Data bind Dropdownlist in asp.net

Fill drop down list dynamically using c# in asp.net.We need to write method inside IspostBack condition to avoid multiple times binding.Below Example using Dictionary collection to fill asp.net dropdownlist,we can fetch data from database as well.

protected void Page_Load(object sender, EventArgs e)

   {

       firstName = "vinod";

       if (!IsPostBack)

       {

           fillDropdownlist();

       }

   }

   public void fillDropdownlist()

   {

       Dictionary<int, string> obj = new Dictionary<int, string>();

       obj.Add(1, "vinod");

       obj.Add(2, "hari");

       

       DropDownList1.DataValueField = "Key";

       DropDownList1.DataTextField = "Value";

       DropDownList1.DataSource = obj;

       DropDownList1.DataBind();

     DropDownList1.Items.Insert(0, "--Select--");

   }

    City : 

    <asp:DropDownList ID="DropDownList1" runat="server">                                   

    </asp:DropDownList>