I am trying create simple shopping Cart (Project) in ASP.NET but having problems. Please help me out, Thanks [on hold]

using System;
using System.Data;
using System.Configuration;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;

namespace WebApplication1
{
    public partial class Cart : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                if (Request.QueryString("ID") = null)
                {
                    Add(Convert.ToInt32 (Request.QueryString[""]));
                    }
                    GridView1.DataSource = GetDataSetCart();
                    GridView1.DataBind ();
            }
        }

private object GetDataSetCart()
{
        throw new NotImplementedException();
}

private void Add(int p)
{
        throw new NotImplementedException();
}

        protected void Button2_Click(object sender, EventArgs e)
        {
            Response.Redirect("Home.aspx");
        }

        public DataSet GetDataSet()
        {
            if (Session["Cart"] == null)
            {
            DataSet ds = new DataSet();
            DataColumn Key = new DataColumn("ID_PRODUCT", System.Type.GetType("System.Int32"), "");
            DataTable dt = new DataTable ("CART");
            dt.Columns.Add(colID);
            dt.Columns.Add("NAME_PRODUCT", System.Type.GetType("System.String"), "");
            dt.Columns.Add("QUANTITY", System.Type.GetType("System.Int32"), "");
            dt.Columns.Add("VALUE", System.Type.GetType("System.Double"), "");
            dt.Columns.Add("SUBTOTAL", System.Type.GetType("System.Double"), "QUANTITY*VALUE");
            dt.Columns.Add("TOTAL", System.Type.GetType("System.Double"), "SUM(SUBTOTAL)");
            //key field
            DataColumn[] keys = new DataColumn[1];
            keys[0] = colID;
            dt.PrimaryKey = keys;
            Session["Cart"] = ds;
            return ds;
            }
            else
            {
                return (Session["Cart"] as DataSet);
            }
        }


        public DataColumn colID { get; set; }
    }

    private void Add(int quantity)
{
    //search database
    SqlConnection con = new SqlConnection(ConfigurationManager.WebShopConnectionStrings"WebShopConnectionString".ToString());
    String aSQL = "select NAME_PRODUCT, VALUE_PRODUCT from PRODUCT where ID_PRODUCT=" + Request.QueryString"ID".ToString();
    try 
{
    //open the connection
    con.Open();
    SqlCommand cmd = new SqlCommand (aSQL, con);
    SqlDataReader reader = cmd.ExecuteReader();
    reader.Read();
    //get data of DataSet in Session
    DataTable dt = GetDataSet().Table 0;
    //find Product
    DataRow row = dt.Rows.Find(Request.QueryString "ID");
    //add line
    if (row ==null)
{
    row = dt.NewRow();
    row"ID_PRODUCT" = Request.QueryString "ID";
    row"NAME_PRODUCT" = reader"NAME_PRODUCT";
    row"QUANTITY" = quantity;
    row"VALUE" = reader"VALUE_PRODUCT";
    dt.Rows.Add(row);
}
else 
{
int qtd = Convert.ToInt32(row "QUANTITY");
qtd = qtd + quantity;
row "QUANTITY" = qtd;
}
}

And i got this errors

Error 1 Expected class, delegate, enum, interface, or struct Line 76 Column 13

private void Add(int quantity) Error 2 Expected class, delegate, enum, interface, or struct Line 79 Column 29

SqlConnection con = new SqlConnection(ConfigurationManager.WebShopConnectionStrings"WebShopConnectionString".ToString()); String aSQL = "select NAME_PRODUCT, VALUE_PRODUCT from PRODUCT where ID_PRODUCT=" + Request.QueryString"ID".ToString(); try Error 3 Expected class, delegate, enum, interface, or struct Line 85 Column 26

SqlCommand cmd = new SqlCommand (aSQL, con); Error 4 Non-invocable member 'System.Web.HttpRequest.QueryString' cannot be used like a method. Line 22 Column 29 if (Request.QueryString("ID") = null)