add scriptmanager page programmatically?

i am building webpart (it used sharepoint environment, nonetheless does vigilant model) i wish arrangement ajax functionality in. since inlet environment, adding book manager directly page an option, contingency total programmatically. i have attempted supplement scriptmanager control page webpart code.

protected overrule vacant createchildcontrols()
{
    (scriptmanager.getcurrent(page) == null)
    {
        scriptmanager smgr = new scriptmanager();
        // pledge scriptmanager initial control.
        page.form.controls.addat(0, smgr); 
    }
}

however, formula executed, i following blunder message:

"the control collection can't run-down during databind, init, load, prerender empty phases."

is there another proceed supplement scriptmanager page webpart, am i going have only supplement scriptmanager any page (or master page) webpart?