一个非常简单的 ASP.NET MVC 示例：长轮询（又叫：反向 AJAX，英文名：Comet）实现

<p>
<input type="button" onclick="longPolling();" value="开始长轮询" />
</p>

<div>
<p>
服务器返回的信息：
</p>
<p id="msgContainer">

</p>
</div>
<script type="text/javascript">
function longPolling()
{
$.getJSON("@Url.Action("GetTime", "DateTime")", function (json)
{
$("#msgContainer").append(json.date + "<br/>");
longPolling();
});
}
</script>

public class DateTimeController : AsyncController
{
public DateTimeController()
{

}

public void GetTimeAsync()
{
//计时器，5秒种触发一次Elapsed事件
System.Timers.Timer timer = new System.Timers.Timer(5000);
//告诉.NET接下来将进行一个异步操作
AsyncManager.OutstandingOperations.Increment();
//订阅计时器的Elapsed事件
timer.Elapsed += (sender, e) =>
{
//保存将要传递给GetTimeCompleted的参数
AsyncManager.Parameters["nowdate"] = e.SignalTime;
//告诉ASP.NET异步操作已完成，进行GetTimeCompleted方法的调用
AsyncManager.OutstandingOperations.Decrement();
};
//启动计时器
timer.Start();
}

public ActionResult GetTimeCompleted(DateTime nowdate)
{
return Json(new { date = nowdate.ToString("HH:mm:ss") + "  Welecom " }, JsonRequestBehavior.AllowGet);
}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Threading.Tasks;
using System.IO;

namespace TestMvc.MvcUI.Controllers
{
public class ArticleController : AsyncController
{
#region 方式 1

public void ArticleAsync(string name)
{
AsyncManager.OutstandingOperations.Increment();
Task.Factory.StartNew(() =>
{
string path = ControllerContext.HttpContext.Server.MapPath(string.Format(@"\articles\{0}.html", name));
using (StreamReader reader = new StreamReader(path))
{
AsyncManager.Parameters["content"] = reader.ReadToEnd();
}
AsyncManager.OutstandingOperations.Decrement();
});
}

public ActionResult ArticleCompleted(string content)
{
return Content(content);
}

#endregion

#region 方式 2

public Task<ActionResult> Article(string name)
{
return Task.Factory.StartNew(() =>
{
string path = ControllerContext.HttpContext.Server.MapPath(string.Format(@"\articles\{0}.html", name));
using (StreamReader reader = new StreamReader(path))
{
AsyncManager.Parameters["content"] = reader.ReadToEnd();
}
}).ContinueWith<ActionResult>(task =>
{
string content = (string)AsyncManager.Parameters["content"];
return Content(content);
});
}

#endregion

#region 方式 3

public Task<ActionResult> Article2(string name)
{
return Task.Factory.StartNew(() =>
{
string path = ControllerContext.HttpContext.Server.MapPath(string.Format(@"\articles\{0}.html", name));
using (StreamReader reader = new StreamReader(path))
{
return reader.ReadToEnd();
}
}).ContinueWith<ActionResult>(task =>
{
return Content((string)task.Result);
});
}

#endregion
}
}