Update list item with managed metadata field returns The security validation for this page is invalid

Question

Using SharePoint 2010 Server
I'm attempting to programtically update a managed metadata field in a document library. I'm able to do it without issue on all other non-managed metadata fields. When I attempt it on a MM field I get the error message
"The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again."
After some digging I realised that this error was being caused because SharePoint was trying to write to the TaxonomyHiddenList list (../sites/mysite/Lists/TaxonomyHiddenList/AllItems.aspx)
When I update a document through the browser with a term (Term01), it shows up in this TaxonomyHiddenList . I can then run my application, apply Term01 to my new document and it works fine. But if I apply Term02 to my new document it gives me the error above.
Summary
My app can read the TaxonomyHiddenList fine but it can not perform an operation that would write to it, resulting in not being able to update the MM field.
Question
Is there any advice on how I can further debug this issue?

Answer 1

Hi,
According to your post, my understanding is that you want to update managed metadata field in document library programmatically.
I have made a simple code demo below to updata managed metadata field in document library, it works like a charm, you can refer to it.
public static void UpdateMSField()
using (SPSite site = new SPSite("http://YourSiteURL"))
using (SPWeb web = site.OpenWeb())
//SPList list = web.Lists.TryGetList("Libs_1");
SPDocumentLibrary lib = (SPDocumentLibrary)web.Lists["Libs_1"];
// No point in proceeding if we can't find the list
if (lib != null)
// add a new item
// SPListItem item = list.AddItem();
SPListItem item = lib.GetItemById(1);
//Console.WriteLine(item.Name);
// get the current taxonomy session, which wraps up all of the
// associated TermStore objects for this SPSite object
TaxonomySession metadataService = new TaxonomySession(site);
// get the taxonomy field
TaxonomyField taxField = item.Fields["MMS_1"] as TaxonomyField;
// get the term store associated with the taxonomy field
TermStore termStore = metadataService.TermStores[taxField.SspId];
// get the actual term set associated with the taxonomy field
TermSet termSet = termStore.GetTermSet(taxField.TermSetId);
// search for the terms we wish to set the field to
var terms = termSet.GetTerms("term_1", true, StringMatchOption.ExactMatch, 1, false);
// if we have found a term populate the field
if (terms.Count > 0)
// set the field to the term(s) we have found
taxField.SetFieldValue(item, terms.First());
// Update the item
item.Update();
Console.WriteLine("success...");
More reference:
http://www.3guysonsharepoint.com/?p=1052
http://vineet-winit.blogspot.com/2013/04/how-to-update-managed-metadata-field-in.html
Thanks & Regards,
Jason
Jason Guo
TechNet Community Support