How to display a popup control in asp.net

// From code behind
protected void Button1_Click(object sender, EventArgs e)
{
   ModalPopupExtender1.Show();
}

yes.you can display it using c#.net;
first you create a object for it and assign properties like is,target control id,popup control id etc.,
call the method show method using this object.
ex:
modalpopupextender pop = new modalpopupextender.
pop.id;
pop.show();

i think this will solve your requirement.