Trouble sending file through IHttpAsyncHandler

asp.net, c#, ihttpasynchandler

Solution

Few remarks about your code:

- You need to call `EndGetFile` on the same service instance on which you called `BeginGetFile`

- 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; } 
}

Problem

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) { string 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; } } } ```

Original source