.net 4.5 webform 提取ModelState错误信息

public class User
    {
        [Required]
        [Display(Name = "用户ID")]
        public int Id { get; set; }
        [Required]
        [StringLength(20)]
        [Display(Name = "用户名")]
        public string UserName { get; set; }
        [Required]
        [StringLength(20)]
        [Display(Name = "用户密码")]
        public string PassWord { get; set; }
    }

var user = new Model.User();
var formValueProvider = new FormValueProvider(ModelBindingExecutionContext);
TryUpdateModel(user, formValueProvider);
//模型验证
if (ModelState.IsValid)
{
     Response.Write(user.UserName);
}

<asp:ValidationSummary runat="server" EnableViewState="True"/>

public static class ModelStateErrorHandler
    {
        /// <summary>
        /// ModelState扩展，取得Dictionary类型的错误信息
        /// </summary>
        /// <param name="errDictionary"></param>
        /// <returns>
        /// Key: 字段名
        /// Value: 字段错误信息
        /// </returns>
        public static Dictionary<string, string> GetModelErrors(this ModelStateDictionary errDictionary)
        {
            var errors = new Dictionary<string, string>();
            foreach (var pair in errDictionary.Where(k => k.Value.Errors.Count > 0))
            {
                var er = string.Join(",", pair.Value.Errors.Select(e => e.ErrorMessage));
                errors.Add(pair.Key, er);
            }
            return errors;
        }
        /// <summary>
        /// ModelState扩展，取得string类型的错误信息
        /// </summary>
        /// <param name="errDictionary"></param>
        /// <returns>
        /// 以|分割错误信息
        /// </returns>
        public static string GetModelErrorsString(this ModelStateDictionary errDictionary)
        {
            var errorsBuilder = new StringBuilder();
            var errors = errDictionary.GetModelErrors();
            foreach (var key in errors)
            {
                errorsBuilder.AppendFormat("{0}:{1}|", key.Key, key.Value);
            }
            return errorsBuilder.ToString();
        }
    }

var errorsDictionary = ModelState.GetModelErrors();
string jsonstr = new JavaScriptSerializer().Serialize(errorsDictionary);
var errorsString = ModelState.GetModelErrorsString();