Saplin Dropdown checkboxes default selectall

I'm trying to have the "Select All" checkbox on the dropdowncheckbox to be selected by default on page load, No luck so far. Any help would be appreciated. I used this on page load, but it's not working.

    foreach(ListItem item in ddChkDiary.Items)
    {
        item.Selected = true;
    }

when I debug, it tells me that there are no items in the dropdowncheckbox(ddChkDiary)