in aspx file errors when I try to run same code in mypage.aspx.cs file

This code runs file in the aspx file ---- "Profile" and "MasterPageFile" are defined in Web.config

<script runat="server">
    protected void Page_PreInit(object sender, EventArgs e)
    {
        if (Request["master"] != null)
        {
            switch (Request["master"])
            {
                case "Dynamic1":
                    Profile.MasterPageFile = "Dynamic1.master";
                 ...