+ c# -> HttpContext.Current.User + directoryentry

Hi everybody

i'm trying to open a directory services connection under the
users' (who is navigating the webpage) credentials, like
impersonation, but i don't succeed, could you help me ?

the problematic line is the following :

DirectoryEntry Ldap = new DirectoryEntry(ConnexionAD,
ADusername, ADpassword);

actually, i would like to retrieve ADusername and ADpassword from
HTTPcontext.current.user becaus the user is authenticated by forms. Is
there a way for doing that ?

thx by advance