Why $event.messageData from serial port event is empty?

Question

I'm trying to receive serial data from Arduino asynchronously via "DataReceived" event, but it $event.messageData is always empty.

Here's my code so far:

$port = New-Object System.IO.Ports.SerialPort COM7,9600,None,8,one
$port.Open()
$port | Format-List -Property *
Register-ObjectEvent -InputObject $port -EventName "DataReceived" -Action {
  $event | Format-List -Property * | Out-Host
}

Here's the output:

PORT

BaseStream             : System.IO.Ports.SerialStream
BaudRate               : 9600
BreakState             : False
BytesToWrite           : 0
BytesToRead            : 0
CDHolding              : False
CtsHolding             : False
DataBits               : 8
DiscardNull            : False
DsrHolding             : False
DtrEnable              : False
Encoding               : System.Text.ASCIIEncoding
Handshake              : None
IsOpen                 : True
NewLine                :

Parity                 : None
ParityReplace          : 63
PortName               : COM7
ReadBufferSize         : 4096
ReadTimeout            : -1
ReceivedBytesThreshold : 1
RtsEnable              : False
StopBits               : One
WriteBufferSize        : 2048
WriteTimeout           : -1
Site                   :
Container              :

EVENT

ComputerName     :
RunspaceId       : 67c40d50-5a9b-4022-974e-0a6a6518c2af
EventIdentifier  : 11
Sender           : System.IO.Ports.SerialPort
SourceEventArgs  : System.IO.Ports.SerialDataReceivedEventArgs
SourceArgs       : {COM7, System.IO.Ports.SerialDataReceivedEventArgs}
SourceIdentifier : 26c24bdb-8085-4e88-828e-44cb7897e416
TimeGenerated    : 6/16/2019 12:52:39 AM
MessageData      :

On the arduino side the code is:

Serial.printlf("ready")

When I use syncronous approach everything works

$port = New-Object System.IO.Ports.SerialPort COM7,9600,None,8,one
$port.Open()
$port.ReadLine()

Output

ready
ready
ready
...

Please, help!

Answer 1

$event.MessageData is not the right property to query - it contains custom data passed to the
-MessageData parameter when raising a custom event via the New-Event cmdlet.

For .NET-originated events, it is generally the automatic $EventArgs variable that contains the event-specific data.

In the specific case of the [System.IO.Ports.SerialPort] type's DataReceived event, however, it seems that the data isn't actually passed as an event argument, and you must instead use the $Sender object to gain access to the originating [System.IO.Ports.SerialPort] instance in order to call its .ReadExisting() method, judging by the sample code in the official docs:

$eventJob = 
  Register-ObjectEvent -InputObject $port -EventName "DataReceived" -Action {
    $Sender.ReadExisting() | Out-Host
  }