How to store a control in Viewstate [Resolved]

I'm trying to store a dropdownlist control in viewstate. Not the selected value, but the control.

protected void ddl_selectedindexchanged(object sender, EventArgs e)
{
ViewState["sender"] = (dropdownlist)sender;
}

That gives an error as dropdownlist is not serializable. So as someone suggested, I tried this

ViewState["sender"] = ddl.ID;

but when trying to retrieve it, the value of ddl is null.

dropdownlist ddl = (dropdownlist)findcontrol(ViewState["sender"].tostring());

what is the right way to do this.

[Resolved]