public class User
{
    public string UserName { get; set; }
    public int Age { get; set; }
    public int UserID { get; set; }
    public static List<User> GetUsers()
    {
      List<User> userList = new List<User>();
      User user = null;
      user = new User();
      user.UserID = 1;
      user.UserName = "小明";
      user.Age = 20;
      userList.Add(user);
      user = new User();
      user.UserID = 2;
      user.UserName = "小红";
      user.Age = 21;
      userList.Add(user);
      user = new User();
      user.UserID = 3;
      user.UserName = "小强";
      user.Age = 22;
      userList.Add(user);
      return userList;
    }
    public static User GetUserById(int userId)
    {
      return GetUsers().SingleOrDefault(u=>u.UserID==userId);
    }
}

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<MvcApplication2.Models.User>" %>
<div>
  <%if (Model != null)
   {%>
  用户名：<%=Model.UserName%><br />
  年龄：<%=Model.Age%>
  <%} %>
</div>

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Site.Master" Inherits="System.Web.Mvc.ViewPage<MvcApplication2.Models.User>" %>
<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
  Index
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
  <h2>Index</h2>
  <%=Html.DropDownList("users", ViewData["users"] as List<SelectListItem>)%>
  <div id="userDetails">
    <%Html.RenderPartial("UserDetails", Model); %>
  </div>
</asp:Content>
<asp:Content ID="Content3" ContentPlaceHolderID="HeadMeta" runat="server">
  <script language="javascript" src="/Scripts/user.js" type="text/javascript"></script>
</asp:Content>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using MvcApplication2.Models;
namespace MvcApplication2.Controllers
{
  public class UserController : Controller
  {
    public ActionResult Index()
    {
      List<SelectListItem> userIdList = new List<SelectListItem>();
      foreach (MvcApplication2.Models.User item in MvcApplication2.Models.User.GetUsers())
      {
        userIdList.Add(new SelectListItem { Text = item.UserName,Value = item.UserID.ToString()});
      }
      ViewData["users"] = userIdList;
      MvcApplication2.Models.User user = MvcApplication2.Models.User.GetUsers().FirstOrDefault();
      return View(user);
    }
    public PartialViewResult UserDetails(int? userId)
    {
      MvcApplication2.Models.User user = null;
      if (userId == null)
      {
        user = MvcApplication2.Models.User.GetUsers().FirstOrDefault();
      }
      else
      {
        user = MvcApplication2.Models.User.GetUserById(userId.Value);
      }
      return PartialView(user);
    }
  }
}

<script language="javascript" type="text/javascript">
    mySite = {
    actions : {
        userDetails: '<%=Url.Action("UserDetails","User")%>'
    }
};
</script>

$(document).ready(function () {
  $("#users").change(function () {
    dropDownChange();
  });
});
function dropDownChange() {
  var userId = $("#users").val();
  $.ajax({
    type: "POST",
    url: mySite.actions.userDetails,
    data: { userId: userId },
    success: function (data) {
      $("#userDetails").html(data);
    }
  });
}