public StockItem(int stockItemId, Supplier supplier, string stockItemName, string description, float costPrice, string category)
 {
     this.StockItemId = stockItemId;
     this.supplier = supplier;
     this.StockItemName = stockItemName;
     this.Description = description;
     this.CostPrice = costPrice;
     this.Category = category;
 }
 public static IStockOrder GetStockOrder(int orderId, DateTime date, Supplier supplierId)
 {
     if (newStockOrder != null)
         {
             return newStockOrder;
         }
         else
         {
             return new StockOrder(orderId,date,supplierId);
         }
 }
 public static IStockItem GetStockItem(int stockItemId, Supplier supplier, string stockItemName, string description, float costPrice, string category)
 {
     if (newStockItem != null)
     {
         return newStockItem;
     }
     else
     {
         return new StockItem(stockItemId, supplier,stockItemName,description,costPrice,category);
     }
 }
 public StockOrder(int orderId, DateTime date, Supplier supplierId)
 {
     this.orderId = orderId;
     this.date = date;
     this.supplierId = supplierId;
 }