ASP.NET

private IEnumerable<Control> GetAllContentPlaceHolders()
{
    Queue<Control> controls = new Queue<Control>();
    foreach (Control c in Page.Controls)
    {
        controls.Enqueue(c);
    }
    while (controls.Count > 0)
    {
        Control c = controls.Dequeue();
        if (c is ContentPlaceHolder)
        {
            yield return c;
        }
        foreach (Control ic in c.Controls)
        {
            controls.Enqueue(ic);
        }
    }
}

foreach (Control c in GetAllContentPlaceHolders())
{
    AddControls(c.Controls, controlList);
}

CheckBox ckbox = DataGrid1.FindControl("yourcheckboxcontrolId");
chkbox.checked = true;