Site Columns Missing From Your Content Types?

I was playing around with site columns and content types in an empty SharePoint solution.  Here’s something interesting I found when I was deploying my package (solution).  You can’t seem to have comment tags in the <FieldRefs> element when you’re defining the fields for your content type.

In my content type below based off the Item content type, I was just trying to add a custom site column (SessionName) I had created.  However, when I went to look at the site columns after deploying it, the only site column was “Title” which came from Item.

<elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <!-- Parent ContentType: Item (0x01) -->
  <contenttype description="Mojo Item Content Type" group="Mojo Custom Content Types" id="0x01006e1d2ee8a874413dbe16d4ca2980cd0e" inherits="TRUE" name="SharePointProject1 - Item" version="0">
    <fieldrefs>
      <!-- Testing -->
      <fieldref id="{36EFEDE7-3F78-4DB1-9A98-C94BF1E57133}" name="SessionName">
    </fieldref></fieldrefs>
  </contenttype>
</elements>

However, after I removed the comments:

<elements xmlns="http://schemas.microsoft.com/sharepoint/">
  <!-- Parent ContentType: Item (0x01) -->
  <contenttype description="Mojo Item Content Type" group="Mojo Custom Content Types" id="0x01006e1d2ee8a874413dbe16d4ca2980cd0e" inherits="TRUE" name="SharePointProject1 - Item" version="0">
    <fieldrefs>
      <fieldref id="{36EFEDE7-3F78-4DB1-9A98-C94BF1E57133}" name="SessionName">
    </fieldref></fieldrefs>
  </contenttype>
</elements>

The “SessionName” site column appeared in my content type after I redeployed the solution. Just something to keep in mind.