为什么User(在User.Identity.Name中)在我的抽象基本控制器中为null？

public void ConfigureServices(IServiceCollection services)
{
    services.AddSession(options =>
    {
        options.IdleTimeout = TimeSpan.FromMinutes(30); //设置会话过期时间为30分钟
        options.Cookie.HttpOnly = true;
        options.Cookie.IsEssential = true;
    });
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    app.UseSession();
}

public class MyController : Controller
{
    private readonly ClaimsPrincipal _user;

    public MyController(ClaimsPrincipal user)
    {
        _user = user;
    }

    public IActionResult Index()
    {
        var username = _user.Identity.Name;
        return View();
    }
}