I needed to enumerate all COM ports in C# and get their name (e.g. COM1, COM2, …) and their description shown in the Windows Device Manager (e.g. “Communications Port”, “RT USB MRTD Reader”). You could find lots of unmanaged code for doing this but I think this C# code could be helpful to people who perform communication through the serial interface in .NET:

using System;
using System.Text;
using ComTypes = System.Runtime.InteropServices.ComTypes;
using System.Runtime.InteropServices;
using System.Collections.Generic;

public class Win32DeviceMgmt
{
    private const UInt32 DIGCF_PRESENT = 0x00000002;
    private const UInt32 DIGCF_DEVICEINTERFACE = 0x00000010;
    private const UInt32 SPDRP_DEVICEDESC = 0x00000000;
    private const UInt32 DICS_FLAG_GLOBAL = 0x00000001;
    private const UInt32 DIREG_DEV = 0x00000001;
    private const UInt32 KEY_QUERY_VALUE = 0x0001;
    private const string GUID_DEVINTERFACE_COMPORT = "86E0D1E0-8089-11D0-9CE4-08003E301F73";

    [StructLayout(LayoutKind.Sequential)]
    private struct SP_DEVINFO_DATA
    {
        public Int32  cbSize;
        public Guid   ClassGuid;
        public Int32  DevInst;
        public UIntPtr Reserved;
    };

    [DllImport("setupapi.dll")]
    private static extern Int32 SetupDiDestroyDeviceInfoList(IntPtr DeviceInfoSet);

    [DllImport("setupapi.dll")]
    private static extern bool SetupDiEnumDeviceInfo(IntPtr DeviceInfoSet, Int32 MemberIndex, ref  SP_DEVINFO_DATA DeviceInterfaceData);

    [DllImport("setupapi.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern bool SetupDiGetDeviceRegistryProperty(IntPtr deviceInfoSet, ref SP_DEVINFO_DATA deviceInfoData,
        uint property, out UInt32 propertyRegDataType, StringBuilder propertyBuffer, uint propertyBufferSize, out UInt32 requiredSize);

    [DllImport("setupapi.dll", SetLastError = true)]
    private static extern IntPtr SetupDiGetClassDevs(ref Guid gClass, UInt32 iEnumerator, IntPtr hParent, UInt32 nFlags);

    [DllImport("Setupapi", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr SetupDiOpenDevRegKey(IntPtr hDeviceInfoSet, ref SP_DEVINFO_DATA deviceInfoData, uint scope,
        uint hwProfile, uint parameterRegistryValueKind, uint samDesired);

    [DllImport("advapi32.dll", CharSet = CharSet.Unicode, EntryPoint = "RegQueryValueExW", SetLastError = true)]
    private static extern int RegQueryValueEx(IntPtr hKey, string lpValueName, int lpReserved, out uint lpType,
        StringBuilder lpData, ref uint lpcbData);

    [DllImport("advapi32.dll", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
    private static extern int RegCloseKey(IntPtr hKey);

    [DllImport("kernel32.dll")]
    private static extern Int32 GetLastError();

    public struct DeviceInfo
    {
        public string name;
        public string decsription;
    }

    public static List<DeviceInfo> GetAllCOMPorts()
    {
        Guid guidComPorts = new Guid(GUID_DEVINTERFACE_COMPORT);
        IntPtr hDeviceInfoSet = SetupDiGetClassDevs(
            ref guidComPorts, 0, IntPtr.Zero, DIGCF_PRESENT | DIGCF_DEVICEINTERFACE);
        if (hDeviceInfoSet == IntPtr.Zero)
        {
            throw new Exception("Failed to get device information set for the COM ports");
        }

        try {
            List<DeviceInfo> devices = new List<DeviceInfo>();
            Int32 iMemberIndex = 0;
            while (true)
            {
                SP_DEVINFO_DATA deviceInfoData = new SP_DEVINFO_DATA();
                deviceInfoData.cbSize = Marshal.SizeOf(typeof(SP_DEVINFO_DATA));
                bool success = SetupDiEnumDeviceInfo(hDeviceInfoSet, iMemberIndex, ref deviceInfoData);
                if (!success)
                {
                    // No more devices in the device information set
                    break;
                }

                DeviceInfo deviceInfo = new DeviceInfo();
                deviceInfo.name = GetDeviceName(hDeviceInfoSet, deviceInfoData);
                deviceInfo.decsription = GetDeviceDescription(hDeviceInfoSet, deviceInfoData);
                devices.Add(deviceInfo);

                iMemberIndex++;
            }
            return devices;
        }
        finally {
            SetupDiDestroyDeviceInfoList(hDeviceInfoSet);
        }
    }

    private static string GetDeviceName(IntPtr pDevInfoSet, SP_DEVINFO_DATA deviceInfoData)
    {
        IntPtr hDeviceRegistryKey = SetupDiOpenDevRegKey(pDevInfoSet, ref deviceInfoData,
            DICS_FLAG_GLOBAL, 0, DIREG_DEV, KEY_QUERY_VALUE);
        if (hDeviceRegistryKey == IntPtr.Zero)
        {
            throw new Exception("Failed to open a registry key for device-specific configuration information");
        }

        StringBuilder deviceNameBuf = new StringBuilder(256);
        try
        {
            uint lpRegKeyType;
            uint length = (uint)deviceNameBuf.Capacity;
            int result = RegQueryValueEx(hDeviceRegistryKey, "PortName", 0, out lpRegKeyType, deviceNameBuf, ref length);
            if (result != 0)
            {
                throw new Exception("Can not read registry value PortName for device " + deviceInfoData.ClassGuid);
            }
        }
        finally
        {
            RegCloseKey(hDeviceRegistryKey);
        }

        string deviceName = deviceNameBuf.ToString();
        return deviceName;
    }

    private static string GetDeviceDescription(IntPtr hDeviceInfoSet, SP_DEVINFO_DATA deviceInfoData)
    {
        StringBuilder descriptionBuf = new StringBuilder(256);
        uint propRegDataType;
        uint length = (uint)descriptionBuf.Capacity;
        bool success = SetupDiGetDeviceRegistryProperty(hDeviceInfoSet, ref deviceInfoData, SPDRP_DEVICEDESC,
            out propRegDataType, descriptionBuf, length, out length);
        if (!success)
        {
            throw new Exception("Can not read registry value PortName for device " + deviceInfoData.ClassGuid);
        }
        string deviceDescription = descriptionBuf.ToString();
        return deviceDescription;
    }

}