redirect to action in void function

Question

If I have

public ActionResult Join(int? id)
{
   if (id == null)
   {
     return RedirectToAction("Index");
   }

   return View();
}

It works well. How can I make this code reusable? I must call it in many actions. I've tried this:

public ActionResult Join(int? id)
{
    isNull(id);

    return View();
}

public void isNull(int? id)
{
    if (id == null)
    {
        RedirectToAction("Index");
    }
}

But it doesn't redirect.

Answer 1

You could do some functional programming:

protected ActionResult WithID(int? arg, Func<int, ActionResult> logic)
{
  if (arg == null)
  {
    return RedirectToAction("Index");
  }

  return logic(arg.Value);
}

invoked like this:

public ActionResult Join(int? arg)
{
  return WithID(arg, (id) => 
  {
    return View();
  });
}