SerialPort BaseStream ReadAsync - CancellationToken 从未取消？

Question

我尝试以异步方式从串行端口读取数据，请记住操作所花费的时间不得超过指定的时间段。我使用的代码：

private async Task<int> Read(byte[] buffer, int offset, int length)
    {
        Console.WriteLine("Reading response to buffer of size {0}, offset {1}, max to cnt {2}", buffer.Length, offset, length);

        int totalBytesRead = 0;

        Console.WriteLine("Begin reading...");

        while (true)
        {
            Console.WriteLine("Re-entering read");

            int bytesRead = 0;

            using (CancellationTokenSource cts = new CancellationTokenSource(new TimeSpan(0, 0, 0, 0, 333))) // 333 ms
            {
                Console.WriteLine("Waiting for read to complete...");

                int tmpOffset = offset + totalBytesRead;
                int tmpLength = length - totalBytesRead;
                Console.WriteLine("tmpOffset: {0}, tmpLength: {1}", tmpOffset, tmpLength);

                try
                {
                    bytesRead = await mPort.BaseStream.ReadAsync(buffer, tmpOffset, tmpLength, cts.Token);
                }
                catch (Exception e)
                {
                    Console.WriteLine("Exception during read: {0}, stack trace: {1}", e.Message, e.StackTrace);
                    break;
                }
                Console.WriteLine("Read completed");
            }

            totalBytesRead += bytesRead;

            Console.WriteLine("Bytes read: {0}, totalBytesRead: {1}, buffer: {2}", bytesRead, totalBytesRead, BitConverter.ToString(buffer, 0, buffer.Length));

            if (bytesRead == 0)
            {
                break;
            }
        }

        Console.WriteLine("Read finished");

        return totalBytesRead;
    }

以下代码将这些消息返回到控制台：

Reading response to buffer of size 256, offset 0, max to cnt 256
Begin reading...
Re-entering read
Waiting for read to complete...
tmpOffset: 0, tmpLength: 256
Read completed
Bytes read: 1, totalBytesRead: 1, buffer: 52-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
Re-entering read
Waiting for read to complete...
tmpOffset: 1, tmpLength: 255
Read completed
Bytes read: 8, totalBytesRead: 9, buffer: 52-45-4A-30-30-36-4A-44-52-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
Re-entering read
Waiting for read to complete...
tmpOffset: 9, tmpLength: 247

最后一条消息后它挂起。当然，应该使用比 333ms 更长的超时时间。对我来说，令牌似乎永远不会被取消，或者至少永远不会捕获这种取消。我 99% 确定设备不会发送更多数据，因此这就是我指望超时触发的原因。我是 C# 新手，所以也许有些东西我不太明白。您能看出任务没有被取消的原因吗？

我尝试复制和调整本文中提出的方法。

Answer 1

我最近遇到了同样的问题，试图通过串行线与一些旧的嵌入式仪器进行通信。我的解决方案是一组如下所示的扩展方法，我不处理整个端口，关闭基本流似乎就足够了，然后如果在取消时它是打开的，我会立即重新打开它。

public static class SerialAsyncExtensions
{
    public static async Task<int> ReadSerialAsync(this SerialPort f_Serial, byte[] f_Buffer, int f_Offset, int f_Count, int f_Delay)
    {
        if (f_Delay > 0)
            await Task.Delay(f_Delay);

        return await f_Serial.BaseStream.ReadAsync(f_Buffer, f_Offset, f_Count);
    }

    public static async Task<int> ReadSerialAsync(this SerialPort f_Serial, byte[] f_Buffer, int f_Offset, int f_Count, int f_Delay, CancellationToken f_Token)
    {
        CancellationTokenRegistration l_Token_Registration = f_Token.Register((param) =>
        {
            SerialPort l_Serial = (SerialPort)param;

            l_Serial.DiscardInBuffer();

        }, f_Serial);

        if (f_Delay > 0)
            await Task.Delay(f_Delay, f_Token);

        try
        {
            int l_Result = await f_Serial.BaseStream.ReadAsync(f_Buffer, f_Offset, f_Count, f_Token);

            return l_Result;
        }
        catch(System.IO.IOException Ex)
        {
            throw new OperationCanceledException("ReadSerialAsync operation Cancelled.", Ex);
        }
        finally
        {
            l_Token_Registration.Dispose();
        }
    }

    public static async Task WriteSerialAsync(this SerialPort f_Serial, byte[] f_Buffer, int f_Offset, int f_Count)
    {
        await f_Serial.BaseStream.WriteAsync(f_Buffer, f_Offset, f_Count);
    }

    public static async Task WriteSerialAsync(this SerialPort f_Serial, byte[] f_Buffer, int f_Offset, int f_Count, CancellationToken f_Token)
    {
        CancellationTokenRegistration l_Token_Registration = f_Token.Register((param) =>
        {
            SerialPort l_Serial = (SerialPort)param;

            l_Serial.DiscardOutBuffer();

        }, f_Serial);

        try
        {
            await f_Serial.BaseStream.WriteAsync(f_Buffer, f_Offset, f_Count, f_Token);
        }
        catch(System.IO.IOException Ex)
        {
            throw new OperationCanceledException("WriteSerialAsync operation Cancelled.", Ex);
        }
        finally
        {
            l_Token_Registration.Dispose();
        }
    }
}