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From a managed class library, I'd like to find out whether the currently executing application is an ASP.NET web application (web forms or MVC) or not.

I have seen different approaches to do so, e.g. by checking one of the following:

  • System.Web.Hosting.HostingEnvironment.IsHosted == true
  • System.Web.HttpContext.Current != null
  • System.Web.HttpRuntime.AppDomainAppId != null
  • System.Web.HttpRuntime.Cache != null
  • checking for a web.config file (note: I don't think this is reliable)

The question is which approach should I be using? Are some of them invalid (i.e. might they return true even when running in a windows app) or are all of them equal?

Update / clarification (sorry if my question wasn't clear enough):

  • I have a managed class library (.net code) which is run by a .net application (obviously)
  • this "host application" can either be an ASP.NET application (e.g. web forms or MVC) or a windows application (e.g. console or win forms)
  • my question is: is there a way to reliably determine from within my class library (at runtime) whether it is running as part of an ASP.NET application?

Note: I know I could implement a different solution (e.g. see comments below or Tomas Lycken's answer), but that is not the point of this question. The class library is already existing, and I'd like to change as little code as possible!

M4N
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    Since it's not easy to detect(f.e. `HttpContext.Current` is `null` in an async thread) , wouldn't it be most reliable to use parameters/constructors in your methods/classes in Class library to indicate what type it is? – Tim Schmelter Aug 10 '11 at 08:23
  • @Tim: that's definitively an alternative (I could also simply add an entry to the application configuration file, e.g. IsWebApp=true|false). But if there is another, reliable way to do it (e.g. one of the above) I'd prefer that. – M4N Aug 10 '11 at 08:38
  • If I needed something like that I would have a initialize method somewhere in my lib that would have to be called at the application startup. This method would set the lib "mode". All other lib methods that need to be aware of the environment would throw an exception if the "mode" is not set. Doing it this way would also help with automated test. – Klinger Aug 10 '11 at 16:27
  • You Can rule out Web.config and HttpContext. – Rune FS Aug 10 '11 at 17:39
  • It's fine for you to tell us to not question your goal - but that only works if your goal has been correctly analyzed. Based on the experience of many here, the analysis on which you're basing your question is faulty, and you really shouldn't want to know this. I have been programming ASP.NET for ten years, and have yet to see a valid reason for code to know this. – John Saunders Aug 10 '11 at 19:15
  • @John: that means only questions with a "valid goal" are allowed on SO? No theoretical/academic question, or question to simple learn something without a direct practical use? Let's say I want to handle some things differently if running in an ASP.NET app than in a windows app. If there is a simple way to find out that would mean practically no code change to my (existing class library). I don't think any details are relevant. Especially since I know I could implement a completely different solution (but maybe I'd prefer not to). – M4N Aug 10 '11 at 19:21
  • It's rare for a question like this to be theoretical. Most likely, you actually do think you need to know this, but don't want to discuss your reasons for that. If you have an example of where it's legitimate to "handle things differently", then please present it. I'm sure we'll find that your question isn't about ASP.NET vs. Windows Applications - it's between strategy 1 and strategy 2, which currently map to ASP.NET vs. Windows applications. – John Saunders Aug 10 '11 at 19:23
  • @John: You're right, the question is not theoretical (to me). I currently use the approach with a app./web.config entry to tell the library which "strategy" to use. I'd like to know if this can be replaced with another approach which does not require user intervention (configuration). I'm sorry, but I don't see why you cannot accept that as a valid question. – M4N Aug 10 '11 at 19:28
  • Because you're view is too limited. _today_ the only choices are those you describe as "web application" vs. "windows application". By concentrating on the strategies, you remove an unnecessary connection. Tomorrow, you might want to use the "web" strategy in what you currently would call a "windows" application. Day after tomorrow, there may be a "windows" application that looks like a "web" application. Break the link. – John Saunders Aug 10 '11 at 19:32
  • @John: you're wrong: today I'm looking for a simple solution (if there is any) which requires as few code changes as possible. The day after tomorrow I might have left the .NET platform - who know, so YAGNI. – M4N Aug 10 '11 at 19:37

4 Answers4

4

Potentially Not reliable. MSDN implies that this will always return a new object if one does not exist already. Thus there are technically times where you can call it and have one not exist before it is called.

    System.Web.Hosting.HostingEnvironment.IsHosted == true

All web environments need a context. What handler is inside that context is what tells you the type of web environment. (MvcHandler for example). Note this can be different types of handlers for the same environment - You can run MVC and web forms together for example. It just depends on what is currently being served and the pipeline it is using.

    System.Web.HttpContext.Current != null

All web apps need an application ID. It is unique and does not change as application pools are restarted.

    System.Web.HttpRuntime.AppDomainAppId != null

I've never seen this, though just logically I can imagine a time where the cache is not used and thus wouldn't be reliable.

    System.Web.HttpRuntime.Cache != null

You're right.

checking for a web.config file (note: I don't think this is reliable)

I use something of this sort within a library. I've found it reliable.

         Page page = (HttpContext.Current != null && HttpContext.Current.Handler != null) ? HttpContext.Current.Handler as Page : null;
         if (HttpRuntime.AppDomainAppId != null && page != null)
         {
            //I'm a web forms application
         }
         else if (HttpRuntime.AppDomainAppId != null && page == null && HttpContext.Current != null) { throw new InvalidOperationException("I'm an MVC application"); }
         else throw new InvalidOperationException("Im not ASP.Net web");
DFTR
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  • HttpContext.Current can be null even when in ASP.NET application because background threads and hence also async code running after .ConfigureAwait(false) may not have a context (or worse, have someone else's context because the receiving thread was released to the thread pool while awaiting for async completion). – David Burg Apr 01 '21 at 05:29
  • @DavidBurg You're correct. I should have clarified that this was only reliable if you're running on the main thread. – DFTR Apr 01 '21 at 17:06
3

I have to question your goal here: Why should the library be aware of what kind of application it is running from?

To me, it sounds like you need to split the relevant part of your library in two parts - one for use with web apps, and one for use with winforms apps. (And possibly a third part, with everything that can be used by both types of apps...)

Tomas Aschan
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    You are of course free to question my goal :-) But that doesn't answer my question! Please see the updated question. – M4N Aug 10 '11 at 19:06
  • Why? If the code is running in ASP.NET it needs to call `System.Web.Hosting.HostingEnvironment.RegisterObject(obj);`. If it's in a WinForms, or other application, it has to find some other way to know that the AppDomain is shutting down. – Ian Boyd Aug 14 '13 at 19:45
0

This might be helpful:

How can I detect if my .NET assembly is running from web site or from a desktop machine?

if(HttpRuntime.AppDomainAppId != null)
{
  //is web app
}
else
{
  //is windows app
}
Community
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Fred
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0

First, I agree that the question is "wrong." Your library shouldn't care. Second, as below, there's a number of heuristics but ultimately no really good way to know. Your question doesn't make it clear why you want to know or a validation for that.

That said, if it were me, I'd probably look at System.Web.HttpContext.Current if I cared about the current request, or System.Web.HttpRuntime.AppDomainAppId if I wanted a general check.

NO GUARANTEES. This is likely a bad idea.

From Stefan on the ASP.NET team:

If it’s a web app, and its running managed code, 99.9999% likelihood its ASP.NET -). If he means something different such as looking at the IIS metabase and trying to figure out if an application is really an ASP.NET app or not – then some kind of heuristics would be needed. For example, for a given application in the IIS config/metbase, get the physical root of the application – then look and see if web.config, *.aspx, *.ashx, etc… exist in the folder, or any of the subfolders. If the answer is yes, then it is likely an ASP.NET application. Unfortunately it is not sufficient to make this determination just off of configuration data stored by IIS. Every application pool has an associated CLR version, even if no managed code ever runs in the application pool. And the second ASP.NET integration with IIS is enabled on a machine, the default module/handler lists all include ASP.NET entries.

In the case of a library, the heuristic I described would be the way to go.

In present times though, what’s “an ASP.NET application” anymore? These days I can stick classic .asp, ASP.NET, static HTML, and php all into the exact same vdir structure and have it all function semi-coherently as a single "application".

Scott Hanselman
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  • (Hi Scott). My library wants to check a config setting, but to open the config file, one needs to call different methods between ASP.NET and a local application. So yes, it would like to known the execution context to open the right config file. – David Burg Apr 01 '21 at 05:33