Properties of a webform causing inline errors using style markup

I’ve been looking at the latest Facebook Developer Toolkit samples and tried to add a property to the Default.aspx.cs using the IFrame sample application like this:

<%=this.MapCanvas %>

Which yielded this error:

Error    28    'ASP.default_aspx' does not contain a definition for 'MapCanvas' and no extension method 'MapCanvas' accepting a first argument of type 'ASP.default_aspx' could be found (are you missing a using directive or an assembly reference?)    C:\Users\SteveT\Documents\Visual Studio 2010\WebSites\FBIFrame\Default.aspx    38    17    C:\...\FBIFrame\

The problem is that the @Page tag reads:

<%@ Page Language="C#" AutoEventWireup="true" MasterPageFile="~/IFrameMaster.Master" CodeBehind="Default.aspx.cs" Inherits="IFrameSample.Default" %>

Changing CodeBehind to CodeFile gets things working.

<%@ Page Language="C#" AutoEventWireup="true" MasterPageFile="~/IFrameMaster.Master" CodeFile="Default.aspx.cs" Inherits="IFrameSample.Default" %>