How to find out if the current application is an ASP.NET web app

Question

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?

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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!

Answer 1

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");

Answer 2

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...)