sql parameter changing Error

Hii

This is my working Code

 strQuery = @"SELECT FROM IBK_CO_USERS where upper(user_id)= upper(:userid) AND co_id= :Com_Id";
 {

                    ocommand = new OracleCommand();
                    if (db.GetConnection().State == ConnectionState.Open)
                    {
                        ocommand.CommandText = strQuery;
                        ocommand.Connection = db.GetConnection();
                        
                        ocommand.Parameters.Add("userid", OracleDbType.Varchar2);
                        ocommand.Parameters["userid"].Value = userID;
                        ocommand.Parameters.Add("Com_Id", OracleDbType.Varchar2);
                        ocommand.Parameters["Com_Id"].Value = Comid;
                        odatareader = ocommand.ExecuteReader();
                        odatareader.Read();

but when i change parameter like this 

 ocommand.Parameters.Add("Com_Id", OracleDbType.Varchar2);
 ocommand.Parameters["Com_Id"].Value = Comid;
 ocommand.Parameters.Add("userid", OracleDbType.Varchar2);
 ocommand.Parameters["userid"].Value = userID;

But it is not working should i set parameter according to query  parameter .why is ti not working changing paramter ??????