Is it possible to search for DataObjects that extend a particular extension?

I'm trying to make a build task that will populate some fields based on whether they extend a custom extension. We don't appear to be able to use DataObject::get() - so is there a generic way of filtering DataObjects, or do I have to specify every DataObject & query them that way?

You should be able to use the 2 native php functions get_declared_classes and is_subclass_of

$i = 0;
foreach (get_declared_classes() as $class) {
        if (is_subclass_of($class , 'DataObject')) {
        echo "<p>Class $class</p>";
                $i++;
        }
}
echo "<p>$i classes extend from DataObject</p>";

Thanks Kirk.

I'm not sure whether this is because I'm running this inside a build task, but consequently I only get Silverstripe's defined classes & none of my own. Do you think it's possibly due to the way that SS lazy loads the ORM & hasn't encountered one of these dataobjects & hence they aren't available?

I did initially think it could be to do with how the order of the build tasks runs.
If you add a your own model you need to run a dev/build for the DB table to be created and you can see this on the output so it must get around to loading all classes eventually.

However looking at framework/model/DatabaseAdmin.php the build task there uses the following line

                // Get all our classes
                SS_ClassLoader::instance()->getManifest()->regenerate();

It might be an idea to look at SS_ClassLoader/manifests or replace the the get_declared_classes with a call to SS_ClassLoader::instance()->getManifest()->regenerate()