Domain security level & Xpath in infopath 2007

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VB

Hi all ,
I publish a form with info path 2007. I put a security level >> domain when i use this code and run it:

Dim mainDomNavigator As XPathNavigator = Me.MainDataSource.CreateNavigator()
Dim userNavigator As XPathNavigator = mainDomNavigator.SelectSingleNode("/my:myFields/my:field1", Me.NamespaceManager)
userNavigator.SetValue("My Test")

It throws a SecurityException that say "check to make sure you have fulfilled all necessary security requirements" and "Use a certificate to obtain the required permission" and "if an assembly implementing the custom security object reference other assemblies, add the reference assemblies to the full trust assembly list"

The exception appear when i use Xpath.
Can any one help me? :rose: