为什么我的MVC验证中ModelState.IsValid总是为true?
public class HomeController : Controller
    {
         public ActionResult Create()
        {
            return View("Create");
        }

        [HttpPost]
        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult Create(Role role)
        {


            if (ModelState.IsValid)
            {
                return this.View("RoleList");
            }

            return this.View(role);
        }
    }

 

public partial class Role
    {
        public bool IsValid
        {
            get { return (GetRuleViolations().Count() == 0); }
        }
        public IEnumerable<RuleViolation> GetRuleViolations()
        {
            if (String.IsNullOrEmpty(RoleID))
                yield return new RuleViolation("RoleID不能为空", "RoleID");
            yield break;
        }

    }

    public class RuleViolation
    {
        public string ErrorMessage { get; private set; }
        public string PropertyName { get; private set; }
        public RuleViolation(string errorMessage, string propertyName)
        {
            ErrorMessage = errorMessage;
            PropertyName = propertyName;
        }
    }

 

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<dynamic>" %>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    Create
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
    <div id="tabDiv" style="margin: 0px 6px 6px 6px;">
        <h2 style="margin: 5px 20px 5px 0px;">