如何判断注册用户是否已经存在（membership验证）

MembershipCreateStatus iStatus;        Membership.CreateUser(username, password, email, question, answer, true, out iStatus);
        ErrirMsgText.Visible = true;
        switch (iStatus)
        {
            case MembershipCreateStatus.Success:
                if (Request.QueryString["ReturnUrl"] != null)
                {
                    FormsAuthentication.RedirectFromLoginPage(username, false);
                }
                else