using System;

using System.Net.Http;

using System.Text.RegularExpressions;

using System.Threading;

using System.Threading.Tasks;

using Microsoft.IdentityModel.Clients.ActiveDirectory;

public class Program

{

    public static async Task Main()

    {

        var cts = new CancellationTokenSource();

        var cancellationToken = cts.Token;

        var subscription = "c3c0a359-4420-4f84-8925-f642e2717296";

        var hostName = "management.azure.com";

        var apiVersion = "2020-08-01";

        var requetUrl = $"https://{hostName}/subscriptions/{subscription}?api-version={apiVersion}";

        var httpClient = new HttpClient();

        var response = await httpClient.GetAsync(requetUrl, cancellationToken);

        var authenticationParameters = await AuthenticationParameters.CreateFromUnauthorizedResponseAsync(response);

​

        var authorizationHeaderRegex = new Regex(@"https://.+/(.+)/?", RegexOptions.Compiled | RegexOptions.CultureInvariant | RegexOptions.IgnoreCase);

        var match = authorizationHeaderRegex.Match(authenticationParameters.Authority);

        var tenantString = match.Success ? match.Groups[1].Value : null;

​

        if (!Guid.TryParse(tenantString, out var tenantId))

        {

            throw new InvalidOperationException($"Received tenant id '{tenantString}' is not valid guid");

        }

        Console.WriteLine(tenantId);

    }

}