How to download an image from web showing the progress of the download in c# using winforms?

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.

Answer 2

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

You have two options:

1. Use WebClient.DownloadDataAsync. This will raise progress events via DownloadProgressChanged and provide a final notification of when the data is available via the DownloadDataCompleted event. At that point, you could assign the image to, say, the PictureBox.

2. If you are downloading an image to eventually display in a PictureBox control then it may even be easier to just go with PictureBox.LoadAsync. This too will provide progress updates via its LoadProgressChanged event and finally LoadCompleted on completion.