ASP.net 页面在导入语句上出现错误，但我确实有参考

你知道为什么在我的MVC2项目中会出现下面的错误吗？即使在项目本身中，我肯定引用了"system.Web.Entity"？

Compiler Error Message: CS0234: The type or namespace name 'Entity' does not exist in the namespace 'System.Data' (are you missing an assembly reference?)

Source Error:

Line 1:  <%@ Page Language="C#" Inherits="System.Web.Mvc.ViewPage<List<Node>>" %>
Line 2:  <%@ Import Namespace="TopologyDAL" %>
Line 3:  <%@ Import Namespace="System.Data.Entity" %>

谢谢

编辑-顺便说一句，如果我删除第3行，则会得到错误:

 Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0012: The type 'System.Data.Objects.DataClasses.EntityObject' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Data.Entity, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

Source Error:

Line 164:    
Line 165:    [System.Runtime.CompilerServices.CompilerGlobalScopeAttribute()]
Line 166:    public class views_node_index_aspx : System.Web.Mvc.ViewPage<List<Node>>, System.Web.SessionState.IRequiresSessionState, System.Web.IHttpHandler {
Line 167:        
Line 168:        private static bool @__initialized;


Source File: c:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\6ec16fd2\a2147d7c\App_Web_index.aspx.1b64bdf1.ajruf7pv.0.cs    Line: 166