从网上copy了一个即时消息的方法，有时候会抛异常导致页面无法访问，错误内容提示为：
未将对象引用设置到对象的实例。 在 System.Web.HttpApplication.CallHandlerExecutionStep.InvokeEndHandler(IAsyncResult ar) 在 System.Web.HttpApplication.CallHandlerExecutionStep.OnAsyncHandlerCompletion(IAsyncResult ar)

using System;
using System.Collections.Generic;
using System.Web;
using System.Threading;

public class AsnyHandler : IHttpAsyncHandler
{
public AsnyHandler() { }

public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
{
    //myAsynResult为实现了IAsyncResult接口的类，当不调用cb的回调函数时，该请求不会返回到给客户端，会一直处于连接状态
    myAsynResult asyncResult = new myAsynResult(context, cb, extraData);
    String content = context.Request.Params["from"];
    if (content != "-1")
    {
        string value = context.Request.Params["to"];
        string events = context.Request.Params["event"];
        if (value == null)
        {
            value = "";
        }
        if (events == null)
        {
            events = "";
        }

        content += "," + value + "," + events;
    }
    //向Message类中添加该消息
    Messages.Instance().AddMessage(content, asyncResult);
    return asyncResult;
}
public void EndProcessRequest(IAsyncResult result)
{

}

public bool IsReusable
{
    get { return false; ; }
}

public void ProcessRequest(HttpContext context)
{
}

}
[Serializable]
public class myAsynResult : IAsyncResult
{
bool _IsCompleted = false;
private HttpContext context;
private AsyncCallback cb;
private object extraData;
public myAsynResult(HttpContext context, AsyncCallback cb, object extraData)
{
this.context = context;
this.cb = cb;
this.extraData = extraData;
}
private string _content;
public string Content
{
get { return _content; }
set { _content = value; }
}

#region IAsyncResult接口
public object AsyncState
{
    get { return null; }
}

public System.Threading.WaitHandle AsyncWaitHandle
{
    get { return null; }
}

public bool CompletedSynchronously
{
    get { return false; }
}
public bool IsCompleted
{
    get { return _IsCompleted; }
}
#endregion

//在Message类中的添加消息方法中，调用该方法，将消息输入到客户端，从而实现广播的功能
public void Send(object data)
{
    try
    {
        context.Response.Write(this.Content);
    }
    catch
    {
        HttpContext.Current.Response.Write("-1");
    }
    if (cb != null)
    {
        cb(this);
    }
    _IsCompleted = true;
}

}

using System;
using System.Collections.Generic;
using System.Web;

/// <summary>
/// Summary description for Messages
/// </summary>
public class Messages
{
//记录所有请求的客户端
List<myAsynResult> clients = new List<myAsynResult>();

#region 实现该类的单例
private static readonly Messages _Instance = new Messages();
public Messages()
{
}
public static Messages Instance()
{
    return _Instance;
}
#endregion

public void AddMessage(string content,myAsynResult asyncResult)
{
    //当传入的内容为"-1"时，表示为建立连接请求，即为了维持一个从客户端到服务器的连接而建立的连接
    //此时将该连接保存到 List<myAsynResult> clients中，待再有消息发送过来时，该连接将会被遍历，并且会将该连接输出内容后，结束该连接
    if (content=="-1")
    {
        clients.Add(asyncResult);
    }
    else
    {
        //将当前请求的内容输出到客户端
        asyncResult.Content = content;
        asyncResult.Send(null);

        //否则将遍历所有已缓存的client,并将当前内容输出到客户端
        foreach (myAsynResult result in clients)
        {
            result.Content = content;
            result.Send(null);
        }

        //清空所有缓存
        clients.Clear();
    }
}

}

web.config配置为：
<system.web>
<httpHandlers>
<add path="/wait.aspx" type="AsnyHandler" verb="GET" />
</httpHandlers>
</system.web>

js方法：
function wait() {
$.get("/wait.aspx", { from: "-1", to: "", event: "" },
function (data, status) {
//有消息提示，与客户端交互
wait();
}, "html"
).fail(function () { wait(); });
}