ExecuteScalar throws NullReferenceException

Question

This code throws a NullReferenceException when it calls ExecuteScalar:

selectedPassengerID = 0;

//SqlCommand command = GenericDataAccess.CreateCommand();


// 2nd test
string connectionString = "";
SqlConnection conn;

connectionString = ConfigurationManager.
    ConnectionStrings["ConnST-MHM"].ConnectionString;
conn = new SqlConnection(connectionString);
SqlCommand command = new SqlCommand();
command.CommandType = CommandType.StoredProcedure ;
command.Connection = conn;
command.CommandText = "SearchForPassenger";

SqlParameter param;

param = command.CreateParameter();
param.ParameterName = "@name";
param.Value = pName; // Session[""];
param.DbType = DbType.String;
command.Parameters.Add(param);

param = command.CreateParameter();
param.ParameterName = "@flightDate";
param.Value = date; 
param.DbType = DbType.String;
command.Parameters.Add(param);

param = command.CreateParameter();
param.ParameterName = "@ticketNo";
param.Value = ticketNumber; 
param.DbType = DbType.Int32;
command.Parameters.Add(param);

int item;

command.Connection.Open();
item = (int)command.ExecuteScalar();

Answer 1

I have encapsulated most of my SQL logic in a DAL. One of these DAL methods pulls scalar Ints using the following logic. It may work for you:

  object temp = cmnd.ExecuteScalar();
  if ((temp == null) || (temp == DBNull.Value)) return -1;
  return (int)temp;

I know that you have entered a lot of code above but I think that this is really the essence of your problem. Good luck!

Answer 2

ExecuteScalar returns null if no records were returned by the query (eg when your SearchForPassenger stored procedure returns no rows).

So this line:

item = (int) command.ExecuteScalar();

Is trying to cast null to an int in that case. That'll raise a NullReferenceException.

As per Mark's answer that just poppped up, you need to check for null:

object o = command.ExecuteScalar();
item = o == null ? 0 : (int)o;