Question

I download an image from an URL asynchronously using WebRequest this way:

public void Download(string url) {      byte[] buffer = new byte[0x1000];   WebRequest request = HttpWebRequest.Create(url);   request.Method = 'GET';   request.ContentType = 'image/gif';    request.BeginGetResponse(result =>   {     WebRequest webRequest = result.AsyncState as WebRequest;      WebResponse response = webRequest.EndGetResponse(result);     ReadState readState = new ReadState()     {       Response = response.GetResponseStream(),       AccumulatedResponse = new MemoryStream(),       Buffer = buffer,     };      readState.Response.BeginRead(buffer, 0,       readState.Buffer.Length, ReadCallback, readState);   }, request); }  public void ReadCallback(IAsyncResult result) {   ReadState readState = result.AsyncState as ReadState;   int bytesRead = readState.Response.EndRead(result);   if(bytesRead > 0)   {     readState.AccumulatedResponse.BeginWrite(readState.Buffer, 0, bytesRead, writeResult =>     {       readState.AccumulatedResponse.EndWrite(writeResult);       readState.Response.BeginRead(readState.Buffer, 0, readState.Buffer.Length, ReadCallback, readState);     }, null);   }   else   {     readState.AccumulatedResponse.Flush();     readState.Response.Close();     pictureBox1.Image = Image.FromStream(readState.AccumulatedResponse);   } }  public class ReadState {   public Stream Response { get; set; }   public Stream AccumulatedResponse { get; set; }   public byte[] Buffer { get; set; } } 

and it works ok, but I would like to show the progress of the download as the browsers do, and not to show only when it finishes.

If I do

pictureBox1.Image = Image.FromStream(readState.AccumulatedResponse); 

before it finishes I get an exception that the picture is not valid, even though it has some data. Is there anyway to show partial data?

Answer 1

JPEG has a special encoding mode called ‘Progressive JPEG’ in which data is compressed in multiple passes of progressively higher detail. Windows 7 has built-in support for this.