C# 验证输入的Account是不是域合法的用户，组或机器。(User, Group, Machine)

            Principal accountInfo;            
            using (PrincipalContext ctx = new PrincipalContext(ContextType.Domain, domain))
            {
                accountInfo = Principal.FindByIdentity(ctx, name);
            }

            if (accountInfo == null)
            {
                return false;
            }

　　　　　　　AccountType accountType
            if (accountInfo is UserPrincipal)
            {
                accountType = AccountType.User;
            }
            else if (accountInfo is GroupPrincipal)
            {
                accountType = AccountType.Group;
            }
            else
            {
                accountType = AccountType.Machine;
            }