public bool MergeFile(string FileName)
    {
    bool rslt = false;
    string partToken = ".part_";
    string baseFileName = FileName.Substring(0, FileName.IndexOf(partToken));
    string trailingTokens = FileName.Substring(FileName.IndexOf(partToken) + partToken.Length);
    int FileIndex = 0;
    int FileCount = 0;
    int.TryParse(trailingTokens.Substring(0, trailingTokens.IndexOf(".")), out FileIndex);
    int.TryParse(trailingTokens.Substring(trailingTokens.IndexOf(".") + 1), out FileCount);
    string Searchpattern = Path.GetFileName(baseFileName) + partToken + "*";
    string[] FilesList = Directory.GetFiles(Path.GetDirectoryName(FileName), Searchpattern);
    if (FilesList.Count() == FileCount) 
    //  merge .. improvement would be to confirm individual parts are there / correctly in sequence, a security check would also be important
    {
    List <SortedFile> MergeList = new List <SortedFile>();
            foreach (string File in FilesList)
            {
            SortedFile sFile = new SortedFile();
            sFile.FileName = File;
            baseFileName = File.Substring(0, File.IndexOf(partToken));
            trailingTokens = File.Substring(File.IndexOf(partToken) + partToken.Length);
            int.TryParse(trailingTokens.Substring(0, trailingTokens.IndexOf(".")), out FileIndex);
            sFile.FileOrder = FileIndex;
            MergeList.Add(sFile);
            }
            var MergeOrder = MergeList.OrderBy(s => s.FileOrder).ToList(); // ensure we merge back in the correct order
            using (FileStream FS = new FileStream(baseFileName, FileMode.Create))
                {
                foreach (var chunk in MergeOrder)
                    {
                    using (FileStream fileChunk = new FileStream(chunk.FileName, FileMode.Open))
                        {
                        fileChunk.CopyTo(FS);
                        }
                    }
                }
            rslt = true;
            }
            return rslt;
      }
    }