public static DataTable SqlDataTable(string sql, IDictionary<string, object> values)
    {
        using (SqlConnection conn = new SqlConnection(GetConnectionString))
        using (SqlCommand cmd = new SqlCommand(sql, conn))
        {
            conn.Open();

            foreach (var item in values)
            {
                cmd.Parameters.AddWithvalue("@" + item.Key, item.Value);  // error
            }
          .......
          ........