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An ExpressionBuilder class generates one expression from another expression.
File: App_Code\LookupExpressionBuilder.cs
using System;
using System.CodeDom;
using System.Web.UI;
using System.ComponentModel;
using System.Web.Compilation;
using System.Xml;
using System.Web.Hosting;
using System.Web.Caching;
namespace MyNamespace
{
public class LookupExpressionBuilder : ExpressionBuilder
{
public override CodeExpression GetCodeExpression(BoundPropertyEntry entry, object parsedData, ExpressionBuilderContext context)
{
CodeTypeReferenceExpression refMe = new CodeTypeReferenceExpression(base.GetType());
CodePrimitiveExpression expression = new CodePrimitiveExpression(entry.Expression);
return new CodeMethodInvokeExpression(refMe, "GetEvalData", new CodeExpression[] { expression });
}
public override object EvaluateExpression(object target, BoundPropertyEntry entry, object parsedData, ExpressionBuilderContext context)
{
return GetEvalData(entry.Expression);
}
public override bool SupportsEvaluate
{
get
{
return true;
}
}
public static string GetEvalData(string expression)
{
XmlDocument lookupDoc = (XmlDocument)HostingEnvironment.Cache["Lookup"];
if (lookupDoc == null)
{
lookupDoc = new XmlDocument();
string lookupFileName = HostingEnvironment.MapPath ("~/Lookup.config");
lookupDoc.Load(lookupFileName);
CacheDependency fileDepend = new CacheDependency(lookupFileName);
HostingEnvironment.Cache.Insert("Lookup", lookupDoc, fileDepend);
}
string search = String.Format("//add[@key='{0}']", expression);
XmlNode match = lookupDoc.SelectSingleNode(search);
if (match != null)
return match.Attributes["value"].Value;
return "[no match]";
}
}
}
File: Web.Config
<configuration>
<system.web>
<compilation>
<expressionBuilders>
<add expressionPrefix="lookup"
type="MyNamespace.LookupExpressionBuilder" />
</expressionBuilders>
</compilation>
</system.web>
</configuration>
File: Lookup.config
<lookup>
<add key="WelcomeMessage" value="Welcome to our Web site!" />
<add key="Copyright" value="All content copyrighted by the company." />
</lookup>
File: ShowLookupExpressionBuilder.aspx
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head id="Head1" runat="server">
<title>Show LookupExpressionBuilder</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Literal ID="Literal1"
Text="<%$ lookup:WelcomeMessage %>"
runat="Server" />
</div>
</form>
</body>
</html>
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