How to get the DomainSid or domain Sid ?

Hi,

I would like to use this:

SecurityIdentifier sidAdmin = new SecurityIdentifier(WellKnownSidType.AccountDomainAdminsSid, domainSid);

But i don't know how to get the domainSid. I tried this:

SecurityIdentifier domainSid = new SecurityIdentifier(WellKnownSidType.BuiltinDomainSid,null);

But when it is applied, an exception indicates that the domainSid is not valid.

How to retrieve it?

Thanks