Add Azure AD authentication to existing ASP.NET Core application

azureauth.cs

// Startup.cs
using Microsoft.IdentityModel.Tokens;

public void ConfigureServices(IServiceCollection services)
{
  services.AddAuthentication().AddOpenIdConnect(c =>
  {
      c.Authority = "https://login.microsoftonline.com/common";
      c.ClientId = "<insert-registered-guid>";
      c.TokenValidationParameters = new TokenValidationParameters
      {
          ValidateIssuer = false
      };
      c.Events.OnTokenValidated = async context =>
      {
          UserManager<ApplicationUser> manager = context.HttpContext.RequestServices.GetService<UserManager<ApplicationUser>>();
          SignInManager<ApplicationUser> signIn = context.HttpContext.RequestServices.GetService<SignInManager<ApplicationUser>>();
          ApplicationUser user = await manager.FindByNameAsync(context.Principal.Identity.Name);
          if (user != null)
          {
              await signIn.SignInAsync(user, false);
          }
      };
  });
}

// HomeController.cs
using Microsoft.AspNetCore.Authentication.OpenIdConnect;

public class HomeController : Controller
{
  [AllowAnonymous]
  public IActionResult LoginWithAzure()
  {
      string redirectUrl = Url.Content("~/");
      return Challenge(new AuthenticationProperties { RedirectUri = redirectUrl }, OpenIdConnectDefaults.AuthenticationScheme);
  }
}