Creé una tarea en segundo plano en mi ASP.NET Core 2.1 siguiendo este tutorial: https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services?view=aspnetcore-2.1# consumir-un-servicio-alcance-en-una-tarea-en-segundo plano

Al compilar me da error:

System.InvalidOperationException: 'No se puede consumir el servicio de ámbito 'MyDbContext' del singleton 'Microsoft.AspNetCore.Hosting.Internal.HostedServiceExecutor'.'

¿Qué causa ese error y cómo solucionarlo?

Tarea de fondo:

 internal class OnlineTaggerMS : IHostedService, IDisposable { private readonly CoordinatesHelper _coordinatesHelper; private Timer _timer; public IServiceProvider Services { get; } public OnlineTaggerMS(IServiceProvider services, CoordinatesHelper coordinatesHelper) { Services = services; _coordinatesHelper = coordinatesHelper; } public Task StartAsync(CancellationToken cancellationToken) { // Run every 30 sec _timer = new Timer(DoWork, null, TimeSpan.Zero, TimeSpan.FromSeconds(30)); return Task.CompletedTask; } private async void DoWork(object state) { using (var scope = Services.CreateScope()) { var dbContext = scope.ServiceProvider.GetRequiredService<MyDbContext>(); Console.WriteLine("Online tagger Service is Running"); // Run something await ProcessCoords(dbContext); } } public Task StopAsync(CancellationToken cancellationToken) { _timer?.Change(Timeout.Infinite, 0); return Task.CompletedTask; } private async Task ProcessCoords(MyDbContext dbContext) { var topCoords = await _coordinatesHelper.GetTopCoordinates(); foreach (var coord in topCoords) { var user = await dbContext.Users.SingleOrDefaultAsync(c => c.Id == coord.UserId); if (user != null) { var now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(); //expire time = 120 sec var coordTimeStamp = DateTimeOffset.FromUnixTimeMilliseconds(coord.TimeStamp).AddSeconds(120).ToUnixTimeMilliseconds(); if (coordTimeStamp < now && user.IsOnline == true) { user.IsOnline = false; await dbContext.SaveChangesAsync(); } else if (coordTimeStamp > now && user.IsOnline == false) { user.IsOnline = true; await dbContext.SaveChangesAsync(); } } } } public void Dispose() { _timer?.Dispose(); } }

Inicio.cs:

 services.AddHostedService<OnlineTaggerMS>();

Programa.cs:

 public class Program { public static void Main(string[] args) { var host = BuildWebHost(args); using (var scope = host.Services.CreateScope()) { var services = scope.ServiceProvider; try { var context = services.GetRequiredService<TutorDbContext>(); DbInitializer.Initialize(context); } catch(Exception ex) { var logger = services.GetRequiredService<ILogger<Program>>(); logger.LogError(ex, "An error occurred while seeding the database."); } } host.Run(); } public static IWebHost BuildWebHost(string[] args) => WebHost.CreateDefaultBuilder(args) .UseStartup<Startup>() .Build(); }

Inicio completo.cs:

 public class Startup { public Startup(IConfiguration configuration) { Configuration = configuration; } public IConfiguration Configuration { get; } // This method gets called by the runtime. Use this method to add services to the container. public void ConfigureServices(IServiceCollection services) { services.AddCors(options => { options.AddPolicy("CorsPolicy", builder => builder.AllowAnyOrigin() .AllowAnyMethod() .AllowAnyHeader() .AllowCredentials()); }); services.AddDbContext<MyDbContext>(options => options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"))); // ===== Add Identity ======== services.AddIdentity<User, IdentityRole>() .AddEntityFrameworkStores<TutorDbContext>() .AddDefaultTokenProviders(); JwtSecurityTokenHandler.DefaultInboundClaimTypeMap.Clear(); // => remove default claims services .AddAuthentication(options => { options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme; options.DefaultScheme = JwtBearerDefaults.AuthenticationScheme; options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme; }) .AddJwtBearer(cfg => { cfg.RequireHttpsMetadata = false; cfg.SaveToken = true; cfg.TokenValidationParameters = new TokenValidationParameters { ValidIssuer = Configuration["JwtIssuer"], ValidAudience = Configuration["JwtIssuer"], IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["JwtKey"])), ClockSkew = TimeSpan.Zero // remove delay of token when expire }; }); //return 401 instead of redirect services.ConfigureApplicationCookie(options => { options.Events.OnRedirectToLogin = context => { context.Response.StatusCode = 401; return Task.CompletedTask; }; options.Events.OnRedirectToAccessDenied = context => { context.Response.StatusCode = 401; return Task.CompletedTask; }; }); services.AddMvc(); services.AddSwaggerGen(c => { c.SwaggerDoc("v1", new Info { Version = "v1", Title = "xyz", }); // Swagger 2.+ support var security = new Dictionary<string, IEnumerable<string>> { {"Bearer", new string[] { }}, }; c.AddSecurityDefinition("Bearer", new ApiKeyScheme { Description = "JWT Authorization header using the Bearer scheme. Example: \"Bearer {token}\"", Name = "Authorization", In = "header", Type = "apiKey" }); c.AddSecurityRequirement(security); }); services.AddHostedService<OnlineTaggerMS>(); services.AddTransient<UsersHelper, UsersHelper>(); services.AddTransient<CoordinatesHelper, CoordinatesHelper>(); } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IServiceProvider serviceProvider, IApplicationBuilder app, IHostingEnvironment env, TutorDbContext dbContext) { dbContext.Database.Migrate(); if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.UseCors("CorsPolicy"); app.UseAuthentication(); app.UseMvc(); app.UseSwagger(); app.UseSwaggerUI(c => { c.SwaggerEndpoint("v1/swagger.json", "xyz V1"); }); CreateRoles(serviceProvider).GetAwaiter().GetResult(); } private async Task CreateRoles(IServiceProvider serviceProvider) { var RoleManager = serviceProvider.GetRequiredService<RoleManager<IdentityRole>>(); var UserManager = serviceProvider.GetRequiredService<UserManager<User>>(); string[] roleNames = { "x", "y", "z", "a" }; IdentityResult roleResult; foreach (var roleName in roleNames) { var roleExist = await RoleManager.RoleExistsAsync(roleName); if (!roleExist) { roleResult = await RoleManager.CreateAsync(new IdentityRole(roleName)); } } var _user = await UserManager.FindByEmailAsync("xxx"); if (_user == null) { var poweruser = new User { UserName = "xxx", Email = "xxx", FirstName = "xxx", LastName = "xxx" }; string adminPassword = "xxx"; var createPowerUser = await UserManager.CreateAsync(poweruser, adminPassword); if (createPowerUser.Succeeded) { await UserManager.AddToRoleAsync(poweruser, "xxx"); } } }