When does the CLR try to load a referenced assembly?

Question

I want to write a small installer app that installs a web site and creates IIS virtual directories. The app should run on Windows XP/Server 2003 (IIS 6) as well as on Vista/2008 (IIS 7).

The problem is: for IIS 6 we create virt dirs by calling WMI/Metabase API, for IIS 7 there is a much better API: Microsoft.Web.Administration, but its assembly is available only on IIS 7 systems.

Naive approach:

...
if (OperatingSystem == old)
{
    call metabase API...
}
else
{
    call Microsoft.Web.Administration...
}
...

Nice, isn't it? But how can I make sure that this does not crash on a old system just while trying to load the Microsoft.Web.Administration DLL? Or is an assembly just loaded, when it is first used? When a method that is calling into the assembly is first used?

I guess, testing does not help without some determinism being guaranteed by CLR/.NET spec.

I am really looking forward to hearing your experiences, hints or solutions for this topic. I have not found anything remotely usable on the web so far.

Answer 1

I have not been able to find the definitive answer as in a specification stating when assemblies must and must not be loaded. However, according to

http://msdn.microsoft.com/en-us/magazine/cc163655.aspx (section "Load Fewer Modules at Startup")

and the book extract at www.informit.com/articles/article.aspx?p=30601&seqNum=5 (extract from "Essential .NET, Volume I: The Common Language Runtime").

the JIT of the CLR will load a needed assembly only when needed to compile a method. Thus, you should move any use of Microsoft.Web.Administration... to a seperate method which is only called when your confident that the assembly exists on the system. That is,

   setup()
   { 
       if ( Operating.System == Old )
          call metabase API
       else
          doIIS7Setup()
   }

   void doIIS7Setup()
   {
     call Microsoft.Web.Administration ....
   }