将查询结果返回到列表c＃

Question

I am working on a project where I am phasing out Entity framework from an existing system.

I got

public List<GSP> GetOwnAirline()
        { 
            var res = from own in entity.GSP where own.Description == "Own Airline" select own;
            return res.ToList();
        }

To bypass this i did

   public List<GSP> GetOwnAirline()
        {

            string get_ = "SELECT * FROM " + Adm.schema_ + "Adm.GSP WHERE Description = 'Own Airline'";
            ccs = new SqlConnection(Adm.COnnectionString);
            //convey transaction to db
            cmd = ccs.CreateCommand();
            ccs.Open();
            cmd.CommandText = get_;
            var res =cmd.ExecuteScalar();
            ccs.Close();

            return res.ToList();
        }

But the.ToList seems not to be recognised in this situation. Where did i go wrong guys ?

Answer 1

ExecuteScalar returns a scalar - like an integer value. It can't be converted to a list.

If you're using Ado.Net you have to either return a DataTable or a DataReader to get the result. There is no way to return a List directly.

Answer 2

There are few problems with your code.

1. ExecuteScalar returns a single value as pointed out by TGH.
2. Basically, you use ToList when you have some sort of Array or IEnumerable. As, in your case, it is a single int value, so, it can't be applied here.
3. You can use ExecuteReader here to accomplish your goal.

Answer 3

Try this:

    public List<GSP> GetOwnAirline()
    {
        List<GSP> lstGSP = new List<GSP>();
        using (ccs = new SqlConnection(Adm.COnnectionString))
        {
            string get_ = "SELECT * FROM " + Adm.schema_ + "Adm.GSP WHERE Description = 'Own Airline'";
            using (SqlCommand cmd = new SqlCommand(get_, ccs))
            {

                ccs.Open();
                SqlDataReader rdr = cmd.ExecuteReader(CommandBehavior.CloseConnection);
                while (rdr.Read())
                {
                    GSP objGSP = new GSP();
                    Fill(objGSP, rdr);//method
                    lstGSP.Add(objGSP);
                }
                ccs.Close();
            }
        }

        return lstGSP;
    }



public static void Fill(object LogicObject, System.Data.SqlClient.SqlDataReader SqlDataReader)
        {
            Dictionary<string, PropertyInfo> props = new Dictionary<string, PropertyInfo>();
            foreach (PropertyInfo p in LogicObject.GetType().GetProperties())
                props.Add(p.Name, p);
            //foreach (System.Data.DataColumn col in Row.Table.Columns)

            for (int i = 0; i < SqlDataReader.FieldCount; i++)
            {
                string name = SqlDataReader.GetName(i);
                if (SqlDataReader[name] != DBNull.Value && props.ContainsKey(name))
                {
                    object item = SqlDataReader[name];
                    PropertyInfo p = props[name];
                    if (p.PropertyType.IsGenericType && p.PropertyType.GetGenericTypeDefinition() == typeof(Nullable<>))
                    {
                        if (p.PropertyType != SqlDataReader.GetFieldType(i))
                            item = Convert.ChangeType(item, p.PropertyType.GetGenericArguments()[0]);

                    }
                    else
                    {
                        if (p.PropertyType != SqlDataReader.GetFieldType(i))
                            item = Convert.ChangeType(item, p.PropertyType);
                    }

                    p.SetValue(LogicObject, item, null);
                }

            }
        }

Answer 4

Exucute scalar will return you scalar. Therefore you will have to create the object of type GSP and add to list. an alternate way of doing this can be

public List<GSP> GetOwnAirline()
    {

        string get_ = "SELECT * FROM " + Adm.schema_ + "Adm.GSP WHERE Description = 'Own Airline'";
        ccs = new SqlConnection(Adm.COnnectionString);
        //convey transaction to db
        cmd = ccs.CreateCommand();
        ccs.Open();
        cmd.CommandText = get_;
        var res =cmd.ExecuteScalar();
        ccs.Close();
        List<GSP> gsp = new List<GSP>();
        GSP temp = new GSP();
        temp.PropertyName = res;
        gsp.Add(temp);
        return gsp;
    }