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.