简单问题，关于ispostback

<script runat="server" language="c#">
void Page_Load()
{ 
if(Page.IsPostBack) //去掉编译出现问题，为什么?
{
Message.Text="You have entered the following: "+ radio1.SelectedItem.Value;
}
 
}
</script>
<html>
<head>
<title>Radio Button List Example</title>
</head>
<body>
<asp:label id="Message" runat="server"/>
<br /><br />
Which city do you wish to look at hotels for?
<br /><br />
<form runat="server">     
    <asp:radiobuttonlist id="radio1" runat="server">
<asp:listitem id="option1" runat="server" value="Madrid"/>
<asp:listitem id="option2" runat="server" value="Oslo"/>
<asp:listitem id="option3" runat="server" value="Lisbon"/>
</asp:radiobuttonlist>     
<br /><br />
<input type="Submit">
</form>
  </body>
 </html>