Default.aspx page Code.
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="page2.aspx.cs" Inherits="Default.aspx" %>
<!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>DropDown Demo with Textbox</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="DropDownList1" style="Width:120px" runat="server"
Enabled="False">
<asp:ListItem>santosh_k</asp:ListItem>
<asp:ListItem>Amit_K</asp:ListItem>
</asp:DropDownList>
</div>
<div>Choose Date:
<asp:TextBox ID="txtDate" style="margin-top:50px" runat="server"></asp:TextBox>
<asp:Calendar ID="Calendar1" runat="server"
onselectionchanged="Calendar1_SelectionChanged1"></asp:Calendar>
</div>
<div>
</div>
</form>
</body>
</html>
Default.aspx.cs Code Behind
protected void Calendar1_SelectionChanged1(object sender, EventArgs e)
{
txtDate.Text = Calendar1.SelectedDate.ToString();
DropDownList1.Enabled = true;
}