Bob responds: Sort of. You can set the runtime to use on a site by site basis interactively in IIS, too; aspnet_regiis is a way to do this in a batch mode for multiple sites. I frankly don’t know if IIS Manager calls aspnet_regiis to do this work for a single site, or if some other mechanism is used. But it makes sense that IIS would do so, because otherwise the IIS snap-in would have to be updated for each new framework release.

Another issue is that you can’t run different versions of the framework within the same app pool; that has to be managed from IIS as far as I know. If anyone has more experience with this, please chime in. Oh — and you have to make sure each version of the .NET framework you want to use has been permitted within the IIS “web services extensions” node. IIS 6 has all of them locked down by default.

Bob responds: Ah, so you’ve encountered the old “installed IIS after .NET” problem, too ;-) Yes, the issue exists and is compounded for later .NET versions because you may have the need to move some or all sites to (say) 2.0 from 1.1. In that case you’d run the 2.0 version of aspnet_regiis — using the -i argument if you want to do this for all sites, or the -sn argument if you want to do it for selected sites. You can list what sites are using what runtimes with aspnet_regiis -lk. Scott Forsyth has an excellent summary post here with more info.