c# - Business Rule - Specified cast is not valid -

i have searched answer question, , can't seem find situation, or haven't grasped yet. working on business rule company's business software , receiving "specified cast not valid" error. know has char variable and/or null, can't figure out how fix or why i'm getting error (it's simple).

below code rule. supposed compare unit_price price1, both fields in table, , replace unit_price price1 if price1 higher, , every row in table.

i had code working until tried add class4. class4 specifies items locked prices. used prevent changing prices of items locked prices. class4 char[8] field in table, , has 2 values, locked , null. issues began.

i have removed references program legal purposes.

forgive me if code messy or inefficient or incorrect. i'm new c# , have been guessing part.

    public class pricecheck : program.extensions.businessrule.rule     {         decimal unit = 0m;         decimal price1 = 0m;         string class4;         datatable datatable1;         boolean bdebugmode = true;         int triggerrow;          public override ruleresult execute()         {             ruleresult result = new ruleresult();             result.success = true;              if (bdebugmode) messagebox.show("starting pricecheck"); //messagebox debugging purposes, shows pricecheck has begun              try             {                 if (this.session.multirow)                 {                     datatable1 = this.data.set.tables["table"]; //set table information pulled , sent                     triggerrow = this.data.triggerrow;                      foreach (datarow row in (internaldatacollectionbase)datatable1.rows) //begin loop rows in datatable1                     {                         unit = datarowextensions.field<decimal>(row, "unit_price"); //retrieve unit price current row                         price1 = datarowextensions.field<decimal>(row, "inv_mast_price1"); //retrieve price1 current row                         class4 = convert.tostring(datarowextensions.field<char?>(row, "class_id4")); //retrives class4 current row                         if (bdebugmode) messagebox.show("unit price " + unit + "\n" + "price 1 " + price1 + "\n" + "class 4" + class4, "debug", messageboxbuttons.okcancel, messageboxicon.information); //messagebox debugging, shows values in unit, list, , class4                          if (class4 == "locked")                         {                             if (bdebugmode) messagebox.show("class 4 locked" + "\n" + "unit price remain same!" + "\n" + "unit price " + unit, "class 4 locked", messageboxbuttons.ok, messageboxicon.exclamation);                         }                         else                         {                             if (unit > 0) //if unit null(0), rule fail, here prevent that...might change item id, depending on if items have 0 unit price , price 1                             {                                 if (bdebugmode) messagebox.show("unit > 0", "unit check", messageboxbuttons.ok, messageboxicon.information); //messagebox debugging, shows if statement has been reached                                  if (unit < price1) //beginning of calculations                                 {                                     if (bdebugmode) messagebox.show("price 1 " + price1, "debug", messageboxbuttons.okcancel, messageboxicon.exclamation); //shows price1 before putting unit_price field                                     datarowextensions.setfield<decimal>(row, "unit_price", price1); //assigns price1 unit_price field                                 }                                 else                                 {                                     if (bdebugmode) messagebox.show("unit price" + unit, "debug", messageboxbuttons.okcancel, messageboxicon.exclamation); //shows unit price before assigning unit_price field                                     datarowextensions.setfield<string>(row, "unit_price", convert.tostring(unit)); //assigns unit unit_price field                                 }                             }                          }                     }                 }             }             catch (exception ex)             {                 result.success = false;                 result.message = ex.message;             }             if (bdebugmode) messagebox.show("ending price check"); //messagebox debugging, shows end of calculations before result returned             return result;         }         public override string getdescription()         {             return "checks prices keep unit price above minimum.";         }         public override string getname()         {             return "price check";         }     }  }