คลิกปุ่มแล้วเก็บข้อมูลลงใน datatable แล้วไปแสดงใน gridview แต่มันเก็บได้แถวเดียวอะครับ แถวมันไม่เพิ่มไปเรื่อยๆ

DataTable dt = new DataTable();

			if (this.Session["Datatable"] == null) {
				
				dt.Columns.Add("P_ID");
				dt.Columns.Add("P_Name");
				dt.Columns.Add("P_Price");
				dt.Columns.Add("P_Quantity");
				this.Session["Datatable"] = dt;
			}
			else {
				dt = (DataTable)this.Session["Datatable"];
			}
			
			if (IsPostBack && RequestHDAddCart != "") {

				string sql;

				sql = "SELECT * FROM T_Product WHERE P_ID = '" + RequestHDAddCart + "'";
				SqlDataAdapter da = new SqlDataAdapter(sql, db);
				DataSet ds = new DataSet();
				da.Fill(ds);

				DataRow dr = dt.NewRow();
				dr["P_ID"] = RequestHDAddCart;
				dr["P_Name"] = ds.Tables[0].Rows[0]["P_Name"].ToString();
				dr["P_Price"] = ds.Tables[0].Rows[0]["P_Price"].ToString();
				dr["P_Quantity"] = 1;

				dt.Rows.Add(dr);

				GridView1.DataSource = dt;
				GridView1.DataBind();

			}

DataTable dt = new DataTable();
DataRow dr = null;

//*** Column ***'
dt.Columns.Add("CustomerID");
dt.Columns.Add("Name");
dt.Columns.Add("Email");
dt.Columns.Add("CountryCode");
dt.Columns.Add("Budget");
dt.Columns.Add("Used");

//*** Rows ***'
dr = dt.NewRow();
dr["CustomerID"] = "C001";
dr["Name"] = "Win Weerachai";
dr["Email"] = "[email protected]";
dr["CountryCode"] = "TH";
dr["Budget"] = "1000000";
dr["Used"] = "600000";
dt.Rows.Add(dr);

Session("myCart") = dt;

DataTable dt = (DataTable)Session("myCart");
DataRow dr = null;

//*** Rows ***'
dr = dt.NewRow();
dr["CustomerID"] = "C003";
dr["Name"] = "Tony Stark";
dr["Email"] = "[email protected]";
dr["CountryCode"] = "US";
dr["Budget"] = "3000000";
dr["Used"] = "600000";
dt.Rows.Add(dr);

Session("myCart") = dt;

this.GridView.DataSource = (DataTable)Session("myCart");
this.DataBid();