Creating a Custom ExpressionBuilder : ExpressionBuilder « Development « ASP.NET Tutorial






  1. ASP.NET Tutorial
  2. Development
  3. ExpressionBuilder
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>








9.19.ExpressionBuilder
9.19.1.Creating a Custom ExpressionBuilder