I'm at Microsoft's Mix09 conference in Las Vegas, where the big news (aside from small matters like the final release of Internet Explorer 8) is the beta release of Silverlight 3.0, Microsoft's browser plug-in that competes with Flash as a platform for rich internet applications.

Silverlight 3.0 is stuffed with new features, one of which is the ability to run outside the browser. Right-click an applet, choose Install onto this computer, and suddenly you have a desktop application complete with a shortcut in the Start menu, or on the Mac desktop. This has triggered a debate over how SLOOB (that's Silverlight Out Of Browser) compares to Adobe's existing AIR (Adobe Integrated Runtime), which supports Flash applications running on the desktop.

Understandably, Adobe folk like Ryan Stewart are emphasising the differences:

"I think AIR and the Silverlight OOB (Out of Browser) are two very different technologies for two very different scenarios...AIR is about letting you take your web application skills to build desktop applications and Silverlight OOB is more about letting you take your Silverlight applications to the desktop. The different models will be different for everyone, but right now AIR gives you a lot more flexibility and more API hooks into the operating system."

The big issue is how much these desktop/web hybrids interact with the local machine. Microsoft makes a point of how SLOOBs run in the same sandbox as they would in the browser, which means the user can install without a security prompt, but also that the application has very access to the user's desktop. The only access to the file system is to a protected area called isolated storage, or via a file dialog that the user controls. Microsoft says there might (or might not) be a way of displaying notifications but that is about it.

AIR applications by contrast run with the same permissions as the user. Installation involves the user consenting to a dialog which usually threatens unrestricted access to the system. Your AIR app can read and write any files that the user can read or write. There is also a notification API, and support for drag-and-drop.

That said, AIR applications are also restricted by design. They cannot be extended with native code, or execute other applications. Adobe has always presented AIR as a way of delivering web applications on the desktop, rather than as an alternative to C++ or .NET for traditional desktop applications. So is it really so different from SLOOB?

Well, some of the AIR features do give it an edge. One is the ability to fire notifications (annoying though this can be). It's something that Microsoft definitely intends to add to SLOOBs, though I get a different answer every time I ask about it; it might not come until Silverlight 4.0. Another difference is that AIR can host HTML as well as Flash content, which is an advantage for applications that depend on both. AIR also includes an embedded relational database engine, SQLite, whereas SLOOB developers will have to roll their own or use XML, though I doubt this is really a big deal. One other thing: Silverlight knows nothing about printing, which is unfortunate when running outside the browser.

Nice features; but I still feel that AIR and SLOOB are close competitors. Both take a cross-platform, rich internet application runtime and make it available from a desktop shortcut and with offline support. That makes a huge difference to users, even if the technical differences between running in or outside the browser are not so great. SLOOBs are going to be attractive to .NET developers because they include the .NET runtime and can be developed in the familiar Visual Studio IDE.

Personally I'd like to see an option to run AIR applications sandboxed, as full file access is more than most of them need. I'd also like an option to run SLOOBs with greater local system access when needed. Why not have the best of both worlds?

In the meantime, I'm expecting both to succeed. For applications that will work as a SLOOB, it makes a great user-friendly, cross-platform alternative to Windows Forms; and we've already seen effective user of AIR for cloud-centric utilities like Twitter clients. Both platforms are also highly effective for visualizing data. Who knows, this approach could become the norm for a wide range of business applications.