/// <summary>
/// 异步方法
/// </summary>
/// <returns></returns>
public  Task<ActionResult> Index()
{
    return Task.Run<ActionResult>(() => {
        using (StreamReader sr = new StreamReader("F:\test.txt"))
        {
            string txt = sr.ReadToEnd();
            return Content(txt);
        }
    });
}

/// <summary>
/// 异步方法，并使用async标注
/// </summary>
/// <returns></returns>
public async Task<ActionResult> Index()
{
    using (StreamReader sr = new StreamReader("F:\test.txt"))
    {
        string txt =await sr.ReadToEndAsync();
        return Content(txt);
    }
        //return View();
}