How to determine an action method is invoked by HTTP GET or POST?

By using HttpMethod property of HttpRequestBase class, you can find out whether an action is invoked by HTTP GET or POST.

public ActionResult Index(int? id)
{
    if (Request.HttpMethod == "GET")
    {
        //TODO:
    }
    else if (Request.HttpMethod == "POST")
    {
        //TODO:
    }
    else
    {
        //TODO:
    }
    return View();
}