ASP.NET MVC HttpPost and SignOn() confusion

As I understand the [HttpPost] attribute, or any POST method for that matter, it is used when state is changed. However, if you set up Forms Authentication with a loginUrl like:

<forms loginUrl="~/Account/LogIn" ...

this will force a redirect when an [Authorize] attribute is encountered. Example:

[Authorize]
public ActionResult AccessPrivateData()
{
    // Should redirect to /Account/LogIn if AuthCookie not set
    // ...
}

So far so good. My problem is that I can't use [HttpPost] for the LogIn action now (because you can't redirect to a POST):

[HttpPost]
public ActionResult LogIn(string username, string password)
{
    // W开发者_如何学编程on't find the URL (/Account/LogIn) if redirected to here...
    // ...
}

but wouldn't a LogIn action indeed change state, warranting a POST? Please someone offer some explanation, and if you can, how you deal with this.

---

You could have two LogIn actions. The redirect will use a GET and get sent to the action that simply renders the login form.

When the form is posted, it will use the method decorated with [HttpPost]

[HttpGet]
public ActionResult Login()
{
  // Render view
}

[HttpPost]
public ActionResult LogIn(string username, string password)
{
  // Process form post
}