With Orcas supporting multi-targeting it'll be common to work in Orcas with .NET 2.0 projects. But when you take a VS 2005 project/solution and open it in Orcas it wants to update to an Orcas solution and projects. Which is fine if you're working on your own thing, but if you share around projects and solutions you might find that you actually would like to keep your projects in VS 2005 format so those that only have VS 2005 can open them.
I figured this would be a problem since Orcas explicitly asks to update your projects to the Orcas project types. But to my surprise I found that an Orcas Project files (2.0 targeted at least) work just fine in Visual Studio 2005 which makes sending off projects to VS 2005 users at least possible.
So I took a look to see what's actually different in the project formats. I looked at several different projects - a Web Application Project, and several Class Library projects that had been ported to Orcas and have been modified over the last few months. All of these projects could be opened with Visual Studio individually.
Sticking the project file into Beyond Compare to see what's different yields only a couple of small version number changes a few apparently optional additional attributes:
Surprising all the 'upgrade' fuss that Orcas makes huh? <s> So project files pretty much work two-way. Even if you open a project in Visual Studio 2005 and save it it leaves the above version changes alone so if you go back to Orcas it just opens without popping up the upgrade dialog again.
Now the Solution file is a different story however. An Orcas Solution file will not open in Visual Studio 2005. What's odd is that it appears that Orcas is actually REMOVING content from the file rather than adding new features to it:
So Solution files can't go back and forth only forward to Orcas. That's not a huge deal as it's relatively easy to recreate a solution file if necessary or even easier to just keep around a second copy of a solution file for VS 2005 if necessary. Solution setting usually don't change to frequently anyway.
However, it's also really, really easy to fix the solution file (at least for the 3 or 4 solutions I tried), by simply changing the headers back to VS 2005 format (ie. change the version to 9.00 and Visual Studio 2005) and you're back in business with VS 2005.
Of course if you start adding Framework 3.5 to your project there's no going back. I think this will be a far more common scenario, but while playing around with Orcas right now the 2.0 targeting is a really nice way to familiarize yourself with Orcas. I know I've been using Orcas almost exclusively for the last couple of months for 2.0 coding and it's worked out great.
Looks like Microsoft really got this right: Making it easy to move between versions. If you remember all the pain that went with upgrading to .NET 2.0 (especially ASP.NET 2.0) Orcas and even .NET 3.5 is going to be a relative painless path by comparison.