public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    private string _userName;

    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options, UserResolverService userService)
        : base(options)
    {
        _userName = userService.GetActualUser();
    }

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
    }
}

public class UserResolverService
{
    private readonly IHttpContextAccessor _accessor;
    public UserResolverService(IHttpContextAccessor accessor)
    {
        _accessor = accessor;
    }

    public string GetActualUser()
    {
        if(_accessor.HttpContext.User.Identity.IsAuthenticated == true)
        {
            return _accessor.HttpContext.User?.Identity?.Name;
        }
        var dummy = "Dummy";
        return dummy;
        //
    }
}

public void ConfigureServices(IServiceCollection services)
    {
        services.AddDbContext<ApplicationDbContext>(options =>
            options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

        services.AddIdentity<ApplicationUser, IdentityRole>()
            .AddEntityFrameworkStores<ApplicationDbContext>()
            .AddDefaultTokenProviders();

        // Add application services.

        services.AddTransient<IEmailSender, EmailSender>();
        services.AddTransient<IHttpContextAccessor, HttpContextAccessor>();
        services.AddTransient<UserResolverService>();

        services.AddMvc();

    }