public class GlobalData
    {

        static GlobalData()
        {
            
            m_orderExample.m_orderDetails.Add(new OrderDetail
            {
                Sku = "1",
                Spec = "白细胞数",
                Number = "WBC",
                Unit = "6.5",
                UnitPrice = "10/L",
                Description = "4.0-10.0",
                Other = ""
            });
            m_orderExample.m_orderDetails.Add(new OrderDetail
            {
                Sku = "2",
                Spec = "淋巴细胞%",
                Number = "LY%",
                Unit = "67.8",
                UnitPrice = "%",
                Description = "20.0-40.0",
                Other = " "
            });
            m_orderExample.m_orderDetails.Add(new OrderDetail
            {
                Sku = "3",
                Spec = "单核细胞%",
                Number = "MO%",
                Unit = "6.4",
                UnitPrice = "%",
                Description = "2.0-9.0",
                Other = " "
            });
            m_orderExample.m_orderDetails.Add(new OrderDetail
            {
                Sku = "4",
                Spec = "中性粒细胞",
                Number = "NE#",
                Unit = "4.4",
                UnitPrice = "%",
                Description = "1.4-7.7",
                Other = " "
            });
            m_orderExample.m_orderDetails.Add(new OrderDetail
            {
                Sku = "5",
                Spec = "红细胞",
                Number = "RBC",
                Unit = "4.78",
                UnitPrice = "%",
                Description = "3.50-5.50",
                Other = " "
            });
            m_orderExample.m_orderDetails.Add(new OrderDetail
            {
                Sku = "6",
                Spec = "血红蛋白",
                Number = "HGB",
                Unit = "149.0",
                UnitPrice = "%",
                Description = "120-160",
                Other = " "
            });

        }

        public static OderData m_orderExample = new OderData
        {
            OrderNo = PublicData.name,
            CustomerName = PublicData.sex,
            ShipAddress = PublicData.age,
            Express = "门诊",
            Freight = PublicData.ID,
        };   
    }

这是第二个类 

 public class OderData
    {
        public string OrderNo { get; set; }
        public string CustomerName { get; set; }
        public string ShipAddress { get; set; }
        public string Express { get; set; }
        public string Freight { get; set; }
      
        public List<OrderDetail> OrderDetails
        {
            get
            {
                return m_orderDetails;
            }
        }
        public List<OrderDetail> m_orderDetails = new List<OrderDetail>();

       }


    public class OrderDetail
    {
        public string Sku { get; set; }
        public string Spec { get; set; }
        public string Number { get; set; }
        public string Unit { get; set; }
        public string UnitPrice { get; set; }
        public string Description { get; set; }
        public string Other { get; set; }
    }

下面这个是函数的调用，其中蓝色的就是我不理解的地方，第一次执行这个函数时，这个对象里面的变量的值可以赋值，之后就不能改变了，这是什么原因？

FlowDocument doc = PrintPreviewWindow.LoadDocumentAndRender("OrderDocument.xaml", GlobalData.m_orderExample, new OrderDocumentRender());

这个是上面这个函数的定义：

 1 public static FlowDocument LoadDocumentAndRender(string strTmplName, Object data, IDoucumentRender renderer = null)
 2         {
 3             FlowDocument doc = (FlowDocument)Application.LoadComponent(new Uri(strTmplName, UriKind.RelativeOrAbsolute));
 4             doc.PagePadding = new Thickness(50);
 5             doc.DataContext = data;
 6             if (renderer != null)
 7             {
 8                 renderer.Render(doc, data);
 9             }
10             return doc;
11         }