authentication using CredentialCache against proxy server problem

[Tom]

Hi all

I'm not able to authenticate against proxy server when I try to pass an
instance of
CredentialCache to WebProxy instance used with HttpWebRequest.WebProxy or
SoapHttpClientProtocol instance.
Everything works well when I pass NetworkCredential instance directly and
unregister
some AuthenticationModules to force choosen type of
authentication. [Negotiate,Kerberos,NTLM which my proxy supports].
The way using CredentialCache works well against IIS hosting my webservice,
but not proxy server. What could cause this problem?

Many thanks for all advices/suggestions

Tom