Why do I get the result zero when I try to get the width of a DropDownList control?

After I click button1, it display 0, why?
How can get correct width of a DropDownList control? Thanks!
 
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %>
<!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>
</head>
<body>
    <form id="form1" runat="server">
    <div>   
        <asp:Button ID="Button1" runat="server" Text="Button" />
        <asp:DropDownList ID="DropDownList1" runat="server"&g ...