HttpEncoder Class

The default ASP.NET implementation for encoding and decoding is the HttpEncoder class. The class contains the core encoding and decoding logic that is used by methods in classes such as HttpUtility, HttpServerUtility, and HttpResponseHeader.

You can inherit from the HttpEncoder class and override its behavior to customize the default encoder/decoder behavior of ASP.NET. You then set the EncoderType property of the HttpRuntimeSection class to configure your custom encoding/decoding class.

Because the HttpEncoder class contains the default base ASP.NET encoding functionality, a custom encoder/decoder class that derives from it can override the built-in ASP.NET encoder/decoder behavior or change only selected aspects of it.

You can configure the custom encoding type for in ASP.NET to replace or supplement the following encoding behavior:

• HTML encoding

• HTML attribute encoding

• URL encoding

• URL path encoding

• HTTP header name and header value encoding

When you create a custom encoder class and override the base methods, the derived encoder might throw an exception from any of the overridden methods. However, in the following cases throwing such an exception could lead to unexpected behavior in ASP.NET:

• If ASP.NET is rendering an error page that is caused by an unhandled exception that was thrown from a custom encoder, ASP.NET does not attempt to encode its error output by calling into the custom encoder. If you allow this, an unhandled exception occurs from the unhandled exception that is the handling error path. To prevent this behavior, the ASP.NET error page always uses the ASP.NET default encoder.

• When ASP.NET is sending HTTP headers to IIS, ASP.NET does not expect unhandled exceptions to occur. Therefore, the standard ASP.NET error page will be rendered (if configuration settings allows this page to be displayed).

The following example shows how to create a custom encoder that routes HTML encoding calls into methods of the anti-XSS library’s Microsoft.Security.Application.AntiXSS type. The anti-XSS library is available for download from download.microsoft.com.

[Visual Basic]

Imports System 
Imports System.Web 
Imports System.Web.Util 
Imports Microsoft.Security.Application 

Public Class AntiXssEncoder Inherits HttpEncoder 
    Public Sub New()
    End Sub 
    
    Protected Overloads Overrides Sub HtmlEncode(ByVal value As String, _
            ByVal output As System.IO.TextWriter)
        output.Write(AntiXss.HtmlEncode(value))
    End Sub 

    Protected Overloads Overrides Sub HtmlAttributeEncode(ByVal _
            value As String, ByVal output As System.IO.TextWriter) 
        output.Write(AntiXss.HtmlAttributeEncode(value))
    End Sub
End Class

[C#]

using System;
using System.Web;
using System.Web.Util;
using Microsoft.Security.Application;

public class AntiXssEncoder : HttpEncoder
{
    public AntiXssEncoder() { }
    protected override void HtmlEncode(string value, 
        System.IO.TextWriter output)
    {
        output.Write(AntiXss.HtmlEncode(value));
    }

    protected override void HtmlAttributeEncode(string value,
        System.IO.TextWriter output)
    {
        output.Write(AntiXss.HtmlAttributeEncode(value));
    }
}

After you have added this code to an application, you can configure the application to use the custom HTTP encoder. The following example from a Web.config file shows how to configure ASP.NET to use the custom http encoder.

<httpRuntime  encoderType="AntiXssEncoder" />

Note
Only one custom encoding type can be configured per application.

.NET Framework

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

Reference