Simple login for multi-domain intranet

// NOTE: implement IDisposable and dispose of this if not null when done.
private DirectoryEntry userSearchRoot = null;
private UserPrincipal FindUserInGlobalContext( string userName )
{
    using (PrincipalSearcher userSearcher = new PrincipalSearcher())
    {
        using (PrincipalContext context
                 = new PrincipalContext( ContextType.Domain ))
        {
            userSearcher.QueryFilter = new UserPrincipal( context );
            DirectorySearcher searcher
                 = (DirectorySearcher)userSearcher.GetUnderlyingSearcher();

            // I usually set the GC path from the existing search root
            // by doing some string manipulation based on our domain
            // Your code would be different.
            string GCPath = ...set GC path..

            // lazy loading of the search root entry.
            if (userSearchRoot == null)
            {
                userSearchRoot = new DirectoryEntry( GCPath );
            }

            searcher.SearchRoot = userSearchRoot;
            using (PrincipalContext gcContext =
                     new PrincipalContext( ContextType.Domain,
                                           null,
                                           GCPath.Replace("GC://",""))
            {
                UserPrincipal userFilter = new UserPrincipal( gcContext );
                userFilter.SamAccountName = userName;
                userSearcher.QueryFilter = userFilter;
                return userSearcher.FindOne() as UserPrincipal;
            }
        }
    }
}