Thank you for your answer. But I have difficulties...
My XML :
<dfs:myFields xmlns:dfs="http://schemas.microsoft.com/office/infopath/2003/dataFormSolution" xmlns:tns="http://tempuri.org/" xmlns:my="http://schemas.microsoft.com/office/infopath/2003/myXSD/2006-12-05T08:48:33" xmlns:msdata="urn:schemas-microsoft-com:xml-msdata" xmlns:_xdns0="http://schemas.microsoft.com/office/infopath/2003/changeTracking" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
My declaration :
XDocument.GetDOM("AfficheAnnonce").setProperty("SelectionNamespaces",'xmlns:tns="http://tempuri.org/"');
And my selectSingleNode :
XDocument.DOM.selectSingleNode('/my:myFields/my:dataFields/tns:GetAnnonce2Response/tns:GetAnnonce2Result/tns::NewDataSet/tns:tbAnnonces/tns:NUM');
Sorry, my English is bad, and sometimes I have problems for details. Thanks a lot !