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
                {
                    FormsAuthentication.SetAuthCookie(username,false);
                    Response.Redirect("~/default.aspx");
                }
                ErrirMsgText.Text = "新用户创建成功！";
                break;
            case MembershipCreateStatus.InvalidUserName:
                ErrirMsgText.Text = "无效的用户名。";
                break;
            case MembershipCreateStatus.DuplicateEmail:
                ErrirMsgText.Text = "重复的电子邮件地址。";
                break;
            case MembershipCreateStatus.InvalidQuestion:
                ErrirMsgText.Text = "无效的密码提示问题。";
                break;
            case MembershipCreateStatus.DuplicateUserName:
                ErrirMsgText.Text = "该用户名已注册，请更换用户名。";
                break;
            case   MembershipCreateStatus.InvalidAnswer:
                ErrirMsgText.Text = "无效的密码答案。";
                break;
            case MembershipCreateStatus.InvalidEmail:
                ErrirMsgText.Text = "无效的电子邮件地址。";
                break;
            case MembershipCreateStatus.InvalidPassword:
                ErrirMsgText.Text = "密码须为７位，并且有１个特殊符号。";
                break;
            default:
                ErrirMsgText.Text = "出现未知错误！请联系网站管理员。";
                break;
        }

MembershipCreateStatus iStatus;
这个东西返回你注册用户时的结果
下面的swicth代码块则根据这个结果给出相应的提示