Why is my ifndef section not working?

Question

I have the following snippet in my program.wxs file:

<?ifndef $(var.TwoOnly) ?>
        <Feature Id="FeatureOne" ... >
          ...
        </Feature>
<?endif ?>

<?ifndef $(var.OneOnly) ?>
        <Feature Id="FeatureTwo" ... >
          ...
        </Feature>
<?endif ?>

I'm compiling this using the following comand:

candle -dOneOnly=1 program.wxs

Looking at the compiled program.wixobj, I can see that Feature Two's declaration is all there, and after running light, both features are listed, and the installer functions exactly as if neither OneOnly or TwoOnly were set. Why is my ifndef block not working correctly?

Note: I also have the following:

<?ifdef $(var.OneOnly) ?>
  <?define productName = One Only ?>
<?else?>
  <?ifdef $(var.TwoOnly) ?>
    <?define productName = Two Only ?>
  <?else?>
    <?define productName = Both ?>
  <?endif?>
<?endif?>

The name, even when OneOnly is set during compile, is displayed as Both.

Edit

I've reduced the complexity of my condition to the minimal case shown in the documentation:

<?ifndef $(var.TwoOnly) ?>
  <?include featureOne.wxi ?>
<?endif?>

This still does not work.

Answer 1

When using ifdef or ifndef, the variable must be stated without $(var.) syntax.

<?ifndef TwoOnly ?>
  <?include featureOne.wxi ?>
<?endif?>

This works.