c# - Adding list in list -

i have class sellstatement

public class sellstatement {     public long billno     public datetime paymentdate;     public list<string>  productname;     public list<double> quantity;     public list<double> rateperquantity; } 

when trying access function getsaledetails

public exception getsaledetails(list<sellstatement> lss1,string query)     {         try         {             (int = 0; < lss1.toarray().length; i++)             {                 query = "select * [product details] [bill no]=@billno";                 com = new sqlcecommand(query, con);                 con.open();                 com.parameters.addwithvalue("@billno",lss1[i].billno);                 sdr = com.executereader();                 while (sdr.read())                 {                     lss1[i].productname.add(sdr.getstring(1));//exception line                     lss1[i].quantity.add(sdr.getdouble(2));                     lss1[i].rateperquantity.add(sdr.getdouble(3));                        }             }             con.close();             return null;         }         catch (exception e)         {             con.close();             return e;         }     } 

null reference exception comes atlss1[i].productname.add(sdr.getstring(1));.i think error might because of null value in @ sdr.getstring(1) checked has value .my friend told me can't change function argument value try copy 1 list other .

 list<sellstatement> lss1 = new list<sellstatement>() ;             lss1.addrange(lss); 

but doesn't me out. not able figure out what's wrong while adding element.

if showed complete sellstatement class in question, reason clear:
never initialized productname, quantity , rateperquantity. null , that's exception telling you.

to fix it, change class this:

public class sellstatement {     public long billno     public datetime paymentdate;     public list<string> productname = new list<string>();     public list<double> quantity = new list<double>();     public list<double> rateperquantity = new list<double>(); } 

please note goes against normal c# design guidelines shouldn't have public fields. consider redesigning class so:

public class sellstatement {     list<string> _productname = new list<string>();     list<double> _quantity = new list<double>();     list<double> _rateperquantity = new list<double>();      public long billno {get; set;}     public datetime paymentdate  {get; set;}     public list<string> productname { { return _productname; } }     public list<double> quantity { { return _quantity; } }     public list<double> rateperquantity { { return _rateperquantity; } } }