Hi Bonjour,
Welcome to Forums!
Instead of using:
IXMLDOMNode dest = newForm.DOM.selectSingleNode("/dsf:myFields/dfs:queryFields/q:Projet/@CodePrj");
Use the below line of code:
IXMLDOMNode dest = newForm.DOM.selectSingleNode("/dfs:myFields/dfs:queryFields/q:Projet/@CodePrj");
Because in the first line of code 'dsf' is undeclared name space. So, it is giving
"Référence à un préfixe d'espace de noms non déclaré : 'dsf'." that error
Hope this will help you to resolve your problem