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ExpressionPrefixAttribute Class

Specifies the prefix attribute to use for the expression builder. This class cannot be inherited.

Namespace:  System.Web.Compilation
Assembly:  System.Web (in System.Web.dll)

[AttributeUsageAttribute(AttributeTargets.Class, AllowMultiple = false)]
[AspNetHostingPermissionAttribute(SecurityAction.LinkDemand, Level = AspNetHostingPermissionLevel.Minimal)]
public sealed class ExpressionPrefixAttribute : Attribute

The ExpressionPrefixAttribute class is used at design time with expressions that are not defined in the configuration file. Use the ExpressionPrefix property to get the name of the prefix that is associated with the ExpressionPrefixAttribute object. An expression builder looks for any statements with the following form:

<%$ [expressionPrefix]:[expressionValue] %>

Then the expression builder, based on the prefix of the expression, generates code for the property assignment. The expressionPrefix parameter refers to a configured expression builder, which is defined in the configuration file or through an ExpressionPrefixAttribute object.

The following code examples demonstrate how to use the ExpressionPrefixAttribute class. The attribute is applied to a custom expression builder that implements the ExpressionBuilder abstract class. This implementation of ExpressionBuilder returns an evaluated statement that is passed to the expression. To run this example, you must first register the custom expression builder in the Web.config file. The first code example demonstrates how to register the custom expression builder in the Web.config file.

              <add expressionPrefix="MyCustomExpression"

The second code example demonstrates how to reference the expression in an .aspx file.

<asp:Label ID="Label1" runat="server" 
Text="<%$ MyCustomExpression:Hello, world! %>" />

The third code example demonstrates how to develop a customized expression builder by deriving from ExpressionBuilder. To run this code example, you must place the class in the App_Code folder.

using System;
using System.CodeDom;
using System.Web.UI;
using System.ComponentModel;
using System.Web.Compilation;
using System.Web.UI.Design;

// Apply ExpressionEditorAttributes to allow the  
// expression to appear in the designer.
public class MyExpressionBuilder : ExpressionBuilder
    // Create a method that will return the result  
    // set for the expression argument. 
    public static object GetEvalData(string expression, Type target, string entry)
        return expression;

    public override object EvaluateExpression(object target, BoundPropertyEntry entry, 
	object parsedData, ExpressionBuilderContext context)
        return GetEvalData(entry.Expression, target.GetType(), entry.Name);

    public override CodeExpression GetCodeExpression(BoundPropertyEntry entry, 
	object parsedData, ExpressionBuilderContext context)
        Type type1 = entry.DeclaringType;
        PropertyDescriptor descriptor1 = TypeDescriptor.GetProperties(type1)[entry.PropertyInfo.Name];
        CodeExpression[] expressionArray1 = new CodeExpression[3];
        expressionArray1[0] = new CodePrimitiveExpression(entry.Expression.Trim());
        expressionArray1[1] = new CodeTypeOfExpression(type1);
        expressionArray1[2] = new CodePrimitiveExpression(entry.Name);
        return new CodeCastExpression(descriptor1.PropertyType, new CodeMethodInvokeExpression(new 
	   CodeTypeReferenceExpression(base.GetType()), "GetEvalData", expressionArray1));

    public override bool SupportsEvaluate
        get { return true; }


Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0