从SerialPort同步读取单个字节？

Question

我正在尝试一次从串行端口读取一个字节。 我的控制台应用程序中包含以下代码：

// Open the serial port in 115200,8N1
using (SerialPort serialPort = new SerialPort("COM1", 115200,
                                              Parity.None, 8,
                                              StopBits.One))
{
    serialPort.Open();

    for (; ; )
    {
        int result = serialPort.ReadByte();
        if (result < 0)
            break;

        Console.WriteLine(result);
    }
}

我期望这会循环，将接收到的字节转储到屏幕上（暂时忽略它们将被打印为整数；稍后再处理）。

但是，它只是阻塞了ReadByte调用，没有任何反应。

我知道我的串行设备正在工作：如果我使用Tera Term，我会看到数据。 如果我使用DataReceived事件，并调用SerialPort.ReadExisting ，那么我可以看到数据。

但是，我并不担心性能（至少现在还没有），并且正在执行的协议在同步处理时效果更好。

所以：我在做什么错？ 为什么ReadByte不返回？

Answer 1

通过执行以下操作，并在每次读取之前调用WaitForData() ，可以使异步行为看起来是同步的：

static SerialPort port;
static AutoResetEvent dataArrived = new AutoResetEvent(false);

static void Main(string[] args) {
  port = new SerialPort(...);
  port.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);
  port.Open();
  WaitForData(1000);
  int data = port.ReadByte();
  Console.WriteLine(data);
  Console.ReadKey();
}

static void WaitForData(int millisecondsTimeout) {
  dataArrived.WaitOne(millisecondsTimeout);
}

static void port_DataReceived(object sender, SerialDataReceivedEventArgs e) {
  dataArrived.Set();      
}

这个答案不像发现和解决根本问题那样“正确”，但是可以作为解决方法的基础。

我已经看到了SerialPort类的一些奇怪之处，包括您描述的行为。 请记住，在辅助线程上调用了DataReceived事件（请参见MSDN ）。 您可以使用锁（）的语义与Monitor.Wait（）和.Pulse（）获得稍好的性能，如所描述这里

如果您很懒，也可以尝试在调用ReadByte之前插入Thread.Sleep（）行（例如200ms），以查看它是否有所作为。 同样，我可能发誓我曾经看到过这样一种情况，即在控制台应用程序中阻塞ReadByte（）的SerialPort被移植到WinForms应用程序而没有有意义的代码更改，问题就消失了。 没有机会进行彻底调查，但是您可以查看在WinForms下是否有更好的运气，然后从那里进行故障排除。

这个答案有点晚了，但我想我会为下一个在这个问题上陷入困境的人而高兴。

编辑：这是一个方便的WaitForBytes(count, timeout)扩展方法，可以很好地过滤掉您描述的“无限阻止”行为。

用法是： port.WaitForBytes(1)等待1个字节的数据到达。 或者为了减少开销，请使用SerialPortWatcher.WaitForBytes(n) 。

using System;
using System.Diagnostics;
using System.IO.Ports;
using System.Threading;

public static class SerialPortExtensions {

  /// <summary>
  /// Wait for a specified number of bytes to arrive on the serial port, or until a timeout occurs.
  /// </summary>
  /// <param name="port">Serial port on which bytes are expected to arrive.</param>
  /// <param name="count">Number of bytes expected.</param>
  /// <param name="millisecondsTimeout">Maximum amount of time to wait.</param>
  /// <exception cref="TimeoutException">Thrown if <paramref name="count"/> bytes are not received
  /// within <paramref name="millisecondsTimeout"/> milliseconds.</exception>
  /// <exception cref="ArgumentNullException">Thrown if <paramref name="port"/> is null.</exception>
  /// <exception cref="ArgumentOutOfRangeException">Thrown if either <paramref name="count"/> or
  /// <paramref name="millisecondsTimeout"/> is less than zero.</exception>
  /// <exception cref="InvalidOperationException">Thrown if the serial port is closed.</exception>
  /// <remarks>This extension method is intended only as an ad-hoc aid.  If you're using it a lot,
  /// then it's recommended for performance reasons to instead instantiate a
  /// <see cref="SerialPortWatcher"/> instance for the lifetime of your SerialPort.</remarks>
  public static void WaitForBytes(this SerialPort port, int count, int millisecondsTimeout) {

    if (port == null) throw new ArgumentNullException("port");
    if (port.BytesToRead >= count) return;

    using (var watcher = new SerialPortWatcher(port)) {
      watcher.WaitForBytes(count, millisecondsTimeout);
    }

  }

  /// <summary>
  /// Wait for a specified number of bytes to arrive on the serial port, or until a timeout occurs.
  /// </summary>
  /// <param name="port">Serial port on which bytes are expected to arrive.</param>
  /// <param name="count">Number of bytes expected.</param>
  /// <exception cref="ArgumentNullException">Thrown if <paramref name="port"/> is null.</exception>
  /// <exception cref="ArgumentOutOfRangeException">Thrown if either <paramref name="count"/> or
  /// <paramref name="millisecondsTimeout"/> is less than zero.</exception>
  /// <exception cref="InvalidOperationException">Thrown if the serial port is closed.</exception>
  /// <exception cref="TimeoutException">Thrown if <paramref name="count"/> bytes are not received
  /// within the number of milliseconds specified in the <see cref="SerialPort.ReadTimeout"/> property
  /// of <paramref name="port"/>.</exception>
  /// <remarks>This extension method is intended only as an ad-hoc aid.  If you're using it a lot,
  /// then it's recommended for performance reasons to instead instantiate a
  /// <see cref="SerialPortWatcher"/> instance for the lifetime of your SerialPort.</remarks>
  public static void WaitForBytes(this SerialPort port, int count) {
    if (port == null) throw new ArgumentNullException("port");
    WaitForBytes(port, count, port.ReadTimeout);
  }

}

/// <summary>
/// Watches for incoming bytes on a serial port and provides a reliable method to wait for a given
/// number of bytes in a synchronous communications algorithm.
/// </summary>
class SerialPortWatcher : IDisposable {

  // This class works primarilly by watching for the SerialPort.DataReceived event.  However, since
  // that event is not guaranteed to fire, it is neccessary to also periodically poll for new data.
  // The polling interval can be fine-tuned here.  A higher number means less wasted CPU time, while
  // a lower number decreases the maximum possible latency.
  private const int POLL_MS = 30;

  private AutoResetEvent dataArrived = new AutoResetEvent(false);
  private SerialPort port;

  public SerialPortWatcher(SerialPort port) {
    if (port == null) throw new ArgumentNullException("port");
    this.port = port;
    this.port.DataReceived += new SerialDataReceivedEventHandler(port_DataReceived);
  }

  public void Dispose() {
    if (port != null) {
      port.DataReceived -= port_DataReceived;
      port = null;
    }
    if (dataArrived != null) {
      dataArrived.Dispose();
      dataArrived = null;
    }
  }

  void port_DataReceived(object sender, SerialDataReceivedEventArgs e) {

    // This event will occur on a secondary thread.  Signal the waiting thread (if any).
    // Note: This handler could fire even after we are disposed.

    // MSDN documentation describing DataReceived event:
    // http://msdn.microsoft.com/en-us/library/system.io.ports.serialport.datareceived.aspx

    // Links discussing thread safety and event handlers:
    // https://stackoverflow.com/questions/786383/c-events-and-thread-safety
    // http://www.codeproject.com/Articles/37474/Threadsafe-Events.aspx

    // Note that we do not actually check the SerialPort.BytesToRead property here as it
    // is not documented to be thread-safe.

    if (dataArrived != null) dataArrived.Set();

  }

  /// <summary>
  /// Blocks the current thread until the specified number of bytes have been received from the
  /// serial port, or until a timeout occurs.
  /// </summary>
  /// <param name="count">Number of bytes expected.</param>
  /// <param name="millisecondsTimeout">Maximum amount of time to wait.</param>
  /// <exception cref="ArgumentOutOfRangeException">Thrown if either <paramref name="count"/> or
  /// <paramref name="millisecondsTimeout"/> is less than zero.</exception>
  /// <exception cref="InvalidOperationException">Thrown if the serial port is closed, or if this
  /// <see cref="SerialPortWatcher"/> instance has been disposed.</exception>
  /// <exception cref="TimeoutException">Thrown if <paramref name="count"/> bytes are not received
  /// within the number of milliseconds specified in the <see cref="SerialPort.ReadTimeout"/> property
  /// of <paramref name="port"/>.</exception>
  public void WaitForBytes(int count, int millisecondsTimeout) {

    if (count < 0) throw new ArgumentOutOfRangeException("count");
    if (millisecondsTimeout < 0) throw new ArgumentOutOfRangeException("millisecondsTimeout");
    if (port == null) throw new InvalidOperationException("SerialPortWatcher has been disposed.");
    if (!port.IsOpen) throw new InvalidOperationException("Port is closed");

    if (port.BytesToRead >= count) return;

    DateTime expire = DateTime.Now.AddMilliseconds(millisecondsTimeout);

    // Wait for the specified number of bytes to become available.  This is done primarily by
    // waiting for a signal from the thread which handles the DataReceived event.  However, since
    // that event isn't guaranteed to fire, we also poll for new data every POLL_MS milliseconds.
    while (port.BytesToRead < count) {
      if (DateTime.Now >= expire) {
        throw new TimeoutException(String.Format(
          "Timed out waiting for data from port {0}", port.PortName));
      }
      WaitForSignal();
    }

  }

  // Timeout exceptions are expected to be thrown in this block of code, and are perfectly normal.
  // A separate method is used so it can be marked with DebuggerNonUserCode, which will cause the
  // debugger to ignore these exceptions (even if Thrown is checkmarked under Debug | Exceptions).
  [DebuggerNonUserCode]
  private void WaitForSignal() {
    try {
      dataArrived.WaitOne(POLL_MS);
    } catch (TimeoutException) { }
  }

}

Answer 2

我认为当缓冲区中没有数据时，您的循环会在第一次运行（启动时）时中断。

if (result < 0)
    break;

以后的循环未运行，并且您在控制台上看不到任何数据。