title: The result written to the file does not work as expected
body: <p>I use multithread to write the result to file. I have 100 results but the number of results saved to the file is only 30 results. What should I do?</p>
  
  <pre><code>    public async Task FileWriteAsync(string text)
      {
          string file = @"uid.txt";
          using (FileStream sourceStream = new FileStream(file, FileMode.Append, FileAccess.Write, FileShare.None, bufferSize: 4096, useAsync: true))
          using (StreamWriter f = new StreamWriter(sourceStream))
          {
              await f.WriteLineAsync(text);
          }
      }
      public void ExFile(int line)
      {
  
          var uid = Regex.Match(txt_ListUID.Lines[line], @"c_user=(.*?);").Groups[1].ToString().Trim();
          string text = uid + "|zxzxzx";
          _ = FileWriteAsync(text)
      }
  </code></pre>