URL Rewriting using ASP.NET for SEO

using System;
using System.Collections.Generic;
using System.Web;
using System.IO;
using System.Web.UI;
using System.Web.SessionState;

public class BetterUrlRewriter : IHttpHandler, IRequiresSessionState
{    
    const string ORIGINAL_PATHINFO = "UrlRewriterOriginalPathInfo";    
    const string ORIGINAL_QUERIES = "UrlRewriterOriginalQueries";     
    
    public void ProcessRequest(HttpContext context)    
    {        
        // Check to see if the specified HTML file actual exists and serve it if so..        
        String strReqPath = context.Server.MapPath(context.Request.AppRelativeCurrentExecutionFilePath);        
        if (File.Exists(strReqPath))        
        {            
            context.Response.WriteFile(strReqPath);            
            context.Response.End();            
            return;        
        }         
        
        // Record the original request PathInfo and QueryString information to handle graceful postbacks        
        context.Items[ORIGINAL_PATHINFO] = context.Request.PathInfo;        
        context.Items[ORIGINAL_QUERIES] = context.Request.QueryString.ToString();         
        
        // Map the friendly URL to the back-end one..        
        String strVirtualPath = "";        
        String strQueryString = "";        
        MapFriendlyUrl(context, out strVirtualPath, out strQueryString);         
        if(strVirtualPath.Length>0)        
        {
            foreach (string strOriginalQuery in context.Request.QueryString.Keys)
            {
                // To ensure that any query strings passed in the original request are preserved, we append these                 
                // to the new query string now, taking care not to add any keys which have been rewritten during the handler..                
                if (strQueryString.ToLower().IndexOf(strOriginalQuery.ToLower() + "=") < 0)                
                {                    
                    strQueryString += string.Format("{0}{1}={2}", ((strQueryString.Length > 0) ? "&" : ""), strOriginalQuery, context.Request.QueryString[strOriginalQuery]);                
                }            
            }             
            
            // Apply the required query strings to the request            
            context.RewritePath(context.Request.Path, string.Empty, strQueryString);             
            // Now get a page handler for the ASPX page required, using this context.              
            Page aspxHandler = (Page)PageParser.GetCompiledPageInstance(strVirtualPath, context.Server.MapPath(strVirtualPath), context);             
            // Execute the handler..            
            aspxHandler.PreRenderComplete += new EventHandler(AspxPage_PreRenderComplete);            
            aspxHandler.ProcessRequest(context);        
        }        
        else        
        {            
            // No mapping was found - emit a 404 response.             
            context.Response.StatusCode = 404;            
            context.Response.ContentType = "text/plain";            
            context.Response.Write("Page Not Found");            
            context.Response.End();        
        }    
    }     
    
    void MapFriendlyUrl(HttpContext context, out String strVirtualPath, out String strQueryString)    
    {        
        strVirtualPath = ""; strQueryString = "";     
            
        // TODO: This routine should examine the context.Request properties and implement        
        //       an appropriate mapping system.        
        //        
        //       Set strVirtualPath to the virtual path of the target aspx page.        
        //       Set strQueryString to any query strings required for the page.         
        
        if (context.Request.Path.IndexOf("FriendlyPage.html") >= 0)        
        {            
            // Example hard coded mapping of "FriendlyPage.html" to "UnfriendlyPage.aspx"             
            
            strVirtualPath = "~/UnfriendlyPage.aspx";            
            strQueryString = "FirstQuery=1&SecondQuery=2";        
        }    
    }     
    
    void AspxPage_PreRenderComplete(object sender, EventArgs e)    
    {        
        // We need to rewrite the path replacing the original tail and query strings..        
        // This happens AFTER the page has been loaded and setup but has the effect of ensuring        
        // postbacks to the page retain the original un-rewritten pages URL and queries.         
        
        HttpContext.Current.RewritePath(HttpContext.Current.Request.Path,                                        
        HttpContext.Current.Items[ORIGINAL_PATHINFO].ToString(),                                        
        HttpContext.Current.Items[ORIGINAL_QUERIES].ToString());    
    }     
    
    public bool IsReusable    
    {        
        get        
        {            
            return true;        
        }    
    }
}