error index out of range exception was unhandled by user code

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You can for example enter the length and width of the web (VS.NET2005), when it runs out of range index error exception was unhandled by user code for chieudai

public class Tinh
    {
         private Int32 _quanhuyen;
         private String _diachiNha;
         private Double _chieudai;
         private Double _chieurong;         
         private Double _dientichDat;
    }
   
      public Int32 Quanhuyen
       {
            get { return _quanhuyen; }
            set { _quanhuyen = value; }
        }
 
       public String DiachiNha
        {
             get { return _diachiNha; }
             set { _diachiNha = value; }
         }

         public Double Chieudai
         {
             get { return _chieudai; }
             set { _chieudai = value; }
         }

         public Double Chieurong
         {
             get { return _chieurong; }
             set { _chieurong = value; }
         }

        public Double DientichDat
        {
           get { return _dientichDat; }
           set { _dientichDat = value; }
        }

    private Tinh DienTich(IDataReader dr, bool GetTotalRecords)
    {
       Tinh obj = new Tinh();
                  obj.Quanhuyen = (dr["Quanhuyen"] is DBNull) ? Int32.MinValue : (Int32)dr["Quanhuyen"];
                  obj.DiachiNha = (dr["DiachiNha"] is DBNull) ? String.Empty : (String)dr["DiachiNha"];
                  // bottom  line this error index out of range exception was unhandled by user code
                  obj.Chieudai = (dr["Chieudai"] is DBNull) ? Double.MinValue : (Double)dr["Chieudai"];   
                  obj.Chieurong = (dr["Chieurong"] is DBNull) ? Double.MinValue : (Double)dr["Chieurong"];
             obj.DientichDat = (dr["DientichDat"] is DBNull) ? Double.MinValue :  (Double)dr["DientichDat"];
                  if (GetTotalRecords)
                     obj.TotalRecords = (dr["TotalRecords"] is DBNull) ? -1 : Globals.ConvertToInt32(dr["TotalRecords"]);

        return obj;
        }

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