Mike Volodarsky's blog

Formerly the core server PM for IIS 7.0 and ASP.NET, now I run LeanSentry.
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IconHandler 2.0: file icons in your ASP.NET applications

Since its release, IconHandler has been a pretty popular module (on its own and with the custom DirectoryListingModule).  Today, I am releasing v2.0 of IconHandler, which contains some much-requested functionality and fixes a few issues that people have reported with the original version.

(I don’t RTFM, take me to download)

Here are the notable changes in v2.0:

1.       Icon pre-generation. The IconHandler now includes IconGen.exe, a tool you can use to export all of the icons registered on your workstation machine, and use those icons on a production server instead of retrieving them on demand from the OS. This way you get the real icons instead of the generic icon for all file types not registered on the server. for More on this later …

2.       Kernel caching. When using pre-generated icons, you can now enable kernel caching for maximum speed. If getting icons dynamically, you can use either response caching or icon caching.

3.       Fixes for several issues. This includes fixes for the “Win32 handle that was passed to Icon is not valid or is the wrong type” and “Object is currently in use elsewhere” exceptions.

I've also been asked for more information on how to deploy and use IconHandler. I’ll describe this below, as well as some general best practices and techniques you should consider when using it.

First, to deploy and use IconHandler:

1)      Place the assemblies in the provided sample application into your application’s /BIN directory. These include ShellIconHandler.dll, and ShellIcons.dll.

2)      Register the <iconHandler> configuration section in your application’s root web.config

3)      Add the handler entry for IIS 7.0 Integrated mode to the <handlers> section, and IIS 6.0/ IIS 7.0 Classic mode to the <httpHandlers> section.

4)      Specify the desired configuration settings for the handler using the <iconHandler> configuration section

Here is a sample web.config file in the root of your application that accomplishes steps 2-4:


  <!-- ShellIconHandler configuration section declaration -->


    <section name="iconHandler" type="Mvolo.ShellIcons.Web.ShellIconHandlerConfigurationSection" />



    <!-- Add IconHandler for IIS 7.0 Integrated mode -->


      <add name="iconhandler" path="geticon.axd" verb="GET" type="Mvolo.ShellIcons.Web.ShellIconHandler" />


    <validation validateIntegratedModeConfiguration="false" />



    <!-- Add IconHandler for IIS 6.0 / IIS 7.0 Classic mode -->


      <add path="geticon.axd" verb="GET" type="Mvolo.ShellIcons.Web.ShellIconHandler" />




  Icon Handler by Mike Volodarsky

  Retrieves the shell icon for the specified file name.


  <iconHandler enabled="true"



              enableServerCaching="true" />


This configuration works on IIS 7.0 for both Integrated and Classic mode apps, as well as on IIS 6.0 and IIS 5.1 (Remove the system.webServer stuff for ASP.NET 1.1).

One cool way to use the IconHandler is with the DirectoryListingModule. Or write your own control.

To test IconHandler:

1)      Make a request to geticon.axd?file=FILENAME

2)      Get back the icon as a PNG file that can be displayed in the browser

IconHandler showing a file icon 

The file parameter can contain a filename or an extension. If the filename is provided, the handler can extract the icon for that specific file if one exists and specifies a custom icon. Generally, it’s preferred to use an extension because it avoids a file system access, AND, makes server caching of icons more efficient. The handler also provides a alwaysUseExtension config setting which instructs it to always use the extension part of the specified path to facilitate this usage.

You can also specify the optional size querystring parameter to select between small and large icons, for example geticon.axd?file=.ppt&size=small will give you a small version of the icon above.

Tweak configuration:

IconHandler provides some configuration settings you can tweak for your app:


You guessed it. If set to false, requests to IconHandler will be rejected.


If set to true, enables caching in the browser for 30 minutes at a time.


If set to true, the icons returned from the OS will be cached and re-used for subsequent requests for the same extension. This only works when asking for icons for an extension (not a file path), or if alwaysUseExtension is true.


If set to true, will cache the response using ASP.NET output cache. Use this as an alternative to enableServerCaching to allow for greater performance or when not using extensions, especially when used together with the kernel cache.


If set to true, the handler always uses the extension part of the file parameter even if it contains a file name or a path. In v2.0, I set this to true by default, because it makes caching more effective and helps avoid the “Win32 handle that was passed to Icon is not valid or is the wrong type” issue.


If set to true, the handler will use the icons generated with IconGen.exe in the /App_Resources/Icons directory of the application instead of retrieving them from the OS. This mode can only be used with extensions, it does not support retrieving icons for specific files. Also, if the extension does not have a pre-generated icon, 404 is returned instead of a generic icon.

Pre-generating icons:

By default, IconHandler retrieves the icons on demand using the shell API SHGetFileInfo. This works well on a Windows Vista or Windows XP workstation that has all the right file types registered to installed applications, but on production servers (typically Windows Server 2003 or 2008) these applications are not installed so you would get generic icons for most file types.

To help with this, IconHandler provides the useSavedIcons config setting that instructs it to find the pre-generated icon for the requested extension in the /App_Resources/Icons directory.

How do these icons get there? Simple, you use the provided IconGen.exe tool to export the icons from your workstation to a directory, and then deploy them to the /App_Resources/Icons directory in your app. Here is an example:

> IconGen.exe c:Icons large

This will export all of the registered icons on your machine to the c:Icons directory as large icons. You can repeat the process using “small” to generate the small icons too.

To export only a specific set of icons that you want to use, provide the path to a text file containing the list of extensions, one per line. This file can look like this:


Here is a cheesy trick I baked into the tool to do this. First, run it for all extensions, and pipe the standard out to a text file:

>IconGen.exe c:icons large > extensions.txt

You’ll end up with extensions.txt containing all of the known extensions on your machine (836 for my Vista machine). Then, delete the lines you don’t want in the file, and re-run the tool with the file provided:

>rmdir c:icons /s/q
>IconGen.exe c:icons large extensions.txt


Caching tips:

The IconHandler supports 3 types of caching: client caching (browser/proxies), server caching (caches the icons obtained from the OS), and response caching (caches the entire response containing the icon).

The recommended way to use this is as follows:

1)      enableClientCaching = true. Each browser will cache each icon it gets for 30 minutes. Why 30 mins, when icons are not likely to ever change? If you are still reading this far, you can ask me to change it J

2)      enableServerCaching = true if using extensions, and fetching icons dynamically, OR

3)      enableResponseCaching = true, if you are not using extensions, OR if you are using saved icons where #2 doesn’t apply. With this, you can also configure kernel caching for max performance.

To enable kernel caching, you need to add the following to your application’s root web.config:



      <outputCache enableKernelCacheForVaryByStar="true" />



This allows ASP.NET to enable kernel caching for all urls that vary by all parameters. Note that this may start kernel caching other pages that weren’t previously, so be aware of that when enabling. Also be aware of the standard cache bloat implications if you request a lot of unique urls.

Here is a obligatory “theoretical” perf comparison of 200 clients making sets of 7 requests for same 7 extensions (theoretical because there is a 100% hit ratio):

Performance test of IconHandler caching modes

1)      No caching: 350-ish RPS

2)      Server bitmap caching (enableServerCaching=true): 1100 RPS

3)      Response caching (useSavedIcons=true, enableServerCaching=false, enableResponseCaching=true): 4400 RPS

4)      Response caching + kernel cache: 9300 RPS

As always, kernel caching has superior performance to other forms of caching. NOTE that at the 9300 RPS, my CPU usage drops from the fully utilized 100% to about 35%. This means that the client is not able to drive the server fast enough, so we can project about 26000 RPS on fully utilized server.

This also effectively makes IconHandler performance the same as native IIS static file handler on cache hits, and since there is a finite number of extensions, real performance on a busy site should approach it as well.

Using the API:

IconHandler comes with ShellIcons.dll, which provides a function that you can use to retrieve icons from the OS by p/invoking into SHGetFileInfo.

You can reference this DLL in your projects and call into the API to get icons (full trust required due to p/invoke). Here is a snippet:

using Mvolo.ShellIcons;

using (IconContainer icon = ShellIcons.GetIconForFile(".ppt", true, true))


    Bitmap b = icon.Icon.ToBitmap();

    // Do stuff with the icon …


The ShellIcons.GetIconForFile method allows you to get icons for a specific file or extension, optionally getting either a large or small icon.

The method returns the icon wrapped in the IconContainer class, which implements CriticalHandle. That means that if you hang on to the icon and forget to dispose it, the CLR will do it even under extreme conditions like thread abort and appdomain unload. But, you can only have so many of the icon handles open before the OS will fail to create any more, so it’s a good idea to dispose of the handle immediately like I show in the snippet above. You can than hang on to the Bitmap after the icon is disposed.

Possible issues to be aware of:

1)      My icons all show up as the generic icon! That means you don’t have file associations for the icons – try pre-generating on a machine that does, and then use saved icons.

2)      IconHandler requires full trust when fetching icons on demand. If you use saved icons, it should run fine under Medium trust.

3)      If  you are fetching icons on demand, and don’t use caching, or are making requests with file paths and not extensions, under heavy load you may cause the ShGetFileInfo API to exceed the number of allowed handles and start returning bogus handles. In this case, IconHandler will throw an exception indicating “You have exceeded the maximum number of open GDI handles. Close some of the existing icon handles to avoid this condition”.

4)      If you are fetching icons on demand, ShGetFileInfo API may end up loading all kinds of DLLs into your process to get icons for various file extensions.  If this is not acceptable, use saved icons instead.

 That’s it. Oh, and here is the download J:

Download: IconHandler 2.0 (DLLs and sample application)

Download: IconGen.exe (tool to pre-generate icons)

Enjoy! And let me know if you hit any issues ...




Mvolo.com moves again

Ever since I started blogging about IIS 7.0, I had the plan to do it on a server that was running the actual technology I was blogging about ... I deviated a little from the plan when I first stated out, making the first few posts using GoDaddy's QuickBlog.

Shortly afterwards, I moved to a self-hosted IIS 7.0 IDX 1 server sitting in my roommate's garage. It was the first server on the internet to run Community Server in Integrated mode (that was a big deal to me back than, as many apps we tested were still broken in Integrated mode).

A while later, good folks at Maximum ASP offered to host my blog on one of their dedicated offerings, which was then running Windows Server 2008 RC0. I moved and never looked back - as much as I love self-hosting, having a web server in your house can be a pain. Like, for example, if the power in your area goes down for 4 days because of a snow storm. Or, when the router decides to reboot and your dynamic ip address updating agent decides to take a vacation at the same time.

Compared to that experience, a dedicated hosted server is hard to beat. You get all the access you need to the machine and none of the hassles of managing it, insuring availability, managing firewalls, etc etc. This came in handy when I set up a custom security sandbox and some performance settings that I wanted on the box. 

So, today mvolo.com makes another move, to a Windows Server 2008 RTM MaximumASP server. I feel funny about having stayed on RC0 for so long, but in reality it was almost identical to RTM for most intents and purposes (minus a few last minute bugs). Now, I can once again continue to claim that I am using the same latest and greatest tech that I write about.

If you notice any availability issues, just stay tuned (and run ipconfig /flushdns) as the DNS records update.