SharePoint 2010: How to set Taxonomy Field Values programmatically

There are now many blogs and forum posts explaining how to add Taxonomy Fields to a list via the UI, via CAML or programmatically. The end result is a Taxonomy Field linked with a hidden text field, with the hidden text field usually taking the name of the Taxonomy Field appended with ‘TaxHTField0’.

The problem is there are also many posts explaining how to set values in Taxonomy Fields programmatically or how to query for list items using a Taxonomy Field, but they usually leave something out with consequences that are not immediately apparent or use unsupported methods.

The Taxonomy Field ultimately requires 3 values to be set correctly, the WSS ID, the term name, and the term GUID.

The term name and GUID are straight-forward assuming you have pre-populated the term store providing the GUIDs rather than letting them get auto-generated, or alternatively you can lookup the term in the term store and find out the GUID.

The WSS ID however has many confused. When a term is used for the first time in a site collection, the term is also added automatically behind-the-scenes to a hidden site collection list called ‘TaxonomyHiddenList’. This among other things acts as a cache of previously used terms in the site collection. The WSS ID of a term is only unique to a site collection, and is the value of the numeric list item ID in the TaxonomyHiddenList.