MVC中的ModelState.IsValid印证总是true

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/MasterPage.Master"    Inherits="System.Web.Mvc.ViewPage" %><asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">    Login</asp:Content><asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">    <h3>        用户登录</h3>    <% using (Html.BeginForm())       { %>    <span class="message">        <%= Html.Encode(ViewData["message"])%>    </span>    <div>        <fieldset>            <legend>登录信息</legend>            <div class="editor-label">                用户名            </div>            <div class="editor-field">                <%= Html.TextBox("username")%>                <%= Html.ValidationMessage("username")%>            </div>            <div class="editor-label">                密码            </div>            <div class="editor-field">                <%= Html.Password("password")%>                <%= Html.ValidationMessage("password")%>            </div>            <p>                <input type="submit" value="登录" />            </p>        </fieldset>    </div>    <% } %></asp:Content>

[HttpPost,ActionName("Login")]        public ActionResult LoginPost(FormCollection collection)        {            userinfo userinfo=new userinfo();            userinfo.username = collection.GetValue("username").AttemptedValue;            userinfo.password = collection.GetValue("password").AttemptedValue;            if (!ModelState.IsValid)                return View();            else            {                userinfo = dal.Login(userinfo.username, userinfo.password);                if (userinfo != null)                {                    Session.Add("username", userinfo.username);                    return RedirectToAction("index");                }                else                {                    ViewData["message"] = "用户名或密码错误，请重新输入！";                    return View();                }            }        }