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Injecting an EF DbContext instance into an IHostedService

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By Egor Quenum
1 min read
Injecting an EF DbContext instance into an IHostedService

Problem

As we all know, there are hardly any APIs that do not use a database to persist their data. ASP.NET makes it easy to connect applications to databases via DbContext, part of Entity Framework.

While working on one of my personal projects, I have encountered the following error when trying to inject DbContext into a hosted background service:

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Cannot consume scoped service 'Microsoft.EntityFrameworkCore.DbContextOptions' from singleton 'Microsoft.Extensions.Hosting.IHostedService'.

This makes sense since at the time I was using AddDbContext() to register my context, which by default registers a context as scoped. IHostedServices are singletons, hence the conflict.

Solution

To fix the issue, I decided to use AddDbContextFactory() registration instead:

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builder.Services.AddDbContextFactory<MonitorDbContext>(options =>
{
    options.UseLazyLoadingProxies();
    options.UseNpgsql(builder.Configuration.GetConnectionString("ChangeMonitor"));
    options.UseSnakeCaseNamingConvention();
});

This method registers a singleton IDbContextFactory and a scoped DbContext which in turn allows us to produce instances of the second on demand via CreateDbContext():

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...

private readonly IDbContextFactory<MonitorDbContext> _contextFactory;

public MonitorJobsRegistrationService(
  ...
  IDbContextFactory<MonitorDbContext> contextFactory)
{
  ...
  _contextFactory = contextFactory;
}

public async Task StartAsync(CancellationToken cancellationToken)
{
  await InitiateScheduler(cancellationToken);

  using (var context = _contextFactory.CreateDbContext())
  {
    var targetEntities = await context.Targets.ToListAsync(cancellationToken);

    if (targetEntities.Count > 0)
    {
      _logger.LogInformation("Existing targets found, count: {TargetCount}. Registering jobs...",
          targetEntities.Count);

      await _jobService.ScheduleAsync(targetEntities.Select(entity => entity.ToTarget()),
          cancellationToken);
    }
  }
}

...

The 2 types registered in WebApplicationBuilder.Services:

Desktop View The 2 types registered in WebApplicationBuilder.Services

The other nice thing about AddDbContextFactory() is that you can still inject scoped DbContext instances directly without using the factory:

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...

private readonly MonitorDbContext _context;
private readonly IMonitorJobService _jobService;

public ResourceService(
  MonitorDbContext context,
  IMonitorJobService jobService)
{
  _context = context;
  _jobService = jobService;
}

...

That’s pretty neat. It makes me wonder whether it’s a good idea to just adopt it as a common practice and always use AddDbContextFactory() by default when first wiring up Entity Framework in new projects.

Useful URLs

software-development, dotnet
dotnet csharp dependency-injection hosted-service
This post is licensed under CC BY 4.0 by the author.