Try adding the System.IO.Ports NuGet package:

<PackageReference Include="System.IO.Ports" Version="7.0.0" />

Also note that the fully qualified name is System.IO.Ports.SerialPort, not System.IO.SerialPort

There is no single built-in Windows command that lists "All Ports + Busy Status" in one view. We effectively have two approaches: the Non-Invasive method (inspecting OS handles) or the Native method (trying to open the port to see if it fails).

Method 1: The Non-Invasive Way

If we want to check port status without physically opening/closing them (which can reset devices), one must inspect the Windows Object Manager handles. This requires looking at who is holding the port, rather than asking the port if it is open.

Option A: Sysinternals Handle (Immediate) This is the industry standard for inspecting port usage. It is not built-in, but can be installed via Winget/Scoop/Choco (winget install Microsoft.Sysinternals.Handle).

Run as Administrator:

handle -a "Device\Serial"

• Empty Result: The port is free.
• Result Found: It lists exactly which process (PID and Name) is hogging the port.

Option B: Native openfiles (Requires Reboot) If one cannot install tools, you can use the native openfiles command, but it requires enabling the system-wide "Maintain Objects List" flag, which costs a small amount of RAM and requires a reboot.

1. Enable (run once): openfiles /local on
2. Reboot.
3. Check ports: openfiles /query /FO TABLE | findstr "Serial"

Method 2: The Native PowerShell Script (Invasive)

If you cannot use external tools or reboot, you must use the "Try/Catch" method. This script iterates through all known ports, attempts to open them, and infers their status based on whether the OS throws an "Access Denied" error.

Note: This is "invasive"—opening the port, even for a millisecond, may cause DTR/RTS lines to toggle, potentially resetting connected hardware (like Arduino/ESP32 boards).

# Get all physically/logically present serial ports
$ports = [System.IO.Ports.SerialPort]::GetPortNames()
$portStatuses = @()

foreach ($port in $ports) {
    $status = 'Available'
    
    try {
        # Attempt to open and immediately close the port
        $testPort = New-Object System.IO.Ports.SerialPort $port
        $testPort.Open()
        $testPort.Close()
    }
    catch {
        # If Open() fails, the port is likely in use by another app (or PS itself)
        $status = 'Busy'
    }

    $portStatuses += [PSCustomObject]@{
        PortName = $port
        Status   = $status
    }
}

# Display cleanly
$portStatuses | Format-Table -AutoSize

Why the original commands were confusing:

• [System.IO.Ports.SerialPort]::getportnames() only queries the registry for existence, so it always lists the port even if it's busy.
• mode only interacts with the driver configuration. If a port is actively locked by another process, mode often fails or hides it, leading to the discrepancy I saw where PowerShell "hides" the port from mode once it opens a handle.

I've seen the strange characters too. My solution is using Regular Expressions to filter out the number portion of the comm port name.

Dim ports As New Devices.Ports
For Each s As String In ports.SerialPortNames
    s = Regex.Replace(s, "\D*(\d+)\D*", "$1")
    Debug.WriteLine(s)
Next

You need the nuget package System.IO.Ports, e.g. in your csproj:

<PackageReference Include="System.IO.Ports" Version="6.0.0" />

How to find these packages

You could find it this way:

1. Check the namespace and assembly on the docs: https://learn.microsoft.com/en-us/dotnet/api/system.io.ports.serialport.getportnames?view=dotnet-plat-ext-6.0. Here both System.IO.Ports. Also double check the platform support, under Applies to.
2. Search for System.IO.Ports on nuget.org: https://www.nuget.org/packages?q=System.IO.Ports and select the Microsoft package