The stream does not support concurrent IO read or write operations.

Here is the function it happens in:

Code:

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public static string UploadFile(string uploadfile, string url, int speedLimit, string fileFormName, string contenttype, NameValueCollection querystring, CookieContainer cookies)
        {
            if (String.IsNullOrEmpty(fileFormName))
            {
                fileFormName = "file";
            }

            if (String.IsNullOrEmpty(contenttype))
            {
                contenttype = "application/octet-stream";
            }

            string postdata = string.Empty;
            if (querystring != null)
            {
                postdata += "?";
                foreach (string key in querystring.Keys)
                {
                    postdata += key + "=" + querystring.Get(key) + "&";
                }
            }
            Uri uri = new Uri(url + postdata);


            string boundary = "----------" + DateTime.Now.Ticks.ToString("x");
            HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(uri);
            webrequest.CookieContainer = cookies;
            webrequest.ContentType = "multipart/form-data; boundary=" + boundary;
            webrequest.Method = "POST";
            webrequest.Timeout = 3600 * 1000;
            webrequest.ReadWriteTimeout = 3600 * 1000;

            // Build up the post message header
            StringBuilder sb = new StringBuilder();
            sb.Append("--");
            sb.Append(boundary);
            sb.Append("\r\n");
            sb.Append("Content-Disposition: form-data; name=\"");
            sb.Append(fileFormName);
            sb.Append("\"; filename=\"");
            sb.Append(Path.GetFileName(uploadfile));
            sb.Append("\"");
            sb.Append("\r\n");
            sb.Append("Content-Type: ");
            sb.Append(contenttype);
            sb.Append("\r\n");
            sb.Append("\r\n");

            string postHeader = sb.ToString();
            byte[] postHeaderBytes = Encoding.UTF8.GetBytes(postHeader);

            // Build the trailing boundary string as a byte array
            // ensuring the boundary appears on a line by itself
            byte[] boundaryBytes = Encoding.ASCII.GetBytes("\r\n--" + boundary + "\r\n");

            FileStream fileStream = new FileStream(uploadfile, FileMode.Open, FileAccess.Read);
            long length = postHeaderBytes.Length + fileStream.Length + boundaryBytes.Length;
            webrequest.ContentLength = length;

            Stream requestStream = webrequest.GetRequestStream();
            requestStream.WriteTimeout = 5 * 3600 * 1000;
            requestStream.ReadTimeout = 5 * 3600 * 1000;

            // Write out our post header
            requestStream.Write(postHeaderBytes, 0, postHeaderBytes.Length);

            // Write out the file contents
            byte[] buffer = new Byte[checked((uint)Math.Min(4096, (int)fileStream.Length))];
            int bytesRead;

            while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) != 0)
            {
                requestStream.Write(buffer, 0, bytesRead); //On this row

                decimal speedDelay = 1000 / (speedLimit / (buffer.Length / (decimal)1000));
                Thread.Sleep((int)Math.Round(speedDelay, MidpointRounding.ToEven));
            }

            // Write out the trailing boundary
            requestStream.Write(boundaryBytes, 0, boundaryBytes.Length);
            WebResponse responce = webrequest.GetResponse();
            Stream s = responce.GetResponseStream();
            StreamReader sr = new StreamReader(s);

            return sr.ReadToEnd();
        }

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I get the exception on the marked row, when I am writing to the stream. I am not sure why I get it but it only happens when I use the function to upload a file to a local server, meaning that it goes at 1 megabyte/s instead of ~100 kB/s.

The weird thing is that it always happens after a certain amount of time. It uploads for like 2 minutes, then I get the error.

Any clue to why this happens?

This error is as it states. There is more than one process trying to access the stream at one time. I can see this might happen if you have more than one user since you have declared the class as static. You need to create a new instance of the class each time or test to ensure the stream isn't currently in use before attempting to use it and handle accordingly.