Sitecore External User builder for Azure Active Directory Integration

CustomExternalUserBuilder.cs

using Microsoft.AspNet.Identity;
using Microsoft.AspNet.Identity.Owin;
using Sitecore.Owin.Authentication.Identity;
using Sitecore.Owin.Authentication.Services;
using Sitecore.SecurityModel.Cryptography;
using System;
using System.Net.Mail;
using System.Security.Claims;
using System.Text;

namespace My.Foundation.Custom.AzureAD
{

  public class CustomExternalUserBuilder : ExternalUserBuilder
  {
    private readonly IHashEncryption _hashEncryption;

    public bool IsPersistentUser
    {
      get;
      set;
    }

    protected ApplicationUserFactory ApplicationUserFactory
    {
      get;
    }

    public CustomExternalUserBuilder(ApplicationUserFactory applicationUserFactory, IHashEncryption hashEncryption)
    {
      _hashEncryption = hashEncryption;
      ApplicationUserFactory = applicationUserFactory;
    }

    public override ApplicationUser BuildUser(UserManager<ApplicationUser> userManager, ExternalLoginInfo externalLoginInfo)
    {
      string userName = CreateUniqueUserName(userManager, externalLoginInfo);
      ApplicationUser applicationUser = ApplicationUserFactory.CreateUser(userName);
      applicationUser.IsVirtual = !IsPersistentUser;
      applicationUser.Email = externalLoginInfo.Email;
      return applicationUser;
    }

    protected virtual string CreateUniqueUserName(UserManager<ApplicationUser> userManager, ExternalLoginInfo externalLoginInfo)
    {
      ClaimsIdentity externalIdentity = externalLoginInfo.ExternalIdentity;
      //AD domain used for Azure
      string domain = "ad";
      string userName = string.Empty;
      if (externalIdentity != null)
      {
        var emailId = externalIdentity.FindFirst("email")?.Value;
        MailAddress addr = new MailAddress(emailId);
        userName = addr.User;
      }
      else
      {
        userName = GetDefaultUserName(userManager, externalLoginInfo);
      }
      return string.Format("{0}\\{1}", domain, userName);
    }

    protected string GetDefaultUserName(UserManager<ApplicationUser> userManager, ExternalLoginInfo externalLoginInfo)
    {
      if (externalLoginInfo == null)
      {
        throw new InvalidOperationException("Unable to retrieve External Login Info from the Identity provider for the given identity");
      }
      string defaultUserName;
      do
      {
        string text = string.Format("{0}{1}{2}", externalLoginInfo.Login.LoginProvider, externalLoginInfo.Login.ProviderKey, "$");
        byte[] bytes = Encoding.ASCII.GetBytes(text);
        byte[] inArray = _hashEncryption.ComputeHash(bytes);
        defaultUserName = Convert.ToBase64String(inArray).Trim('=').Replace("/", string.Empty).Replace("+", string.Empty);
        defaultUserName = defaultUserName.Substring(0, Math.Min(defaultUserName.Length, 10));
      }
      while (userManager.FindByName(defaultUserName) != null);
      return defaultUserName;
    }
  }
}