Question

我试图一次从串行端口读取一个字节。我的控制台应用程序中有以下代码：

// 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);
    }
}

我期待这个循环，将收到的字节转储到屏幕上（暂时忽略它们将被打印为整数;我稍后会处理它。）

然而，它只是阻止了ReadByte调用，没有任何反应。

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

然而，我并没有对性能感到困扰（至少，还没有），并且我正在实现的协议在同步处理时效果更好。

那么：我做错了什么？为什么ReadByte没有回来？

Answer 1

您可以通过执行类似的操作使异步行为看起来同步，并在每次读取之前调用WaitForData()：

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();      
}

这个答案并不像发现和解决潜在问题那样“正确”，但可能是解决方法的基础。

我在SerialPort类中看到了一些奇怪的东西，包括你描述的行为。请记住，在辅助线程上调用DataReceived事件（请参阅MSDN）。使用带有Monitor.Wait（）和.Pulse（）的lock（）语义可以获得略微更好的性能，如here所述

如果你很懒，你也可以在调用ReadByte之前尝试插入一个Thread.Sleep（）行（例如200ms）来查看它是否有所作为。我也可以发誓我曾经看过一个案例，在控制台应用程序中的ReadByte（）上阻塞的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。没有机会彻底调查，但你可以看看你是否在WinForms下有更好的运气，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难过的人打电话。

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“无限阻塞”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，请改用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，你的循环在第一次运行时（在开始时）中断。

if (result < 0)
    break;

后循环未运行，您在控制台上看不到任何数据。

同步从SerialPort读取单个字节？

2 个答案: