Trouble sending file through IHttpAsyncHandler

I'm using a IHttpHandler to call a webservice and return the resulting byte[] to the client as a downloaded file attachment. This works fine, but when I tried changing the IHttpHandler to a IHttpAsyncHandler, the file download dialog shows, but the file does not start/finish downloading. What am I doing wrong?

<%@ WebHandler Language="C#" Class="PreviewPDF"  %>

using System;
using System.Web;

public class PreviewPDF : IHttpAsyncHandler
{
    public void ProcessRequest(HttpContext context)
    {
    } 

    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)
    {
        s开发者_如何转开发tring data = "some data";

        using (WebService.RequestService service = new WebService.RequestService())
        {
            AsyncCallback callback = new AsyncCallback(EndProcessRequest);
            return service.BeginGetFile(data, callback, context);
        }
    }
    public void EndProcessRequest(IAsyncResult result)
    {
        HttpContext context = result.AsyncState as HttpContext;
        byte[] wsoutput;
        using (WebService.RequestService service = new WebService.RequestService())
        {
            wsoutput = service.EndGetFile(result);
        }

        context.Response.ContentType = "application/octet-stream";
        context.Response.ContentEncoding = System.Text.Encoding.Unicode;
        context.Response.AddHeader("Content-Disposition", "attachment; filename=attachment.pdf");
        using (System.IO.MemoryStream ms = new System.IO.MemoryStream(wsoutput))
        {
            ms.WriteTo(context.Response.OutputStream);
        }
        context.Response.Flush();
    }


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

Few remarks about your code:

1. You need to call EndGetFile on the same service instance on which you called BeginGetFile
2. You need to pass cb as the AsyncCallBack instead of EndProcessRequest

Here's the code with these remarks taken into account:

private class State
{
    public HttpContext Context { get; set; }
    public RequestService Service { get; set; }
}

public void ProcessRequest(HttpContext context)
{
    throw new NotImplementedException();
} 

public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, Object extraData)
{
    // Don't use using block or it will dispose the service before you can call EndGetFile
    var state = new State
    {
        Service = new RequestService(),
        Context = context
    };
    // Pass cb here and not EndProcessRequest
    return state.Service.BeginGetFile(cb, state);
}

public void EndProcessRequest(IAsyncResult result)
{
    State state = result.AsyncState as State;
    // Be carefull as this may throw: it is best to put it in a try/finally block
    // so that you dispose properly of the service
    byte[] buffer = state.Service.EndGetFile(result);
    state.Service.Dispose();
    state.Context.Response.ContentType = "application/octet-stream";
    state.Context.Response.AddHeader("Content-Disposition", "attachment; filename=attachment.pdf");
    // Write directly into the output stream, and don't call Flush
    state.Context.Response.OutputStream.Write(buffer, 0, buffer.Length);
}

public bool IsReusable 
{ 
    get { return false; } 
}