views:

1006

answers:

1

I have a string that I Gzip on the server and download to a client using the WebClient class. When I try to uncompress it, I get the error message that the Magic Number is missing. I have tried both the GZipStream class and the ICSharpLib methods of solving this, so I'm at a loss.

The compression/decompression works if I omit the step of downloading via the WebClient (using DownloadData which returns the data as byte[]), so I can only assume that there is some problem with the data getting truncated or corrupted some how, but since it's compressed data, I'm not sure how to debug this.

Here's the code snippet that seems to be the offending portion:

   byte[] response
   try {
        response = client.DownloadData(Constants.GetSetting("SyncServer"));
   } catch {
        MessageBox.Show("There was a problem synchronizing the data. Please try verify the supplied credentials or try again later.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        return;
   }

   int rows = SQLiteAPI.ImportStatHistoryXML(CurrentUser.User, myCampus, Convert.ToBase64String(response));

public static int ImportStatHistoryXML(Person tempPerson, Campus tempCampus, string xmlFile) {
      byte[] encryptedFile = Convert.FromBase64String(xmlFile);
      MemoryStream memStream = new MemoryStream(encryptedFile);
      memStream.ReadByte();
      GZipInputStream stream = new GZipInputStream(memStream);
      MemoryStream memory = new MemoryStream();
      byte[] writeData = new byte[4096];
      int size;

      while (true) {
       size = stream.Read(writeData, 0, writeData.Length);
       if (size > 0) {
        memory.Write(writeData, 0, size);
       } else {
        break;
       }
      }
      stream.Close();
      memory.Position = 0;
      StreamReader sr = new StreamReader(memory);
      string decompressed = sr.ReadToEnd();
      DataSet tempSet = new DataSet();
      StringReader xmlReader = new StringReader(decompressed);
      tempSet.ReadXml(xmlReader);
      DataTable statTable = tempSet.Tables["Stats"];
...more unrelated processing of the table
}

Any help would be appreciated. P.S. I'm using the Base64 string to be able to pass back and forth across the web. This may in fact be the area I am messing up in since I've not done web requests and responses between a desktop app and a web service before.

+4  A: 

First, I don't think the snippet is valid, because DownloadString returns (as expected) a String.

Now, do I understand right that it works correctly when you use DownloadData and incorrectly when you use DownloadString? That makes sense because it is not valid to decode Gzip data as Unicode.

EDIT:

Okay, the ToBase64String and FromBase64String should be okay. But if you can avoid it and pass the byte[] directly, that would be good.

public static int ImportStatHistoryXML(Person tempPerson, Campus tempCampus, byte[] compressedFile) {

Then you would get rid of the first line of the function (the decode from base64). Note we're renaming encryptedFile to compressedFile.

The line:

memStream.ReadByte();

should not be there. You are reading a byte and discarding it. If everything is as we expect that byte is 0x1F, part of the gzip magic number.

Then, I think you're using the wrong gzip class. You want GZipStream. It is constructed like:

GZipStream stream = new GZipStream(memStream, CompressionMode.Decompress);

Then, you use StreamReader directly on that:

StreamReader sr = new StreamReader(stream);

It would help if you knew the encoding, but hopefully the default will be correct. Then it seems correct from there. So, overall we get the below.

public static int ImportStatHistoryXML(Person tempPerson, Campus tempCampus, byte[] compressedFile) {
    MemoryStream memStream = new MemoryStream(compressedFile);
    GZipStream gzStream = new GZipStream(memStream, CompressionMode.Decompress);
    StreamReader sr = new StreamReader(gzStream);
    string decompressed = sr.ReadToEnd();
    DataSet tempSet = new DataSet();
    StringReader xmlReader = new StringReader(decompressed);
    tempSet.ReadXml(xmlReader);
    DataTable statTable = tempSet.Tables["Stats"];

    //...
}
Matthew Flaschen
You're right. Sorry that was me trying a last ditch effort to see if I could straighten out what was missing.
Jared
Thanks. Now the code makes more sense, but you didn't answer my question.
Matthew Flaschen
I have the string base64 encoded? Will that not work? How does one then pass the Gzip'ed data to and from a web service?
Jared
Jared, you're really not helping me understand the problem. You still haven't answered my question. Another is, when are you actually decoding the gzip data? There's obviously no reference to GZipStream or ICSharpLib in your short snippet. You may or may not have reason to use Base64 (probably not). I'm still trying to figure out what you're doing.
Matthew Flaschen
Thanks for sticking with me on this one.Ok, I've added the decompression code. I'd be thrilled not to use the string conversions but I'm not sure how to upload/download the data without it (I've never done desktop-web transfers, always just desktop or just ASP.Net, so a totally new area for me.)
Jared
Hey, I improved ImportStatHistoryXML. I hope this is helping.
Matthew Flaschen
Thanks. This got me far enough to to debug that the real problem was the web service returning extra content than just the compressed data, so thanks!
Jared
@Matthew: Im gonna give you an upvote, just for your persistence.
mizipzor