为什么BindAttribute应该声明为static

Question

    private string attrNames= "ID,Title,ReleaseDate,Genre,Price";

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit([Bind(Include = attrNames)] Movies movies)
    {
        if (ModelState.IsValid)
        {
            db.Entry(movies).State = EntityState.Modified;
            db.SaveChanges();
            return RedirectToAction("Index");
        }
        return View(movies);
    }

为什么应该将attrNames声明为静态？这是正确的使用方法，只是试验

* .Net MVC 5新手