A lot of applications like to know what version of the operating system they are running on. Sometimes (especially older applications) it's so they can flat-out refuse to run if you're still on, say Windows 98. This is now frowned on, by the way, and if you want a Windows 7 logo you must not refuse to run based on a version check. Other times (and this is bad too) it's as part of some arcane roll-your-own strategy. For example someone I know planned to work out the default hard code paths for user documents (C:\Documents and Settings\ ... vs C:\users\... ) based on OS versions. This is insane! There's already a function you can call that will get you that path, and it works even if the user has changed it from the default.

But there are still legitimate reasons for version checking. If you're using Vista or Windows7 light-up features like the Task Dialog, Restart and Recovery, or Taskbar Jump Lists, you'd better make sure you're on an OS that supports those features. It's pretty easy from managed code: just call System.Environment.OSVersion.Version.Major and System.Environment.OSVersion.Version.Minor. From native code, GetVersionEx() does the same job. Then you have to do some comparing. That's where things can get weird.

For example, a huge incompatibility bucket for Vista was applications whose code checked that the version was exactly 5.1. If not, it would pop up a message box saying it only ran on XP SP2 or better. I've seen these apps in the wild and I just can't keep from laughing. The error message itself says "or better", and that was presumably the intent, but the code is checking for equality. The magic of >= fixes this "incompatibility" bug in the code, and often these applications don't need any other fixes to make them work on newer OS versions. (That's why the logo people frown on refusing to run. How can you know you won't work on a later OS that doesn't exist yet?)

Even if you grasp the magic of >=, comparing major and minor versions can bite you. Larry Osterman shows us this code:

// Example in C#.
 
internal bool SupportsTaskProgress() {
    if (System.Environment.OSVersion.Version.Major >= 6) {
        if (System.Environment.OSVersion.Version.Minor >= 1) {
            return true;
        }
    }
    return false;
}

He rightly points out it will return false if the version is 7.0, or 8.0, or any other .0 that's larger than 6. (Yes, Windows 7 returns 6.1 for the version #, that's a long story and well covered elsewhere.)

Version checking is hard. If you can, use a library that does it for you. Vista Bridge, for example, not only wraps up cool new OS features for easy access from managed code, it also does some checking to see if the feature is there for you or not. Don't reinvent wheels. Also, check for the feature rather than the OS if you can. Larry discusses this in his post for Taskbar features. Any feature you plan to use, you should know how to check for. I hope to post some more links and examples on this soon.

Kate