<%@ Page Language="C#" AutoEventWireup="true" Codevirtual="iframe.aspx.cs" 
Inherits="iframe" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script language="javascript" src="JScript.js">
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
 <title>Untitled Page</title>
</head>
<body><form id="form1" runat="server">
 <div>
 <asp:TextBox ID="TextBox1" runat="server">
 </asp:TextBox>
 <asp:TextBox ID="TextBox2" runat="server" 
 style="top: 66px; left: 10px; position: absolute; height: 22px; width: 
 128px">
 </asp:TextBox>
 <asp:Button ID="Button1" runat="server" Text="Button" 
 style="top: 120px; left: 12px; position: absolute; height: 26px; width: 
 56px" OnClientClick="uro()"/>
 </div>
 </form></body>
</html>

Step 2 Add a JavaScript file from Solution Explorer window3

The code is written in JavaScript file is

function uro()
{
alert(parent.document.location.href);
}
function url()
{
alert(document.location.href);
}

Below code written in the design page to attached the JavaScript file to your project

 <script language="javascript" src="JScript.js">
</script>

Step 3. Add another page from new item selection menu. The design code of this page is (url.aspx)

<%@ Page Language="C#" AutoEventWireup="true" Codevirtual="url.aspx.cs" 
Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script language="javascript" src="JScript.js">
</script>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
 <title>url</title>
</head>
<body>
 <form id="url" runat="server">
 <div>
 <asp:Button ID="Button1" runat="server" Text="Button" 
 onclientclick="url()"/>
 <iframe src="iframe.aspx" align="middle" height="350px" 
 name="Comments">
 </iframe>
 </div>
 </form>
</body>
</html>

(追記) (追記ここまで)