asp.net 2.0中的 自定义表达式 ExpressionBuilder 有人用过吗？求助（200分）

现在正在学习.net 2.0 做了一个自定义表达式，主要是为了解决用access数据库的时候，connectionStrings必须写绝对路径这个很讨厌的问题。
实际运行时没问题，但是在vs2005中设计的时候没办法返回正确的值，所以GridView等设计的时候很不方便，请各位兄弟帮忙。asp.net和C#版各一个，总共200分重谢下面是ExpressionBuilder类
using System;
using System.Collections.Generic;
using System.Text;
using System.Web.Configuration;
using System.Web.Compilation;
using System.Collections.Specialized;
using System.CodeDom;namespace Huolx.Library
{
    [ExpressionPrefix("DB")]
    [ExpressionEditor("DBExpressionBuilderEditor")]
    class DBExpressionBuilder: System.Web.Compilation.ExpressionBuilder
    {
        public override System.CodeDom.CodeExpression GetCodeExpression(System.Web.UI.BoundPropertyEntry entry, object parsedData, ExpressionBuilderContext context)
        {
            string param = entry.Expression;
            if (string.Compare(param, "ConnStr", true) == 0)
            {
                return new CodePrimitiveExpression(bulidConnStr(WebConfigurationManager.AppSettings));
            }
            else if (string.Compare(param, "Type", true) == 0)
            {
                return new CodePrimitiveExpression(WebConfigurationManager.AppSettings["DataBaseType"]);
            }            return new CodePrimitiveExpression(bulidConnStr(WebConfigurationManager.AppSettings));        }        public override object EvaluateExpression(object target, System.Web.UI.BoundPropertyEntry entry, object parsedData, ExpressionBuilderContext context)
//按照MSDN中的说明，只要实现这个方法，就可以在设计的时候返回计算过的值了，可是……
        {
            string param = entry.Expression;
            if (string.Compare(param, "ConnStr", true) == 0)
            {
                return new CodePrimitiveExpression(bulidConnStr(WebConfigurationManager.AppSettings));
            }
            else if (string.Compare(param, "Type", true) == 0)
            {
                return new CodePrimitiveExpression(WebConfigurationManager.AppSettings["DataBaseType"]);
            }            return bulidConnStr(WebConfigurationManager.AppSettings);
            //return @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source="D:\My Documents\Visual Studio 2005\WebSites\baoming\data\bm.mdb"";
        }        public override bool SupportsEvaluate
        {
            get { return true; }
        }        private string bulidConnStr(NameValueCollection appSettings)
        {
            string connStr = "";
            string _dbType = appSettings["DataBaseType"];            if (String.Compare(_dbType, "ACCESS", true, System.Globalization.CultureInfo.CurrentCulture) == 0)
            {
                string dbname = appSettings["DataBaseFilePath"] + appSettings["DataBaseFileName"];
                System.Web.UI.Page page = new System.Web.UI.Page();                connStr = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=" + page.Server.MapPath(dbname) + ";";
            }
            else if (String.Compare(_dbType, "MSSQL", true, System.Globalization.CultureInfo.CurrentCulture) == 0)
            {
                connStr = appSettings["SQL"];
            }
            return connStr;
        }
    }\\后面是ExpressionEditor类，翻了半天MSDN，加了这个类还是不管用    [ExpressionPrefix("DB")]
    [ExpressionEditor(typeof(Huolx.Library.DBExpressionBuilderEditor))]
    public class DBExpressionBuilderEditor : System.Web.UI.Design.ExpressionEditor
    {        public override object EvaluateExpression(string expression, object parseTimeData, Type propertyType, IServiceProvider serviceProvider)
        {
            //string param = expression;
            //if (string.Compare(param, "ConnStr", true) == 0)
            //{
            //    return new CodePrimitiveExpression(bulidConnStr(WebConfigurationManager.AppSettings));
            //}
            //else if (string.Compare(param, "Type", true) == 0)
            //{
            //    return new CodePrimitiveExpression(WebConfigurationManager.AppSettings["DataBaseType"]);
            //}            return new CodePrimitiveExpression(bulidConnStr(WebConfigurationManager.AppSettings));        }
        private string bulidConnStr(NameValueCollection appSettings)
        {
            string connStr = "";
            string _dbType = appSettings["DataBaseType"];            if (String.Compare(_dbType, "ACCESS", true, System.Globalization.CultureInfo.CurrentCulture) == 0)
            {
                string dbname = appSettings["DataBaseFilePath"] + appSettings["DataBaseFileName"];
                System.Web.UI.Page page = new System.Web.UI.Page();                connStr = "Provider=Microsoft.Jet.OLEDB.4.0; Data Source=" + page.Server.MapPath(dbname) + ";";
            }
            else if (String.Compare(_dbType, "MSSQL", true, System.Globalization.CultureInfo.CurrentCulture) == 0)
            {
                connStr = appSettings["SQL"];
            }
            return connStr;
        }
    }
}
后面是web.config
<?xml version="1.0"?>
<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
 <appSettings>
<!--=数据库类型可以为"MSSQL"或者"ACCESS"==-->
<add key="DataBaseType" value="ACCESS"/>
<!--=====ACCESS数据库相对路径=================-->
<add key="DataBaseFilePath" value="./App_Data/"/>
<!--=====ACCESS数据库名称=================-->
<add key="DataBaseFileName" value="bm.mdb"/>
<!--=====SQL连接字符串====================-->
<add key="SQL" value="Data Source=.\SQLEXPRESS;Initial Catalog=bmSQL;Integrated Security=True"/>
<!--=======================================-->
<add key="ACCESS" value="=Microsoft.Jet.OLEDB.4.0;Data Source=&quot;data\bm.mdb&quot;"/>
<!--=======================================-->
</appSettings>
<system.web>
<authentication mode="Forms"/>
<compilation defaultLanguage="c#" debug="true">
<expressionBuilders>
<add expressionPrefix="DB" type="Huolx.Library.DBExpressionBuilder"/>
</expressionBuilders>
<assemblies>
<add assembly="System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A"/></assemblies></compilation>
</system.web>
</configuration>后面是aspx页中引用的部分
       <asp:GridView ID="GridView1" runat="server" DataSourceID="SqlDataSource1">
        </asp:GridView>
        <asp:Label ID="Label1" runat="server" Text="<%$ DB:ConnStr %>"></asp:Label>
        <br />
        <br />
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ DB:ConnStr %>" ProviderName="System.Data.OleDb" SelectCommand="SELECT * FROM [peixun]"></asp:SqlDataSource>