Unable to read file after download multi connections in C#

See more:

connection

Can anyone tell me download multi connection mechanism of IDM?How it downloads to temporary files and join them.
how to download a file from internet[^] I using this code to create a program to download one file with 8 connection. I create one connection to download some MB (I divide MB for connection by using webRequest.AddRange(start,end)) and save it with 01.file, 02.file, 03.file. After download successfull but when I Join files, I can't watch video file. Please help me :(
Here is my code:

try
 {
            HttpWebRequest webRequest;
            HttpWebResponse webResponse;
            IWebProxy proxy = null;
            webRequest = (HttpWebRequest)WebRequest.Create(url);
            webRequest.AddRange((int)start, (int)end);
            webRequest.Timeout = 240 * 1000;
            webRequest.Proxy = proxy; //SA??? or DefineProxy
            webResponse = (HttpWebResponse)webRequest.GetResponse();
            fs = new FileStream(
                fileName, FileMode.Append, FileAccess.Write);
            long fileLength = webResponse.ContentLength;
            Stream strm = webResponse.GetResponseStream();
            int SeekPosition = (int)start;
            Console.WriteLine(name);
            strm.ReadTimeout = 60 * 1000;
            int arrSize = 18 * 1024 * 1024; //SA???                 
            byte[] barr = new byte[arrSize];
            long bytesCounter = 0;
            while (true)
            {
                int actualBytes = 0;
                try
                {
                    actualBytes = strm.Read(barr, 0, arrSize);
                }
                catch
                {

                }
                if (actualBytes <= 0)
                {
                    if (fileLength == actualBytes)
                        break;
                    else
                    {
                        if (tryCounter == 0)
                        {
                            tryCounter++;
                            dStart = DateTime.Now;
                            continue;
                        }

                        if (dStart.AddMinutes(1) >= DateTime.Now)
                            break;
                    }
                }

                fs.Write(barr, 0, actualBytes);
                bytesCounter += actualBytes;
                Transfered = bytesCounter;
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine("Error: {0}. Message: {1}", name, ex.Message);
        }
        finally
        {
            fs.Flush();
            fs.Close();
        }