Enumerate monitor connections by WMI.

MonitorHelper.cs

using System.Collections.Generic;
using System.Linq;
using System.Management;

public static class MonitorHelper
{
    public static IEnumerable<MonitorConnection> EnumerateMonitorConnections()
    {
        var @class = new ManagementClass(@"root\wmi:WmiMonitorConnectionParams");

        foreach (var @object in @class.GetInstances().Cast<ManagementObject>())
        {
            yield return new MonitorConnection(
                (bool)@object["Active"],
                (string)@object["InstanceName"],
                (D3DKMDT_VIDEO_OUTPUT_TECHNOLOGY)(uint)@object["VideoOutputTechnology"]);
        }
    }
}

public record MonitorConnection(bool IsActive, string InstanceName, D3DKMDT_VIDEO_OUTPUT_TECHNOLOGY OutputTechnology);

public enum D3DKMDT_VIDEO_OUTPUT_TECHNOLOGY : int
{
    D3DKMDT_VOT_UNINITIALIZED = -2,
    D3DKMDT_VOT_OTHER = -1,
    D3DKMDT_VOT_HD15 = 0,
    D3DKMDT_VOT_SVIDEO = 1,
    D3DKMDT_VOT_COMPOSITE_VIDEO = 2,
    D3DKMDT_VOT_COMPONENT_VIDEO = 3,
    D3DKMDT_VOT_DVI = 4,
    D3DKMDT_VOT_HDMI = 5,
    D3DKMDT_VOT_LVDS = 6,
    D3DKMDT_VOT_D_JPN = 8,
    D3DKMDT_VOT_SDI = 9,
    D3DKMDT_VOT_DISPLAYPORT_EXTERNAL = 10,
    D3DKMDT_VOT_DISPLAYPORT_EMBEDDED = 11,
    D3DKMDT_VOT_UDI_EXTERNAL = 12,
    D3DKMDT_VOT_UDI_EMBEDDED = 13,
    D3DKMDT_VOT_SDTVDONGLE = 14,
    D3DKMDT_VOT_MIRACAST = 15,
    D3DKMDT_VOT_INDIRECT_WIRED = 16,
    D3DKMDT_VOT_INTERNAL = unchecked((int)0x80000000),
    D3DKMDT_VOT_SVIDEO_4PIN = D3DKMDT_VOT_SVIDEO,
    D3DKMDT_VOT_SVIDEO_7PIN = D3DKMDT_VOT_SVIDEO,
    D3DKMDT_VOT_RF = D3DKMDT_VOT_COMPOSITE_VIDEO,
    D3DKMDT_VOT_RCA_3COMPONENT = D3DKMDT_VOT_COMPONENT_VIDEO,
    D3DKMDT_VOT_BNC = D3DKMDT_VOT_COMPONENT_VIDEO,
}