protected void DropDownList_Change(object sender, EventArgs e)
        {
            int count = 0;
            foreach (User u in udc.Users)
            {
                count++;
            }

            String value = user_list.SelectedValue;
            int index = user_list.SelectedIndex;
            user_list.Items.Clear();
            foreach (User u in udc.Users)
            {
                if (u.Aktywny == true)
                {
                    ListItem li = new ListItem(u.Nazwa);
                    user_list.Items.Add(li);
                }
            }
            if (index >= count)
            {
                index = 0;
            }
            user_list.SelectedIndex = index;

        }