Thanks for your responses,
It is OK to make the link to the dll in VS studio, it works well.
The problem is when I ask for a preview from VS studio and I click the button responsible to open the windows form from the DLL, an infopath windows appears with the following error message :
System.Security.SecurityException
Security error.
at Form.Form.BrowseButton_OnClick(DocActionEvent e)
at Microsoft.Office.Interop.InfoPath.SemiTrust._ButtonEventSink_SinkHelper.OnClick(DocActionEvent pEvent)
After that the form will be full trusted (I use the refForm Tool with an urn)
Do you have any ideas ?
Thank you very much
Christophe