On ADD TO CART button show item from Datalist to Gridview.

C# Code

protected void AddToCart(object sender, EventArgs e)
    {
        string id = "";
        Button btn = (Button)sender;
        DataListItem item = (DataListItem)btn.NamingContainer;
        id += (item.FindControl("ItemID") as Label).Text;
        string query = "SELECT * FROM tablename where Id IN(" + id.TrimEnd(',') + ")";
        DataTable dt = GetData(query);
        if (Session["dt"] == null)
        {
            Session["dt"] = dt;
            GridView1.DataSource = dt;
            GridView1.DataBind();
            lblTotalItems.Text = "" + (GridView1.DataSource as DataTable).Rows.Count;
        }
        else
        {
            DataTable dt1 = Session["dt"] as DataTable;
            for (int i = 0; i < dt.Rows.Count; i++)
            {
                dt1.Rows.Add(dt.Rows[i][0], dt.Rows[i][1], dt.Rows[i][2], dt.Rows[i][3]);
            }
            GridView1.DataSource = dt1;
            lblTotalItems.Text = " " + (GridView1.DataSource as DataTable).Rows.Count;
            GridView1.DataBind();
        }
    }