public static void TaskFromIOStreamAsync(string fileName)
{
  int chunkSize = 4096;
  byte[] buffer = new byte[chunkSize];

  FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read, chunkSize, true);

  Task<int> task = fileStream.ReadAsync(buffer, 0, buffer.Length);
  task.ContinueWith((readTask) =>
  {
    int amountRead = readTask.Result;
    //必須在ContinueWith中釋放文件流 
    fileStream.Dispose();
    Console.WriteLine($"Async(Simple) Read {amountRead} bytes");
  });
}

public static Task<long> AsynchronousRead(string fileName)
{
  int chunkSize = 4096;
  byte[] buffer = new byte[chunkSize];
  //創(chuàng)建一個(gè)返回的偽Task對(duì)象
  TaskCompletionSource<long> tcs = new TaskCompletionSource<long>();

  MemoryStream fileContents = new MemoryStream();//用于保存讀取的內(nèi)容
  FileStream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read, FileShare.Read, chunkSize, true);
  fileContents.Capacity += chunkSize;//指定緩沖區(qū)大小。好像Capacity會(huì)自動(dòng)增長，設(shè)置與否沒關(guān)系，后續(xù)寫入多少數(shù)據(jù)，就增長多少

  Task<int> task = fileStream.ReadAsync(buffer, 0, buffer.Length);
  task.ContinueWith(readTask => ContinueRead(readTask, fileStream, fileContents, buffer, tcs));
  //在ContinueWith中循環(huán)讀取，讀取完成后，再返回tcs的Task
  return tcs.Task;
}

/// <summary>
/// 繼續(xù)讀取數(shù)據(jù)
/// </summary>
/// <param name="task">讀取數(shù)據(jù)的線程</param>
/// <param name="fileStream">文件流</param>
/// <param name="fileContents">文件存放位置</param>
/// <param name="buffer">讀取數(shù)據(jù)緩存</param>
/// <param name="tcs">偽Task對(duì)象</param>
private static void ContinueRead(Task<int> task, FileStream fileStream, MemoryStream fileContents, byte[] buffer, TaskCompletionSource<long> tcs)
{
  if (task.IsCompleted)
  {
    int bytesRead = task.Result;
    fileContents.Write(buffer, 0, bytesRead);//寫入內(nèi)存區(qū)域。似乎Capacity會(huì)自動(dòng)增長
    if (bytesRead > 0)
    {
      //雖然看似是一個(gè)新的任務(wù)，但是使用了ContinueWith，所以使用的是同一個(gè)線程。
      //沒有讀取完，開啟另一個(gè)異步繼續(xù)讀取
      Task<int> newTask = fileStream.ReadAsync(buffer, 0, buffer.Length);
      //此處做了一個(gè)循環(huán)
      newTask.ContinueWith(readTask => ContinueRead(readTask, fileStream, fileContents, buffer, tcs));
    }
    else
    {
      //已經(jīng)全部讀取完，所以需要返回?cái)?shù)據(jù)
      tcs.TrySetResult(fileContents.Length);
      fileStream.Dispose();
      fileContents.Dispose();//應(yīng)該是在使用了數(shù)據(jù)之后才釋放數(shù)據(jù)緩沖區(qū)的數(shù)據(jù)
    }
  }
}

const int ReadSize = 256;

/// <summary>
/// 從文件中獲取字符串
/// </summary>
/// <param name="path">文件路徑</param>
/// <returns>字符串</returns>
public static Task<string> GetStringFromFile(string path)
{
  FileInfo file = new FileInfo(path);
  byte[] buffer = new byte[1024];//存放數(shù)據(jù)的緩沖區(qū)

  FileStream fileStream = new FileStream(
    path, FileMode.Open, FileAccess.Read, FileShare.None, buffer.Length,
    FileOptions.DeleteOnClose | FileOptions.Asynchronous);

  Task<int> task = Task<int>.Factory.FromAsync(fileStream.BeginRead, fileStream.EndRead,
    buffer, 0, ReadSize, null);//此參數(shù)為BeginRead需要的參數(shù)

  TaskCompletionSource<string> tcs = new TaskCompletionSource<string>();

  task.ContinueWith(taskRead => OnReadBuffer(taskRead, fileStream, buffer, 0, tcs));

  return tcs.Task;
}

/// <summary>
/// 讀取數(shù)據(jù)
/// </summary>
/// <param name="taskRead">讀取任務(wù)</param>
/// <param name="fileStream">文件流</param>
/// <param name="buffer">讀取數(shù)據(jù)存放位置</param>
/// <param name="offset">讀取偏移量</param>
/// <param name="tcs">偽Task</param>
private static void OnReadBuffer(Task<int> taskRead, FileStream fileStream, byte[] buffer, int offset, TaskCompletionSource<string> tcs)
{
  int readLength = taskRead.Result;
  if (readLength > 0)
  {
    int newOffset = offset + readLength;
    Task<int> task = Task<int>.Factory.FromAsync(fileStream.BeginRead, fileStream.EndRead,
      buffer, newOffset, Math.Min(buffer.Length - newOffset, ReadSize), null);

    task.ContinueWith(callBackTask => OnReadBuffer(callBackTask, fileStream, buffer, newOffset, tcs));
  }
  else
  {
    tcs.TrySetResult(System.Text.Encoding.UTF8.GetString(buffer, 0, buffer.Length));
    fileStream.Dispose();
  }
}

/// <summary>
/// 同步方法的壓縮
/// </summary>
/// <param name="lstFiles">文件清單</param>
public static void SyncCompress(IEnumerable<string> lstFiles)
{
  byte[] buffer = new byte[16384];
  foreach(string file in lstFiles)
  {
    using (FileStream inputStream = File.OpenRead(file))
    {
      using (FileStream outputStream = File.OpenWrite(file + ".compressed"))
      {
        using (System.IO.Compression.GZipStream compressStream = new System.IO.Compression.GZipStream(outputStream, System.IO.Compression.CompressionMode.Compress))
        {
          int read = 0;
          while((read=inputStream.Read(buffer,0,buffer.Length))>0)
          {
            compressStream.Write(buffer, 0,read);
          }
        }
      }
    }
  }
}

/// <summary>
/// 異步方法的文件壓縮
/// </summary>
/// <param name="lstFiles">需要壓縮的文件</param>
/// <returns></returns>
public static async Task AsyncCompress(IEnumerable<string> lstFiles)
{
  byte[] buffer = new byte[16384];
  foreach(string file in lstFiles)
  {
    using (FileStream inputStream = File.OpenRead(file))
    {
      using (FileStream outputStream = File.OpenWrite(file + ".compressed"))
      {
        using (System.IO.Compression.GZipStream compressStream = new System.IO.Compression.GZipStream(outputStream, System.IO.Compression.CompressionMode.Compress))
        {
          int read = 0;
          while ((read = await inputStream.ReadAsync(buffer, 0, buffer.Length)) > 0)
          {
            await compressStream.WriteAsync(buffer, 0, read);
          }
        }
      }
    }
  }
}