dropdownlist with auto-post back in mvc

public class User
 {
     public int Id { set; get; }
     public string Name { get; set; }
     public int CompanyId { get; set; }
 }

public class HomeController : Controller
   {
       private const string companyIdHiddenValue = "companyIdHiddenValue";

       private List<User> Users;

       public HomeController()
       {
           // don't get all user list here. I just added for demonstration.
           this.Users = new List<User>()
                            {
                                new User(){Id = 1, Name = "User1", CompanyId = 1},
                                new User(){Id = 1, Name = "User2", CompanyId = 5},
                                new User(){Id = 1, Name = "User3", CompanyId = 4},
                                new User(){Id = 1, Name = "User4", CompanyId = 3},
                                new User(){Id = 1, Name = "User5", CompanyId = 2},
                                new User(){Id = 1, Name = "User2", CompanyId = 1},
                                new User(){Id = 1, Name = "User3", CompanyId = 5},
                                new User(){Id = 1, Name = "User4", CompanyId = 2},
                                new User(){Id = 1, Name = "User5", CompanyId = 2},
                                new User(){Id = 1, Name = "User2", CompanyId = 3},
                                new User(){Id = 1, Name = "User3", CompanyId = 4},
                                new User(){Id = 1, Name = "User4", CompanyId = 4},
                                new User(){Id = 1, Name = "User5", CompanyId = 2},
                                new User(){Id = 1, Name = "User2", CompanyId = 5},
                                new User(){Id = 1, Name = "User3", CompanyId = 1},
                                new User(){Id = 1, Name = "User4", CompanyId = 1},
                                new User(){Id = 1, Name = "User5", CompanyId = 1},
                            };
       }

       public ActionResult List()
       {
           this.GetCompany();
           return this.View();
       }

       [HttpPost]
       public ActionResult List(string companyId)
       {
           this.GetCompany(companyId);
           var cId = Convert.ToInt32(companyId, CultureInfo.InvariantCulture);
           return this.PartialView(this.Users.Where(x => x.CompanyId == cId).ToList());
       }

       private void GetCompany(string companyId = null)
       {
           var companyList = new List&lt;SelectListItem>
           {
               new SelectListItem
                   {
                       Selected = true,
                       Text = "Select",
                       Value = string.Empty,
                   }
           };

           companyList.AddRange(new[]
                                    {
                                        new SelectListItem { Text = "Comapny1", Value= "1"},
                                        new SelectListItem { Text = "Comapny2", Value = "2"},
                                        new SelectListItem { Text = "Comapny3", Value = "3"},
                                        new SelectListItem { Text = "Comapny4", Value = "4"},
                                        new SelectListItem { Text = "Comapny5", Value = "5"}
                                    });

           var firstOrDefault = companyList.FirstOrDefault(x => x.Value == companyId);

           if (firstOrDefault != null)
           {
               firstOrDefault.Selected = true;
           }

           ViewBag.CompanyDropDown = companyList;
       }
   }

@using MvcApplication1.Models
@model List<mvcapplication1.models.user>

@{ Html.RenderPartial("UserFilter"); }

@Html.Hidden("companytIdHiddenValue")
<div id="divUserList">
    @{Html.RenderPartial("UserList", Model ?? new List<user>());}
</user></div>
</mvcapplication1.models.user>

@using MvcApplication1.Models
@model List<user>
<table>
    <tr>
        <th>Id</th>
        <td>Name</td>
        <td>Comapny Id</td>
    </tr>
    @foreach (var user in Model)
    {
        <tr>
            <td>@user.Id</td>
            <td>@user.Name</td>
            <td>@user.CompanyId</td>
        </tr>  
    }
</table>
</user>

<script src="~/Scripts/jquery-1.8.2.js"></script>
<script type="text/javascript">
    $(document).ready(function () {
        $("#CompanySelection").change(function () {
            $("#companyId").val($(this).val());
        });
    });

    function CheckUserValidation() {
        if ($('#companyId').val() == undefined || $('#companyId').val() == '') {
            alert('please select a company');
        }
    }

    function successUserAjax() {
        alert('ajax completed');

    }

    function onAjaxFailure() {
        alert('please try again');
    }
</script>

@using (Ajax.BeginForm("List", "Home", new AjaxOptions { UpdateTargetId = "divUserList", HttpMethod = "POST", OnSuccess = "successUserAjax", OnFailure = "onAjaxFailure" }))
{
    <ul>
        <li>
            <label>Company:<label>
            @Html.DropDownList("CompanySelection", (IEnumerable<SelectListItem>)ViewBag.CompanyDropDown)
            @Html.Hidden("companyId")
        </label></label></li>
    </ul>
    <input type="submit" value="Get" name="filter" onclick="return CheckUserValidation();" />
}



</script>