A function written before realizing OpenRA has FindActorsInCircle.

bool IsInRangeOf.cs

bool IsInRangeOf(Actor actorInQuestion, Actor sourceActor, bool use3D = false)
{
        // convert distance (px -> cells)
        var distX = (sourceActor.Location.CenterPosition.X - actorInQuestion.Location.CenterPosition.X) / 1024;
        var distY = (sourceActor.Location.CenterPosition.Y - actorInQuestion.Location.CenterPosition.Y) / 1024;
        var distZ = (sourceActor.Location.CenterPosition.Z - actorInQuestion.Location.CenterPosition.Z) / 1024;
 
        // convert Info.Range (px -> cells)
        var range = (Info.Range * 1024);
 
        if (use3D)
                if (distX < range && distY < range && distZ < range)
                        return true;
 
        if (distX < range && distY < range)
                return true;
 
        return false;
}