Using XmlSerializer to create an element with attributes and a value but no sub-element

Hopefully this should be an easy answer for someone out there (and possibly a dupe), but I can't seem to figure it out.

I need to output an element that looks like this:

<Quantity foo="AB" bar="CD">37</Quantity>

I know how to get this:

  <Quantity foo="AB" bar="CD">
    <qty>37</qty>
  </Quantity>

with a Quantity class containing

public int qty;    
[XmlAttribute]
public string foo;

[XmlAttribute]
public string bar;

but then of course whatever variable I insert the quantity into becomes its own sub-element.

On the other hand, if I make the Quantity a variable in the parent element, then I can set the value and get

<Quantity>37</Quantity>

but then I don't know how to get the attributes.

I would be very surprised if there weren't a simple way to do this with XmlSerializer, but I don't know it yet. Any ideas?