c# - C# で 3 つのファイルを非同期的に読み取る

Question

3 つのファイルを非同期で読み取り、それらを 1 つのファイルに書き込んでいます。問題は、LAST ファイルの内容のみを書き込むことです。何故ですか？私のコード:メインのクラス:

    public static FileReader filereader1=  new FileReader("file1.txt");
    public static FileReader filereader2 = new FileReader("file2.txt");
    public static FileReader filereader3 = new FileReader("file3.txt");
    public static FileWriter towrite = new FileWriter(writefile.txt");

public static void readFileAsync(object obj)
{
  var filereader = obj as FileReader;
  readf = new ReadFile(filereader.readLine);
  if (filereader!=null)
  {
    while (!filereader.IsFinished)
    {
      IAsyncResult ar = readf.BeginInvoke(new AsyncCallback(myCallback),null);
    }

   }
 }
public static void myCallback(IAsyncResult iar)
{
  AsyncResult ar = (AsyncResult)iar;
  ReadFile readf = (ReadFile)ar.AsyncDelegate;
  string line = readf.EndInvoke(iar);
  writefile.writeLine(line);
 }
 static void Main(string[] args)
 {
    readFileAsync(filereader1);
    readFileAsync(filereader2);
    readFileAsync(filereader3);
    Console.ReadKey();
    filereader1.Dispose();
    filereader2.Dispose();
    filereader3.Dispose();
    writefile.Dispose();

クラス FileReader:

    public static StreamReader toRead { get; set; }
    public string FilePath { get; set; }

    public FileReader(string path)
    {
        FilePath = path;
        toRead = new StreamReader(path);
    }
    public string ReadLine()
    {
        string line=null;

            if (!toRead.EndOfStream)
            {
                line = toRead.ReadLine();
                return line;
            }
            return null;


    }
    public bool IsFinished
    {
        get
        {
            if (toRead != null)
            {
                return toRead.EndOfStream;
            }
            return true;
       }

    }
    public void Dispose()
    {
        if (toRead != null)
        {
            toRead.Close();
        }
    }
}

クラスFileWriter：{

    public StreamWriter toWrite { get; set; }
    public static object locker = new object();
    public string FilePath { get; set; }

    public FileWriter(string path)
    {
        FilePath = path;
        toWrite = new StreamWriter(path);
    }

    public void writeLine(string line)
    {
        if (toWrite != null)
        {
            lock (locker)
            {
                toWrite.WriteLine(line);
            }
        }
    }

    public void Dispose()
    {
        if (toWrite != null)
        {
            toWrite.Close();
        }
    }
}