[Authorize]
[HttpGet]
public object Foo()
{
  return DateTime.Now.ToString();
}

public class MyAuthHandler : IAuthenticationHandler
  {
    public const string SchemeName = "MyAuth";

    AuthenticationScheme _scheme;
    HttpContext     _context;

    /// <summary>
    /// 初始化认证
    /// </summary>
    public Task InitializeAsync(AuthenticationScheme scheme, HttpContext context)
    {
      _scheme = scheme;
      _context = context;
      return Task.CompletedTask;
    }

    /// <summary>
    /// 认证处理
    /// </summary>
    public Task<AuthenticateResult> AuthenticateAsync()
    {
      var req   = _context.Request.Query;
      var isLogin = req["isLogin"].FirstOrDefault();

      if (isLogin != "true")
      {
        return Task.FromResult(AuthenticateResult.Fail("未登陆"));
      }

      var ticket = GetAuthTicket("test", "test");
      return Task.FromResult(AuthenticateResult.Success(ticket));
    }

    AuthenticationTicket GetAuthTicket(string name, string role)
    {
      var claimsIdentity = new ClaimsIdentity(new Claim[]
      {
        new Claim(ClaimTypes.Name, name),
        new Claim(ClaimTypes.Role, role),
      }, "My_Auth");

      var principal = new ClaimsPrincipal(claimsIdentity);
      return new AuthenticationTicket(principal, _scheme.Name);
    }

    /// <summary>
    /// 权限不足时的处理
    /// </summary>
    public Task ForbidAsync(AuthenticationProperties properties)
    {
      _context.Response.StatusCode = (int)HttpStatusCode.Forbidden;
      return Task.CompletedTask;
    }

    /// <summary>
    /// 未登录时的处理
    /// </summary>
    public Task ChallengeAsync(AuthenticationProperties properties)
    {
      _context.Response.StatusCode = (int)HttpStatusCode.Unauthorized;
      return Task.CompletedTask;
    }
  }

app.UseAuthentication();
app.UseAuthorization();

services.AddAuthentication(options =>
  {
    options.AddScheme<MyAuthHandler>(MyAuthHandler.SchemeName, "default scheme");
    options.DefaultAuthenticateScheme = MyAuthHandler.SchemeName;
    options.DefaultChallengeScheme  = MyAuthHandler.SchemeName;
  });