%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
!DOCTYPE html>
html xmlns="http://www.w3.org/1999/xhtml">
head runat="server">
title>/title>
/head>
body>
form id="form1" runat="server">
asp:DropDownList ID="DropDownListYesNo" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
/asp:DropDownList>
asp:TextBox ID="TextBox1" runat="server" Enabled="true">/asp:TextBox>
/form>
/body>
/html>

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
Data_Binding();
}
}
private void Data_Binding()
{
this.DropDownListYesNo.DataSource = GetData().Select(yn => new { value = yn }).ToList();
this.DropDownListYesNo.DataTextField = "value";
this.DropDownListYesNo.DataBind();
}
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
switch (this.DropDownListYesNo.SelectedItem.Text)
{
case "YES":
this.TextBox1.Enabled = true;
break;
case "NO":
this.TextBox1.Enabled = false;
this.TextBox1.Text = string.Empty;
break;
}
}
private Liststring> GetData()
{
Liststring> yn = new Liststring>();
yn.Add("YES");
yn.Add("NO");
return yn;
}
}