Question

I’m using the HttpWebRequest class asynchronously as seen below (its a Windows application)

private void StartWebRequest(string url)
{
   HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
   request.BeginGetResponse(new AsyncCallback(FinishWebRequest), request);
}

private void FinishWebRequest(IAsyncResult result)
{
   HttpWebResponse response = (result.AsyncState as HttpWebRequest).EndGetResponse(result) as HttpWebResponse;
   Stream responseStream = response.GetResponseStream();
   int num = 100000;
   byte[] buffer = new byte[num];
   int offset = 0;

   while ((num2 = responseStream.Read(buffer, offset, 1000)) != 0)
   {
      offset += num2;
   }

   MemoryStream stream = new MemoryStream(buffer, 0, offset);
   Bitmap bitmap = (Bitmap)Image.FromStream(stream);
   bitmap.Save(@"z:\new.jpg");
   response.Close();
   responseStream.Close();
   stream.Close();
}

But I want to give some parameters to the AsyncCallback delegate from StartWebRequest method, is it possible? Because I want to take picture name as parameter like :

bitmap.Save(@"z:\MYPARAMATERVALUE.jpg");

Answer 1

The simplest approach here is to use an anonymous function (an anonymous method or lambda expression), like this:

private void StartWebRequest(string url, string filename)
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
    request.BeginGetResponse(result => {
        FinishWebRequest(result, request, filename);
    }, null); // Don't need the state here any more
}

private void FinishWebRequest(IAsyncResult result,
                              HttpWebRequest request,
                              string filename)
{
    using (HttpWebResponse response =
             (HttpWebResponse) request.EndGetResponse(result))
    {
        // Use filename here
    }
}

You can do all of this manually, of course, but you’d need to create a new class to capture the filename, create an instance of that class, and then use a method in that class for the callback delegate. That’s exactly what the compiler does for you with the anonymous function.