Question

Is MvcApplication singleton in a MVC 3 Web Application? Why did I find 3 instances?

My steps: (VS2010 SP1 + ASP.NET MVC 3 Tools Update)

1. create a default Internet MVC 3 Appliction (with the account sample)
2. Add a MvcApplication() constructor in global.asax, and set a breakpoint in it
3. Press F5 to debug

I found the constructor was called 3 times!

Best regards,

Zach@Shine

===Edit===

Correction:

2 instances, not 3 were found for 1 web app.
After a careful check, I found my solution contains 2 web apps, after I removed one of them, I can find 2 instances for the left app.

Here is the log (the numbers are the MvcApplication instances’ hashcode)

MvcApplication(): 41516761
Application_Start(): 41516761
 MvcApplication(): 56552081
 Application_AuthenticateRequest(): 56552081
 Application_AuthenticateRequest(): 56552081
 Application_AuthenticateRequest(): 56552081
 MvcApplication_Disposed(): 56552081
Application_End(): 41516761

As shown, one instance is used to handle Start, End events, and the other one is used to handle requests.

Answer 1

MvcApplication inherits from HttpApplication from the normal asp.net below is a great link to understanding the asp.net pipeline. I have copied the relevant section from the article.

In short – no MvcApplication is not singleton but there is only ever one present with each request.

http://www.west-wind.com/presentations/howaspnetworks/howaspnetworks.asp

Each request is routed to an HttpApplication object. The
HttpApplicationFactory class creates a pool of HttpApplication objects
for your ASP.NET application depending on the load on the application
and hands out references for each incoming request. The size of the
pool is limited to the setting of the MaxWorkerThreads setting in
machine.config’s ProcessModel Key, which by default is 20.   The pool
starts out with a smaller number though; usually one and it then grows
as multiple simulataneous requests need to be processed. The Pool is
monitored so under load it may grow to its max number of instances,
which is later scaled back to a smaller number as the load drops.  
HttpApplication is the outer container for your specific Web
application and it maps to the class that is defined in Global.asax.
It’s the first entry point into the HTTP Runtime that you actually see
on a regular basis in your applications. If you look in Global.asax
(or the code behind class) you’ll find that this class derives
directly from HttpApplication: