If you chose the contract first approach then you need to ensure that any element declaration that should use SwaRef encoding simply refers to wsi:swaRef schema type:

 
<element name="data" type="wsi:swaRef" 
xmlns:wsi="http://ws-i.org/profiles/basic/1.1/xsd"/>
 

Any wsi:swaRef schema type would then be mapped to DataHandler.