how to use dropdownlist in asp.net
front end /user inter face 
="ProductOrderByCustomer.aspx
<%@ Page Language="C#"
AutoEventWireup="true"
CodeFile="ProductOrderByCustomer.aspx.cs"
Inherits="ProductOrderByCustomer"
%>
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style type="text/css">
        .style1
        {
            width:
100%;
        }
        .style2
        {
            height:
23px;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <table class="style1">
            <tr>
                <td align="center" colspan="2">
                    PRODUCT / SERVICES ENQUIRE</td>
            </tr>
            <tr>
                <td class="style2">
                    Customer Name :</td>
                <td class="style2">
                    <asp:TextBox ID="TextBox1" runat="server" Width="216px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    Email Id :</td>
                <td>
                    <asp:TextBox ID="TextBox2" runat="server" Width="216px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    Mobile No :</td>
                <td>
                    <asp:TextBox ID="TextBox3" runat="server" Width="139px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    Land Line :</td>
                <td>
                    <asp:TextBox ID="TextBox4" runat="server" Width="138px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    State    City    Area    :</td>
                <td>
                    <asp:DropDownList ID="DropDownList1" runat="server">
                        <asp:ListItem>-Slelect
State-</asp:ListItem>
                        <asp:ListItem>Bihar</asp:ListItem>
                        <asp:ListItem>Odisha</asp:ListItem>
                        <asp:ListItem>West
Bengal</asp:ListItem>
                        <asp:ListItem>Jharkhand</asp:ListItem>
                    </asp:DropDownList>
    
                    <asp:DropDownList ID="DropDownList2" runat="server">
                        <asp:ListItem>-Select
City-</asp:ListItem>
                        <asp:ListItem>Patna</asp:ListItem>
                        <asp:ListItem>Darbhanga</asp:ListItem>
                        <asp:ListItem>Madhubani</asp:ListItem>
                        <asp:ListItem>BBSR</asp:ListItem>
                        <asp:ListItem>Cuttack</asp:ListItem>
                        <asp:ListItem>Balasore</asp:ListItem>
                        <asp:ListItem>Bhadrak</asp:ListItem>
                    </asp:DropDownList>
    
                    <asp:DropDownList ID="DropDownList3" runat="server">
                        <asp:ListItem>-Select
Area-</asp:ListItem>
                        <asp:ListItem>Link
Road</asp:ListItem>
                        <asp:ListItem>Badambadi</asp:ListItem>
                    </asp:DropDownList>
                </td>
            </tr>
            <tr>
                <td>
                     </td>
                <td>
                             
                </td>
            </tr>
            <tr>
                <td>
                    Product / Services :</td>
                <td>
                      
Catagory :
                    <asp:DropDownList ID="DropDownList4" runat="server" Height="20px" 
                        onselectedindexchanged="DropDownList4_SelectedIndexChanged"
onclick="DropDownList4_SelectedIndexChanged"
                        Width="160px" AutoPostBack="True">
                    </asp:DropDownList>
 <asp:Button ID="Button5" runat="server" onclick="Button5_Click" Text="Description" />
 <asp:TextBox ID="TextBox8" runat="server" Height="57px" TextMode="MultiLine" Width="227px" 
                        ></asp:TextBox>
  
                    <asp:Button ID="Button4" runat="server" Text="Select" />
                </td>
            </tr> 
            <tr>
                <td>
                     </td>
                <td>
                               
                    <asp:TextBox ID="TextBox5" runat="server" Height="60px" TextMode="MultiLine" 
                        Width="468px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    Special Interest :</td>
                <td>
                    <asp:TextBox ID="TextBox6" runat="server" Height="53px" TextMode="MultiLine" 
                        Width="305px"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                     </td>
                <td>
                    <br />
                    <asp:Button ID="Button1" runat="server" Height="33px" onclick="Button1_Click" 
                        Text="Submit" Width="92px"
/>
                                     
                </td>
            </tr>
        </table>
    </div>
    </form>
</body>
</html>
back end /database  
ProductOrderByCustomer.aspx.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
using System.Web.Security;
using System.Net;
using System.Net.Mail;
public partial
class ProductOrderByCustomer
: System.Web.UI.Page
{
    SqlConnection
con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbConnection"].ConnectionString);
    protected void Page_Load(object
sender, EventArgs e)
    {
        con.Open();
        if
(!IsPostBack)
{
BindContrydropdown();
}
}
    protected void BindContrydropdown()
    {
        SqlCommand
cmd = new SqlCommand("select DISTINCT Catagory from ProductByAdmin",
con);
        SqlDataReader
dr = cmd.ExecuteReader();
        if
(dr.Read() == true)
        {
           DropDownList4.DataTextField = "Catagory";
            DropDownList4.Items.Add( dr["Catagory"].ToString());
           //
ListBox1.DataTextField = "Catagory";
           //
ListBox1.Items.Add(dr["Catagory"].ToString());
        }
        dr.Close();
    }
    protected void Button1_Click(object
sender, EventArgs e)
    {
    }
    protected void DropDownList4_SelectedIndexChanged(object sender, EventArgs
e)
    {
    }
    protected void Button5_Click(object
sender, EventArgs e)
    {
        if
(TextBox8.Text == "" ||
TextBox8.Text == null)
        {
            TextBox8.Text =
DropDownList4.SelectedItem.ToString();
        }
        else
        {
            TextBox8.Text = TextBox8.Text + "\n" +
DropDownList4.SelectedValue.ToString();
        }
    }
}
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