How to pass Viewdata value to masterpage in Razor MVC4

Question

I use Directoryservices for login in My page. I need to pass the username to my masterpage to display the username in all the pages.

I got the username and stored it in a ViewData. How to pass the viewdata value in masterpage.
My code :

 [HttpPost]
    public ActionResult Index(LoginModels model)
    {
        if (ModelState.IsValid)
        {
            string DisplayUserName = string.Empty;
            string LoginUser = model.Userid;
            string LoginPassword = model.Password;    
            string  name = model.UserName
            if (ValidateActiveDirectoryLogin(LoginUser, LoginPassword, out DisplayUserName) == true)
            {
                model.UserName = DisplayUserName;
                ViewData["UserName"] = "Welcome" + DisplayUserName;
                return RedirectToAction("Index", "MPP", new { UserID = LoginUser });
            }          
            else
            {
                ModelState.AddModelError("","Invalid Username or Password");
            }               
        }
        return View();          
    }

In Layout page :

   @{  @ViewData["UserName"]   }

I tried the following way to display the Username. But it throws nullexception.
EDIT :

@foreach (var m in IEnumerable<SampleECommerce.Models.LoginModels>)ViewData["UserName"])
{ 
    @m.UserName
} 

Answer 1

There are some misunderstandings, like if you set ViewData["UserName"] to a string value you get a IEnumerable<SampleECommerce.Models.LoginModels>. Here is another solution:

Put this to layout page:

<span>@{Html.RenderAction("actionname", "controllername");}</span>

And in related action:

 public ActionResult actionname() {
        string result = getusername();
        return Content(result);
    }


[NoneAction]
private string getusername(){
    return (Membership.GetUser()!= null) ? Membership.GetUser().UserName : "Guest";
}