Populate textboxes from a dropdownlist

Thank you so much for your quick and very helpful answer - it works perfectly!

Thank you for the help. It does not fully work for me though. I am getting the error message "The name 'Top_Site_NumDropDown' does not exist in the current context". Below is your code changed to reflect my fields etc.. Have I missed something?


ASPX PAGE

<%@ Page Title="Add New Project" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="True" CodeBehind="ProjectReviewAdd.aspx.cs" Inherits="Project_Review.ProjectReviewAdd" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>



CS CODE BEHIND

using System;
using System.Collections;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Data;
using System.Data.SqlClient;
using System.Web.Services.Protocols;
using System.Xml.Linq;
using System.Web.Services;

 public partial class _Default : System.Web.UI.Page
        {
            string strConnString = ConfigurationManager.ConnectionStrings["SqlDataSourceSite"].ConnectionString;
            string str;
            SqlCommand com;
            protected void Page_Load(object sender, EventArgs e)
            {
                Top_Site_NumDropDown.AutoPostBack = true;
                SqlConnection con = new SqlConnection(strConnString);

                if (!IsPostBack)
                {
                    Top_Site_NumDropDown.Items.Add("Site_Num");
                    con.Open();
                    str = "select Site_Num, Site_Name, Site_Address, Site_County, Site_Post_Code from SCSite";
                    com = new SqlCommand(str, con);
                    SqlDataReader reader = com.ExecuteReader();
                    while (reader.Read())
                    {
                        Top_Site_NumDropDown.Items.Add(reader["Site_Num"].ToString());
                    }
                    reader.Close();
                    con.Close();
                }
            }

            protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
            {
                SqlConnection con = new SqlConnection(strConnString);
                con.Open();
                str = "select Site_Num, Site_Name, Site_Address, Site_County, Site_Post_Code from SCSite where id='" + Top_Site_NumDropDown.SelectedItem.Text + "'";
                com = new SqlCommand(str, con);
                SqlDataReader reader = com.ExecuteReader();
                while (reader.Read())
                {
                    Top_Site_NameTextBox.Text = reader["Site_Name"].ToString();

                }
                reader.Close();
                con.Close();
            }
        }

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="Dropdownlist_click_data_in_textbox._Default" %>





   


   

   

                    onselectedindexchanged="DropDownList1_SelectedIndexChanged">
       

       
   

   
   

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
namespace Dropdownlist_click_data_in_textbox
{
    public partial class _Default : System.Web.UI.Page
    {
        string strConnString = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        string str;
        SqlCommand com;
        protected void Page_Load(object sender, EventArgs e)
        {
            DropDownList1.AutoPostBack = true;
            SqlConnection con = new SqlConnection(strConnString);

            if (!IsPostBack)
            {
                DropDownList1.Items.Add("Choose id");
                con.Open();
                str = "select * from  test";
                com = new SqlCommand(str, con);
                SqlDataReader reader = com.ExecuteReader();
                while (reader.Read())
                {
                    DropDownList1.Items.Add(reader["id"].ToString());
                }
                reader.Close();
                con.Close();
            }
        }

        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection(strConnString);
            con.Open();
            str = "select * from test where id='" + DropDownList1.SelectedItem.Text + "'";
            com = new SqlCommand(str, con);
            SqlDataReader reader = com.ExecuteReader();
            while (reader.Read())
            {
                TextBox1.Text = reader["name"].ToString();
               
            }
            reader.Close();
            con.Close();
        }
    }
}