I don't know, how to remove/disable default AspNetCore.Identity.UserValidator. UserManager has 2 UserValidators now: Microsoft.AspNetCore.Identity.UserValidator and Penize_eShop.MyUserValidator
Thanks for help
public void ConfigureServices(IServiceCollection services) { // Add framework services. services.AddApplicationInsightsTelemetry(Configuration); services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"))); services.AddIdentity<MyUser, MyRole>(options => { options.Password.RequiredLength = 6; options.Password.RequireDigit = false; options.Password.RequireLowercase = false; options.Password.RequireUppercase = false; options.Password.RequireNonAlphanumeric = false; }) .AddEntityFrameworkStores<ApplicationDbContext>() .AddUserStore<MyUserStore>() .AddUserManager<MyUserManager>() .AddRoleManager<MyRoleManager>() .AddErrorDescriber<MyErrorDescriber>() .AddDefaultTokenProviders() .AddUserValidator<MyUserValidator<MyUser>>() ; services.AddScoped<SignInManager<MyUser>, MySignInManager>(); services.AddScoped<IPasswordHasher<MyUser>, MyPasswordHasher>(); services.AddMvc(); // Add application services. services.AddTransient<IEmailSender, AuthMessageSender>(); services.AddTransient<ISmsSender, AuthMessageSender>(); } 
services.AddTransient<IUserValidator<ApplicationUser>, CustomUserValidator>();