未初始化的串行端口属性是否像使用默认构造函数时一样给出默认值？

如果使用默认构造函数或无参数构造函数创建串行端口，则会为该端口指定默认值。从文档中 https://msdn.microsoft.com/en-us/library/s14dyf47(v=vs.110).aspx:

// Create a new SerialPort object with default settings.
_serialPort = new SerialPort();

当未指定任何属性值时，此构造函数使用默认属性值。例如，DataBits 属性默认为 8，Parity 属性默认为 None 枚举值，StopBits 属性默认为 1，默认端口名称为 COM1。

有几个构造函数 https://msdn.microsoft.com/en-us/library/system.io.ports.serialport(v=vs.110).aspx

使用带参数的构造函数时：
使用带参数构造函数和使用默认构造函数时，不在串口构造函数参数内的属性是否具有相同的默认值？

问题在这个问题 https://stackoverflow.com/questions/39294081/serial-port-communication-on-the-same-windows-machine-not-working，有趣的答案、评论和随后的聊天讨论让我提出了这个问题。

你可以看一下它的源代码 http://referencesource.microsoft.com/#System/sys/system/io/ports/SerialPort.cs,0288da67ff57609d并查看构造函数和归档定义。

您将看到所有重要的属性，包括PortName, BaudRate, DataBits, Parity and StopBits有一个默认值，如果它们不存在于构造函数参数中，则将使用默认值。

还有一些其他重要的属性，它们也有默认值，但它们不存在于构造函数中，但在某些情况下，它们的默认值可能不好。

例如对于HandShake默认值为Handshake.None;而您可能想将其设置为Handshake.XOnXOff, Handshake.RequestToSend or Handshake.RequestToSendXOnXOff。对于这个特定的属性，您可能需要参考 Hans 的comments https://stackoverflow.com/questions/39297839/are-uninitialised-serial-port-attributes-given-default-values-as-they-are-when-u#comment98625879_39297839.

看一下构造函数：

public SerialPort(System.ComponentModel.IContainer container)
{
    container.Add(this);
}
public SerialPort()
{
}
// Non-design SerialPort constructors here chain, 
//using default values for members left unspecified by parameters
public SerialPort(string portName) 
    : this (portName, defaultBaudRate, defaultParity, defaultDataBits, defaultStopBits)
{
}
public SerialPort(string portName, int baudRate) 
    : this (portName, baudRate, defaultParity, defaultDataBits, defaultStopBits)
{
}
public SerialPort(string portName, int baudRate, Parity parity)
    : this (portName, baudRate, parity, defaultDataBits, defaultStopBits)
{
}
public SerialPort(string portName, int baudRate, Parity parity, int dataBits)
    : this (portName, baudRate, parity, dataBits, defaultStopBits)
{
}
public SerialPort(string portName, int baudRate, Parity parity, 
     int dataBits, StopBits stopBits)
{
    this.PortName = portName;
    this.BaudRate = baudRate;
    this.Parity = parity;
    this.DataBits = dataBits;
    this.StopBits = stopBits;
}

以下是这些字段和默认值定义：

// ---------- default values -------------*

private const int defaultDataBits = 8;
private const Parity defaultParity = Parity.None;
private const StopBits defaultStopBits = StopBits.One;
private const Handshake defaultHandshake = Handshake.None;
private const int defaultBufferSize = 1024;
private const string defaultPortName = "COM1";
private const int defaultBaudRate = 9600;
private const bool defaultDtrEnable = false;
private const bool defaultRtsEnable = false;
private const bool defaultDiscardNull = false;
private const byte defaultParityReplace = (byte) '?';
private const int defaultReceivedBytesThreshold = 1;
private const int defaultReadTimeout = SerialPort.InfiniteTimeout;
private const int defaultWriteTimeout = SerialPort.InfiniteTimeout;
private const int defaultReadBufferSize = 4096;
private const int defaultWriteBufferSize = 2048;
private const int maxDataBits = 8;
private const int minDataBits = 5;
private const string defaultNewLine = "\n";

private const string SERIAL_NAME = @"\Device\Serial";

// --------- members supporting exposed properties ------------*
private int baudRate = defaultBaudRate;
private int dataBits = defaultDataBits;
private Parity parity = defaultParity;
private StopBits stopBits = defaultStopBits;
private string portName = defaultPortName;
// ASCII is default encoding for modem communication, etc.
private Encoding encoding = System.Text.Encoding.ASCII; 
private Decoder decoder = System.Text.Encoding.ASCII.GetDecoder();
private int maxByteCountForSingleChar = System.Text.Encoding.ASCII.GetMaxByteCount(1);
private Handshake handshake = defaultHandshake;
private int readTimeout = defaultReadTimeout;
private int writeTimeout = defaultWriteTimeout;
private int receivedBytesThreshold = defaultReceivedBytesThreshold;
private bool discardNull = defaultDiscardNull;
private bool dtrEnable = defaultDtrEnable;
private bool rtsEnable = defaultRtsEnable;
private byte parityReplace = defaultParityReplace;
private string newLine = defaultNewLine;
private int readBufferSize = defaultReadBufferSize;
private int writeBufferSize = defaultWriteBufferSize;