Error while closing ftp stream in C#

Question

Dear all

I'm trying to cancel a downloading operation. My scenario is as follows:

When the user clicks on Cancel Download Button so this action throws exception in Download function which is as follows:

        try
        {
            reqFTP = (FtpWebRequest)FtpWebRequest.Create(new Uri("ftp://" + uri + "/" + fileName));
            reqFTP.Method = WebRequestMethods.Ftp.DownloadFile;
            reqFTP.UseBinary = true;
            reqFTP.Credentials = new NetworkCredential(ftpUserID, ftpPassword);
            reqFTP.UsePassive = true;

            response = (FtpWebResponse)reqFTP.GetResponse();
            ftpStream = response.GetResponseStream();
            _isItOutputStream = true;
            string dataLengthString = response.Headers["Content-Length"];
            int dataLength = 0;
            if (dataLengthString != null)
            {
                dataLength = Convert.ToInt32(dataLengthString);
            }

            long cl = response.ContentLength;
            int bufferSize = 4048;
            int readCount;
            byte[] buffer = new byte[bufferSize];
            readCount = ftpStream.Read(buffer, 0, bufferSize);
            outputStream = new FileStream(filePath + "\\" + fileName, FileMode.Create);
            bool first = true;
            while (readCount > 0)
            {
                outputStream.Write(buffer, 0, readCount);
                _actualDownloaded += readCount;
                if (this.InvokeRequired)
                {
                    ProgressBarDel _progressDel = new ProgressBarDel(ProgressBar);
                    this.Invoke(_progressDel, new object[] { _actualDownloaded, first });
                }
                first = false;
                readCount = ftpStream.Read(buffer, 0, bufferSize);
            }
            ftpStream.Close();
            outputStream.Close();
            response.Close();
            _isItOutputStream = false;
            return true;
        }
        catch (Exception ee)
        {
            _downloadException = ee.Message;
            if (response != null)
            {

                outputStream.Close();
                ftpStream.Close();
                response.Close();
            }
            return false;
        }

In the line " ftpStream.Close() " here where the exception is thrown...

The exception text is:

The remote server returned an error: (450) File unavailable (e.g., file busy)

where it opens a file to download as what i wrote " outputStream = new FileStream(filePath + "\\" + fileName, FileMode.Create);" i wont to close this stream and to close the response in order if the user did as follows: download -> cancel download -> download -> cancel download -> download

if this scenario happened the application hungs up. i don't know how to close the stream and the response so i can stop downloading then delete the created file in order to download again.

thnx

Answer 1

The line that seems to be causing your execption is the one below and is as a result of the FTP server not finding the file you want. This is a situation that could easily happen again so it may be worth putting a try/catch block around that individual line or using a catch on the specific exception it is generating.

ftpStream = response.GetResponseStream();

At the moment in your catch block you are using outputStream.Close() but if the exception occurs on the line above then outputStream will be null. At the very least you should change the order of closing your objects in your catch block and check them for null.

if(response != null) {
    response.Close();
}

if(ftpStream != null) {
    ftpStream.Close();
}

if(outputStream != null) {
    outputStream.Close();
}

Answer 2

Ok first off, put your closes in a finally block.

So make sure your code is like this to ensure the streams get closed no matter what.

try
{
   //Your existing logic
   //Don't need to close the streams/etc
}
catch
{
   //Just do what you need to with the exception
}
finally
{
   //This is always called
   if (response != null)
      response.Close();
   if(outputStream != null)
      outputStream.Close();
   if(ftpStream != null)
      ftpStream.Close();
}

Wrap this line in a try catch

ftpStream = response.GetResponseStream();

This should be fine as long as those Close calls get called, where's your cancel logic? I would expect to see a flag on the loop that is set when the user clicks a button. e.g.

while (readCount > 0 && !_shouldExit)
{
   outputStream.Write(buffer, 0, readCount);
  _actualDownloaded += readCount;
  if (this.InvokeRequired)
  {
    ProgressBarDel _progressDel = new ProgressBarDel(ProgressBar);
    this.Invoke(_progressDel, new object[] { _actualDownloaded, first });
  }
  first = false;
  readCount = ftpStream.Read(buffer, 0, bufferSize);
}