private int _commoditytypenumber;
		private int? _unitnumber;
		private string _region;
		private decimal _commodityprice;
		private decimal _purchaseprice;
		private string _barcode;
        private  commoditytype commoditytype; //实体类a
        private  unit unit;//实体类b

上面字段都封装了

SuperMarket.Model.commodity model=new SuperMarket.Model.commodity();
if(row["TypeName"]!=null && row["TypeName"].ToString()!="")
				{
                    model.Commoditytype.TypeName = row["TypeName"].ToString();

                }

这里报错说没有将对象实例化。命名初始化了model但是好像其中包含的实体类A没有初始化