Handling ASP.NET PostBacks with URL Rewriting

Recently a collegue develop crossed with this strange error Validation of viewstate MAC failed… when posting a contacts form. Strangely the error only happened when using the re-written url off the contacts page (contacts/), calling contacts.aspx directly worked flawlessly. This is a C# port based on ScottGu’s solution.

A few minutes after I’ve remembered that I had to strugle with this issue when implementing a website using url-rewriting with IIRF (it was a requirement). The problem is that when using url-rewriting the <form> control does not render the proper url but the re-written one. The solution is to write a form control adapter and I found, at the time I had the problem, the right solution from ScottGu’s. Since my language of choice is C# I’m going to share the solution right here:

Step 1 - Create a FormRewriterControlAdapter class

FormRewriterControlAdapter.cs file:

using System;
using System.Data;
using System.Configuration;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

public class FormRewriterControlAdapter : System.Web.UI.Adapters.ControlAdapter { protected override void Render(HtmlTextWriter writer) { base.Render(new RewriteFormHtmlTextWriter(writer)); } }

public class RewriteFormHtmlTextWriter : HtmlTextWriter { public RewriteFormHtmlTextWriter(HtmlTextWriter writer) : base(writer) { this.InnerWriter = writer.InnerWriter; }

public RewriteFormHtmlTextWriter(System.IO.TextWriter writer) : base (writer)
    base.InnerWriter = writer;

public override void WriteAttribute(string name, string value, bool fEncode)
    // If the attribute we are writing is the "action" attribute, and we are not on a sub-control,
    // then replace the value to write with the raw URL of the request - which ensures that we'll
    // preserve the PathInfo value on postback scenarios

    if (name == "action")
        HttpContext context = HttpContext.Current;

        if (context.Items["ActionAlreadyWritten"] == null)
            // Because we are using an url reweriting HttpModule, we will use the
            // Request.RawUrl property within ASP.NET to retrieve the origional URL
            // before it was re-written.  You'll want to change the line of code below
            // if you use a different URL rewriting implementation.

            // value = context.Request.RawUrl;
            value = context.Request.ServerVariables["HTTP_X_REWRITE_URL"];

            // Indicate that we've already rewritten the <form>'s action attribute to prevent
            // us from rewriting a sub-control under the <form> control

            context.Items["ActionAlreadyWritten"] = true;

    base.WriteAttribute(name, value, fEncode);


Step 2 - Add adapter to App_Browsers

  1. Add ASP.NET folder if it does not exists;
  2. Create Form.browser.

Form.browser file:

    <browser refID=“Default”>
            <adapter controlType=“System.Web.UI.HtmlControls.HtmlForm”
                adapterType=“FormRewriterControlAdapter” />

I hope you’ll find this information useful.

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