querying WMI WIN32_SerialPort locks up serial port

Question

Running the below code locks up my com port, and I would LOVE to know why. I have found a couple unanswered questions on SO that are similar...

I am using Bluetooth adapters (RN-41) to make a serial connection. The adapters SPP create two virtual com ports (This is common to all adapters we have tested). Windows labels them as Incoming and Outgoing, but the Incoming is the only one that actually carries serial data.

That leaves me with 'extra' virtual serial ports. I populate a dropdown with the available Com ports so the user can select the port to connect to from the list.

Prior to that, I am filtering that list to remove the 'fake' com ports. Here is my code:

public string checkBluetoothCom()
        {
            string bluetoothComm = "";
            List<string> badComPorts = new List<string>();
            ManagementClass processClass = new ManagementClass("WIN32_SerialPort");

            ManagementObjectCollection Ports = processClass.GetInstances();

            foreach (ManagementObject property in Ports)
            {
                if (property.GetPropertyValue("Name").ToString().Contains("Bluetooth"))
                {
                    string hardwareAddress = property.GetPropertyValue("PNPDeviceID").ToString().Split('&')[4].Split('_')[0];
                    if (!hardwareAddress.All(c => c == '0'))
                    {
                        bluetoothComm = property.GetPropertyValue("DeviceID").ToString();
                    }
                    else
                    {
                        badComPorts.Add(property.GetPropertyValue("DeviceID").ToString());
                    }
                }
            }

            Console.WriteLine("Removing {0} items from comm port list", badComPorts.Count);
            ports.RemoveAll(item => badComPorts.Contains(item));

            return bluetoothComm;
        }

The problem is that after this runs my Com Port locks up and I cannnot make any connection until I remove my bluetooth adapter and restart the remote device (RN-41). Any thoughts?

Probably because this is a synchronous query. Same thing happens for USB mouting while querying win32_LogicalDisk. You likely need to switch to an asynchronous WQL query. msdn.microsoft.com/en-us/library/cc143292.aspx — David-SkyMesh
That was correct @David-SkyMesh. If you add that as an answer I'll accept it. And thank you for the direction, very helpful! — Matthew Hilgenfeld
@MatthewHilgenfeld Would you be able to provide the code you have that performs the asynchronous query and fixes the problem? — JacobD

Synck Synck · Accepted Answer · 2015-11-16T12:45:47

If you query "Win32_PnPEntity" instead of "Win32_SerialPort" and check for the COM port ClassGuid {4d36e978-e325-11ce-bfc1-08002be10318}, the port isn't locked up or accessed.

PowerShell example:

Get-WmiObject -Query "SELECT Name,DeviceID FROM Win32_PnPEntity WHERE ClassGuid=`"{4d36e978-e325-11ce-bfc1-08002be10318}`""

This gives a list of all the COM ports as can be found in the device manager.

Source: Finding information about all serial devices connected through USB in C#

querying WMI WIN32_SerialPort locks up serial port

2 Answers