Create Dynamic Master Page

if you want create your dynamic masterpage in your website you can just use this kind of codepart at your code.

Session["masterpage"] = "Master2.master";
Response.Redirect(Request.Url.ToString());

void Page_PreInit(Object sender, EventArgs e)
{
if (Session["masterpage"] != null)
MasterPageFile = (String)Session["masterpage"];
}