How do I get the available wifi APs and their signal strength in .net?

If the platform is Windows10, you can use Microsoft.Windows.SDK.Contracts package to access all available wifis.

First, install Microsoft.Windows.SDK.Contracts package from nuget.

Then, you can use next code to get ssid and signal strength.

var adapters = await WiFiAdapter.FindAllAdaptersAsync();
foreach (var adapter in adapters)
{
    foreach (var network in adapter.NetworkReport.AvailableNetworks)
    {
        Console.WriteLine($"ssid: {network.Ssid}");
        Console.WriteLine($"signal strength: {network.SignalBars}");
    }
}

It is a wrapper project with managed code in c# at http://www.codeplex.com/managedwifi

It supports Windows Vista and XP SP2 (or later version).

sample code:

using NativeWifi;
using System;
using System.Text;

namespace WifiExample
{
    class Program
    {
        /// <summary>
        /// Converts a 802.11 SSID to a string.
        /// </summary>
        static string GetStringForSSID(Wlan.Dot11Ssid ssid)
        {
            return Encoding.ASCII.GetString( ssid.SSID, 0, (int) ssid.SSIDLength );
        }

        static void Main( string[] args )
        {
            WlanClient client = new WlanClient();
            foreach ( WlanClient.WlanInterface wlanIface in client.Interfaces )
            {
                // Lists all networks with WEP security
                Wlan.WlanAvailableNetwork[] networks = wlanIface.GetAvailableNetworkList( 0 );
                foreach ( Wlan.WlanAvailableNetwork network in networks )
                {
                    if ( network.dot11DefaultCipherAlgorithm == Wlan.Dot11CipherAlgorithm.WEP )
                    {
                        Console.WriteLine( "Found WEP network with SSID {0}.", GetStringForSSID(network.dot11Ssid));
                    }
                }

                // Retrieves XML configurations of existing profiles.
                // This can assist you in constructing your own XML configuration
                // (that is, it will give you an example to follow).
                foreach ( Wlan.WlanProfileInfo profileInfo in wlanIface.GetProfiles() )
                {
                    string name = profileInfo.profileName; // this is typically the network's SSID

                    string xml = wlanIface.GetProfileXml( profileInfo.profileName );
                }

                // Connects to a known network with WEP security
                string profileName = "Cheesecake"; // this is also the SSID
                string mac = "52544131303235572D454137443638";
                string key = "hello";
                string profileXml = string.Format("<?xml version=\"1.0\"?><WLANProfile xmlns=\"http://www.microsoft.com/networking/WLAN/profile/v1\"><name>{0}</name><SSIDConfig><SSID><hex>{1}</hex><name>{0}</name></SSID></SSIDConfig><connectionType>ESS</connectionType><MSM><security><authEncryption><authentication>open</authentication><encryption>WEP</encryption><useOneX>false</useOneX></authEncryption><sharedKey><keyType>networkKey</keyType><protected>false</protected><keyMaterial>{2}</keyMaterial></sharedKey><keyIndex>0</keyIndex></security></MSM></WLANProfile>", profileName, mac, key);

                wlanIface.SetProfile( Wlan.WlanProfileFlags.AllUser, profileXml, true );
                wlanIface.Connect( Wlan.WlanConnectionMode.Profile, Wlan.Dot11BssType.Any, profileName );
            }
        }
    }
}

I am doing it running a command netsh wlan show networks mode=bssid from C# code.

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace ConsoleApp2
{
    class AccessPoint
    {
        public string SSID { get; set; }
        public string BSSID { get; set; }
        public byte Signal { get; set; }
    }

    class Program
    {
        private static async Task Main(string[] args)
        {
            var apList = await GetSignalOfNetworks();

            foreach (var ap in apList)
            {
                WriteLine($"{ap.BSSID} - {ap.Signal} - {ap.SSID}");
            }

            Console.ReadKey();
        }

        private static async Task<AccessPoint[]> GetSignalOfNetworks()
        {
            string result = await ExecuteProcessAsync(@"C:\Windows\System32\netsh.exe", "wlan show networks mode=bssid");

            return Regex.Split(result, @"[^B]SSID \d+").Skip(1).SelectMany(network => GetAccessPointFromNetwork(network)).ToArray();
        }

        private static AccessPoint[] GetAccessPointFromNetwork(string network)
        {
            string withoutLineBreaks = Regex.Replace(network, @"[\r\n]+", " ").Trim();
            string ssid = Regex.Replace(withoutLineBreaks, @"^:\s+(\S+).*$", "$1").Trim();

            return Regex.Split(withoutLineBreaks, @"\s{4}BSSID \d+").Skip(1).Select(ap => GetAccessPoint(ssid, ap)).ToArray();
        }

        private static AccessPoint GetAccessPoint(string ssid, string ap)
        {
            string withoutLineBreaks = Regex.Replace(ap, @"[\r\n]+", " ").Trim();
            string bssid = Regex.Replace(withoutLineBreaks, @"^:\s+([a-f0-9]{2}(:[a-f0-9]{2}){5}).*$", "$1").Trim();
            byte signal = byte.Parse(Regex.Replace(withoutLineBreaks, @"^.*(Signal|Sinal)\s+:\s+(\d+)%.*$", "$2").Trim());

            return new AccessPoint
            {
                SSID = ssid,
                BSSID = bssid,
                Signal = signal,
            };
        }

        private static async Task<string> ExecuteProcessAsync(string cmd, string args = null)
        {
            var process = new Process()
            {
                StartInfo = new ProcessStartInfo
                {
                    FileName = cmd,
                    Arguments = args,
                    RedirectStandardInput = false,
                    RedirectStandardOutput = true,
                    RedirectStandardError = true,
                    UseShellExecute = false,
                    CreateNoWindow = true,
                    StandardOutputEncoding = Encoding.UTF8,
                }
            };

            process.Start();

            string result = await process.StandardOutput.ReadToEndAsync();

            process.WaitForExit();

#if DEBUG
            if (result.Trim().Contains("The Wireless AutoConfig Service (wlansvc) is not running."))
            {
                return await GetFakeData();
            }
#endif

            return result;
        }

        private static async Task<string> GetFakeData()
        {
            var assembly = Assembly.GetExecutingAssembly();
            var resourceName = "ConsoleApp2.FakeData.txt";

            using (Stream stream = assembly.GetManifestResourceStream(resourceName))
            using (StreamReader reader = new StreamReader(stream))
            {
                return await reader.ReadToEndAsync();
            }
        }

        private static void WriteLine(string str)
        {
            Console.WriteLine(str);
        }
    }
}

Use Native Wifi APIs, present on all Vista and XP SP3 systems. XP SP2 has a different API with which you can do the same thing.

How to enumerate networks

How to get signal strength