How to restrict an action method to be invoked only by HTTP GET, POST, PUT or DELETE?

By default, each and every action method can be invoked by any HTTP request (i.e. GET, PUT, POST, and DELETE). But you can restrict an action to be invoked only by a specific HTTP request by applying HttpGet or HttpPost or HttpPut or HttpDelete attribute.
If you want to restrict an action method for HTTP Get request only then decorate it with HttpGet action method selector attribute as given below:

[HttpGet]
public ActionResult Index()
{
    //TODO:
    return View();
}