TIP: How to Handle Form Postbacks when Url Rewriting

Url Rewriting is great and I love it a lot, you get to create nice readable (and memorable) urls without having to create hundred of files.  However, there can be issues. 

For example, when you post back to a page where the url has been rewritten, it won’t be to the nice alias, it will be to the direct page.aspx?id=blah location.  Which is a bit nasty.

To stop this, I make use of a Control Adapters which alters the rendering logic of the HtmlForm control on every ASP.Net page, to amend the action to be the nice alias.

   1: using System;

   2: using System.Collections.Generic;

   3: using System.Linq;

   4: using System.Text;

   5: using System.Web.UI.HtmlControls;

   6: using System.Web;

   7:  

   8: namespace MartinOnDotNet.Web.ControlAdapters

   9: {

  10:     /// <summary>

  11:     /// Control adapter to ensure the form action persists the rewritten url

  12:     /// </summary>

  13:     public class FormActionRewriterControlAdapter : System.Web.UI.Adapters.ControlAdapter

  14:     {

  15:         /// <summary>

  16:         /// Overrides the <see cref="M:System.Web.UI.Control.OnPreRender(System.EventArgs)"/> method for the associated control.

  17:         /// </summary>

  18:         /// <param name="e">An <see cref="T:System.EventArgs"/> that contains the event data.</param>

  19:         protected override void OnPreRender(EventArgs e)

  20:         {

  21:             HtmlForm form = Control as HtmlForm;

  22:             if (form != null && HttpContext.Current != null)

  23:             {

  24:                 form.Action = HttpContext.Current.Request.RawUrl;

  25:             }

  26:             base.OnPreRender(e);

  27:         }

  28:  

  29:         /// <summary>

  30:         /// Generates the target-specific markup for the control to which the control adapter is attached.

  31:         /// </summary>

  32:         /// <param name="writer">The <see cref="T:System.Web.UI.HtmlTextWriter"/> to use to render the target-specific output.</param>

  33:         protected override void Render(System.Web.UI.HtmlTextWriter writer)

  34:         {

  35:  

  36:             base.Render(new RewriteFormActionHtmlTextWriter(writer));

  37:         }

  38:     }

  39: }

You'll notice that the control adapter needs a special HtmlTextWriter implementation to do the actual heavy lifting:

   1: using System;

   2: using System.Collections.Generic;

   3: using System.Linq;

   4: using System.Text;

   5: using System.Web;

   6: using System.Web.UI;

   7: using System.IO;

   8:  

   9: namespace MartinOnDotNet.Web.ControlExtensions

  10: {

  11:     /// <summary>

  12:     /// Specialised <see ref="HtmlTextWriter" /> that handles populating the form action attribute appropriately for

  13:     /// url rewriting

  14:     /// </summary>

  15:     public class RewriteFormActionHtmlTextWriter : HtmlTextWriter

  16:     {

  17:         private bool _haveAlreadyWritten;

  18:  

  19:         /// <summary>

  20:         /// Initializes a new instance of the <see cref="RewriteFormActionHtmlTextWriter"/> class.

  21:         /// </summary>

  22:         /// <param name="writer">The writer.</param>

  23:         public RewriteFormActionHtmlTextWriter(TextWriter writer) : base(writer) { InnerWriter = writer; }

  24:        

  25:         /// <summary>

  26:         /// Initializes a new instance of the <see cref="RewriteFormActionHtmlTextWriter"/> class.

  27:         /// </summary>

  28:         /// <param name="writer">The writer.</param>

  29:         public RewriteFormActionHtmlTextWriter(HtmlTextWriter writer) : base(writer) { InnerWriter = writer.InnerWriter; }

  30:  

  31:         /// <summary>

  32:         /// Writes the specified markup attribute and value to the output stream, and, if specified, writes the value encoded.

  33:         /// </summary>

  34:         /// <param name="name">The markup attribute to write to the output stream.</param>

  35:         /// <param name="value">The value assigned to the attribute.</param>

  36:         /// <param name="fEncode">true to encode the attribute and its assigned value; otherwise, false.</param>

  37:         public override void WriteAttribute(string name, string value, bool fEncode)

  38:         {

  39:             if (string.Equals(name, "action", StringComparison.OrdinalIgnoreCase) && !_haveAlreadyWritten)

  40:             {

  41:  

  42:                 value = HttpContext.Current.Request.RawUrl;

  43:                 _haveAlreadyWritten = true;

  44:  

  45:             }

  46:             base.WriteAttribute(name, value, fEncode);

  47:         }

  48:     }

  49: }

This is then all registered in your web application with a simple FormAdapter.Browser file in the App_Browsers folder of your Web App.

   1: <browsers>

   2:   <browser refID="Default">

   3:     <controlAdapters>

   4:       <adapter controlType="System.Web.UI.HtmlControls.HtmlForm"

   5:                adapterType="MartinOnDotNet.Web.ControlAdapters.FormActionRewriterControlAdapter" />

   6:             

   7:     </controlAdapters>

   8:   </browser>

   9: </browsers>

These clases/config can now be reused across multiple projects quickly and easily.

Enjoy.