Using the FileResolver to allow virtual application paths ( ~ ) in any file

#region Usings

using System;
using System.IO;
using System.Configuration;
using System.Text;
using System.Web;
using System.Web.Configuration;
using System.Web.Caching;

#endregion

namespace FileResolverDemoWeb
{
	public class FileResolver : IHttpHandler
	{
		/// <summary>
		/// File cache item used to store file content & date entered into cache
		/// </summary>
		internal class FileCacheItem
		{
			internal string Content;
			internal DateTime DateEntered = DateTime.Now;

			internal FileCacheItem(string content)
			{
				this.Content = content;
			}
		}

		private FileCacheItem UpdateFileCache(HttpContext context, string filePath)
		{
			string content;

			using(FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read))
			{
				using(StreamReader sr = new StreamReader(fs))
				{
					content = sr.ReadToEnd();
					sr.Close();
				}

				fs.Close();
			}

			//Get absolute application path
			string relAppPath = HttpRuntime.AppDomainAppVirtualPath;
			if(!relAppPath.EndsWith("/"))
				relAppPath += "/";

			//Replace virtual paths w/ absolute path
			content = content.Replace("~/", relAppPath);

			FileCacheItem ci = new FileCacheItem(content);

			//Store the FileCacheItem in cache w/ a dependency on the file changing
			CacheDependency cd = new CacheDependency(filePath);
			context.Cache.Insert(filePath, ci, cd);
			return ci;
		}

		public void ProcessRequest(HttpContext context)
		{
			string absFilePath = context.Request.PhysicalPath.Replace(".ashx", "");
			
			//If a tilde was used in the page to this file, replace it w/ the app path
			if(absFilePath.IndexOf("~\\") > -1)
				absFilePath = absFilePath.Replace("~", "").Replace("\\\\", "\\");

			if(!File.Exists(absFilePath))
			{
				context.Response.StatusCode = 404;
				return;
			}

			FileCacheItem ci = (FileCacheItem)context.Cache[absFilePath];
			if(ci != null)
			{
				if(context.Request.Headers["If-Modified-Since"] != null)
				{
					try
					{
						DateTime date = DateTime.Parse(context.Request.Headers["If-Modified-Since"]);

						if(ci.DateEntered.ToString() == date.ToString())
						{
							//Don't do anything, nothing has changed since last request
							context.Response.StatusCode = 304;
							context.Response.StatusDescription = "Not Modified";
							context.Response.End();
							return;
						}
					}
					catch(Exception){}
				}
				else
				{
					//In the event that the browser doesn't automatically have this header, add it
					context.Response.AddHeader("If-Modified-Since", ci.DateEntered.ToString());
				}
			}
			else
			{
				//Cache item not found, update cache
				ci = UpdateFileCache(context, absFilePath);
			}

			context.Response.Cache.SetLastModified(ci.DateEntered);
			context.Response.ContentType = "text/" + GetContentType(Path.GetExtension(absFilePath));
			context.Response.Write(ci.Content);
			context.Response.End();
		}

		/// <summary>
		/// Gets the appropriate content type for a specified extension
		/// </summary>
		private string GetContentType(string ext)
		{
			switch(ext.ToLower())
			{
				case ".css":
					return "css";
					break;
				case ".xml":
					return "xml";
					break;
				case ".js":
					return "javascript";
					break;
				default:
					return "plain";
					break;
			}
		}

		#region IHttpHandler Members

		public bool IsReusable
		{
			get
			{
				return true;
			}
		}

		#endregion
	}
}