ASP.NET Tutorial/Development/ExpressionBuilder
Creating a Custom ExpressionBuilder
<source lang="csharp">
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.ruponentModel; using System.Web.rupilation; 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">
<asp:Literal ID="Literal1"
Text="<%$ lookup:WelcomeMessage %>"
runat="Server" />
</form>
</body> </html></source>
