ASP.NET

if(!IsPostBack)
{
foreach (short vKey in city.Keys)
                {
                    dropdownlist1.Items.Add(city[vKey].cityName);
                    dropdownlist1.DataTextField = city[vKey].cityName;
                    dropdownlist1.DataValueField = (city[vKey].id_city).ToString();
                    
                }
}

protected void Page_Load(object sender, EventArgs e)
{
	if (!IsPostBack)
	{
		foreach (short vKey in city.Keys)
		{
			dropdownlist1.Items.Add(city[vKey].cityName);
			dropdownlist1.DataTextField = city[vKey].cityName;
			dropdownlist1.DataValueField = (city[vKey].id_city).ToString();
		}
	}
}

dropdownlist1.Items.Add(new ListItem(city[vKey].cityName, (city[vKey].id_city).ToString()));

private void ListControlCollections()
{
    ArrayList controlList = new ArrayList();
    AddControls(Page.Controls,controlList);

    foreach (string str in controlList)
    {
        Response.Write(str + "<br/>");
    }
    Response.Write("Total Controls:" + controlList.Count);
}

private void AddControls(ControlCollection page,ArrayList controlList)
{
    foreach (Control c in page)
    {
        if (c.ID != null)
        {
            controlList.Add(c.ID);
        }

        if(c.HasControls())
        {
            AddControls(c.Controls, controlList);
        }
    }
}