HtmlHelper类位于System.Web.Mvc.Html之中主要有七个静态类组成：
FormExtensions - BeginForm, BeginRouteForm, EndForm
InputExtensions - CheckBox, CheckBoxFor, Hidden, HiddenFor, Password, PasswordFor, RadioButton, RadioButtonFor, TextBox, TextBoxFor
LinkExtensions - ActionLink, RouteLink
SelectExtensions - DropDownList, ListBox, ListBoxFor
TextAreaExtensions - TextArea, TextAreaFor
ValidationExtensions - Validate, ValidateFor, ValidationMessage, ValidationMessageFor, ValidationSummary
RenderPartialExtensions - RenderPartial，显示用户控件