Dependency injection in HostedService failing

Question

User-959394753 posted

Hi,

I am busy on a asp net core application where I need to perform a background task on my database once a day. So reading https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services?view=aspnetcore-3.1&tabs=visual-studio I made a background service that's working fine.  But in order to do something with it I need to access my database with the database context.  I wanted to use Dependency injection like I do with all my other classes. But then I get a runtime error : HTTP Error 500.30 ANCM In-Process start failure.

The relevant code:

 public class BackgroundService : IHostedService, IDisposable
    {
        private readonly Data.ApplicationDbContext _DB;
        private int executionCount = 0;
        private readonly ILogger<BackgroundService> _logger;

        private Timer _timer;

        //public BackgroundService(Data.ApplicationDbContext DB, ILogger<BackgroundService> logger) //this fails!!
        public BackgroundService( ILogger<BackgroundService> logger)  //This works
        {
            _logger = logger;
           // _DB = DB;
        }
....

Can somebody tell me what I do wrong? 

thanks i advance.

Rob

Answer 1

User-474980206 posted

how did you register the db context? the constructor only support singleton objects. If you need scoped object, then you need to create a scoped service:

   https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services?view=aspnetcore-3.1&tabs=visual-studio

 

Answer 2

User-959394753 posted

Hi Bruce, I registered my DB Context in the Startup as follow:

public void ConfigureServices(IServiceCollection services)
        {
            ...

            services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(
                    Configuration.GetConnectionString("DefaultConnection")));
...

public void Configure(IApplicationBuilder app,
          IHostingEnvironment env,
          ApplicationDbContext context,
          RoleManager<ApplicationRole> roleManager,
          UserManager<ApplicationUser> userManager
)

Answer 3

User-474980206 posted

but that a transient scope (every request gets a unique dbcontext), not a singleton. you should register dbcontext factory as a singleton.

factory:

  https://docs.microsoft.com/en-us/ef/core/miscellaneous/cli/dbcontext-creation

Answer 4

User-959394753 posted

OK, I know the services,addSingleton<> but where to put the options??

 //services.AddDbContext<ApplicationDbContext>(options =>
 //    options.UseSqlServer(
 //        Configuration.GetConnectionString("DefaultConnection")));

            services.AddSingleton<Data.ApplicationDbContext>(options =>
               options.UseSqlServer(                                 //Syntax error!!
                   Configuration.GetConnectionString("DefaultConnection")));

What is exactly the correct syntax ??

Answer 5

User-474980206 posted

for the singleton, you create an instance of an object. depending on your needs, you pass the factory, or you call the factory and pass an dbcontext instance created by the factory. see the factory example link above. 

Answer 6

User-959394753 posted

Sorry Bruce,

It's like Chinese to me.

what is a factory? how do I pass one?

Do I need to change my ApplicationDbContext class? 

All I want is to run a Linq query once a day.

I appreciate your help but my level of knowledge is not the same

Answer 7

User-474980206 posted

the sample looks pretty simple:

    public class BloggingContextFactory : IDesignTimeDbContextFactory<BloggingContext>
    {
        public BloggingContext CreateDbContext(string[] args)
        {
            var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
            optionsBuilder.UseSqlite("Data Source=blog.db");

            return new BloggingContext(optionsBuilder.Options);
        }
    }

then its:

      services.AddSingleton<BloggingContextFactory>(new BloggingContextFactory());

then in service:

          public BackgroundService( ILogger<BackgroundService> logger, BloggingContextFactory factory)

then when the service needs a context:

using (var dbcontext = factory.CreateDbContext(args))
{
}

note: as a Factory is a major concept in DI, you should study the DI pattern (inversion of control)

hint: you should add a connection string to Factory constructor that you can be used by CreateDbContext(). I'd probably use a delegate for the option builder, so it can be set in startup.cs

Answer 8

User-959394753 posted

Thanks Bruce,

the light is slowly starting to shine.

What I did:

1. I have created a new class RoosterContextFactory, containing:

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Design;
using Microsoft.EntityFrameworkCore.Infrastructure;

namespace RoosterApp.Data
{
    public class RoosterContextFactory : IDesignTimeDbContextFactory<ApplicationDbContext>
    {
        public ApplicationDbContext CreateDbContext(string[] args)
        {
            var optionsBuilder = new DbContextOptionsBuilder<ApplicationDbContext>();
            optionsBuilder.UseSqlServer("Data Source= Server=xxxxxxxxxxx; Database=xxxxxx; User Id=xxxxxx ; Password = xxxxx");

            return new ApplicationDbContext(optionsBuilder.Options);
        }
    }

then in my startup I added the line:

            services.AddSingleton<RoosterContextFactory>(new RoosterContextFactory());

And then in my BackgroundService where I need to access the database :

using (var dbcontext = RoosterContextFactory.CreateDbContext(args)) //This args argument is not accepted
                {
			// I will enter my Linq query here.
                }

Here, the argument args is not accepted. I have tried to leave it out, or enter an empty string. But it keeps saying "The name 'args' does not exist in the current context".

I have read the note that the args parameter is unused, But it will not build like this. do you have a solution for this?

second question: the database connection string is now copy - past from my Appsettings file. is there a neat way to prevent this and refer to the appsettingsfile directly?

Regards

Rob 

Answer 9

User-854763662 posted

Hi Rob warning ,

You could create scoped service in BackgroundService instead of registering it in Constructor.The doc of Consuming a scoped service in a background task has explained it.

public class BackgroundService : IHostedService, IDisposable
{
        private int executionCount = 0;
        private readonly ILogger<BackgroundService> _logger;
        private readonly IServiceProvider _provider;

        private Timer _timer;


        public BackgroundService(ILogger<BackgroundService> logger,IServiceProvider provider)  //This works
        {
            _logger = logger;
            _provider = provider;
        }

      ...

        private void DoWork(object state)
        {
            using (var scope = _provider.CreateScope())
            {
                var dbcontxt = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();
                var data = dbcontxt.Employee.ToList();            

            }
            var count = Interlocked.Increment(ref executionCount);

            _logger.LogInformation(
                "Timed Hosted Service is working. Count: {Count}", count);
        }

      ...
}

Best Regards,

Sherry

Answer 10

User-959394753 posted

Tanks Cherry,

Your input has directed me into the right direction. The injection of IServiceProvider works as you stated. But the syntax to create a scope does not work this way in core 2.2.

After a lot of reading I came up with the following:

private void DoWork(object state)
        {using (var scope = ServiceProviderServiceExtensions.CreateScope(_provider))
                {
                    var DBcontext = scope.ServiceProvider.GetRequiredService<ApplicationDbContext>();
                    List<Mutation> mut = (from m in DBcontext.Mutations
                                          where m.DateStartOn < DateTime.Today
                                          where m.ApprovedOn == null
                                          select m).ToList();

                    foreach (Mutation M in mut)
...

Now everything works as it should.