Proper way to register HostedService in ASP.NET Core. AddHostedService vs AddSingleton

Question

What is the proper way to register a custom hosted service in ASP.NET Core 2.1? For example, I have a custom hosted service derived from BackgroundService named MyHostedService. How should I register it?

public IServiceProvider ConfigureServices(IServiceCollection services)
{           
    //...
    services.AddSingleton<IHostedService, MyHostedService>();
}

or

public IServiceProvider ConfigureServices(IServiceCollection services)
{           
    //...
    services.AddHostedService<MyHostedService>();
}

?

Here we can see the first case, but here there is a second case.

Are these methods equal?

Answer 1

Update

Somewhere between .Net Core 2.2 and 3.1 the behavior has changed, AddHostedService is now adding a Singleton instead of the previous Transient service. Credit - Comment by LeonG

public static class ServiceCollectionHostedServiceExtensions
{
    /// <summary>
    /// Add an <see cref="IHostedService"/> registration for the given type.
    /// </summary>
    /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
    /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
    /// <returns>The original <see cref="IServiceCollection"/>.</returns>
    public static IServiceCollection AddHostedService<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicConstructors)] THostedService>(this IServiceCollection services)
        where THostedService : class, IHostedService
    {
        services.TryAddEnumerable(ServiceDescriptor.Singleton<IHostedService, THostedService>());

        return services;
    }

    /// <summary>
    /// Add an <see cref="IHostedService"/> registration for the given type.
    /// </summary>
    /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
    /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
    /// <param name="implementationFactory">A factory to create new instances of the service implementation.</param>
    /// <returns>The original <see cref="IServiceCollection"/>.</returns>
    public static IServiceCollection AddHostedService<THostedService>(this IServiceCollection services, Func<IServiceProvider, THostedService> implementationFactory)
        where THostedService : class, IHostedService
    {
        services.TryAddEnumerable(ServiceDescriptor.Singleton<IHostedService>(implementationFactory));

        return services;
    }
}

Reference ServiceCollectionHostedServiceExtensions

---

Original Answer

They are similar but not completely

AddHostedService is part of Microsoft.Extensions.Hosting.Abstractions.

It belongs to Microsoft.Extensions.Hosting.Abstractions in the ServiceCollectionHostedServiceExtensions class

using Microsoft.Extensions.Hosting;

namespace Microsoft.Extensions.DependencyInjection
{
    public static class ServiceCollectionHostedServiceExtensions
    {
        /// <summary>
        /// Add an <see cref="IHostedService"/> registration for the given type.
        /// </summary>
        /// <typeparam name="THostedService">An <see cref="IHostedService"/> to register.</typeparam>
        /// <param name="services">The <see cref="IServiceCollection"/> to register with.</param>
        /// <returns>The original <see cref="IServiceCollection"/>.</returns>
        public static IServiceCollection AddHostedService<THostedService>(this IServiceCollection services)
            where THostedService : class, IHostedService
            => services.AddTransient<IHostedService, THostedService>();
    }
}

Note it is using Transient life time scope and not Singleton

Internally the framework add all the hosted services to another service (HostedServiceExecutor)

public HostedServiceExecutor(ILogger<HostedServiceExecutor> logger, 
    IEnumerable<IHostedService> services) //<<-- note services collection
{
    _logger = logger;
    _services = services;
}

at startup that is a singleton via the WebHost Constructor.

_applicationServiceCollection.AddSingleton<HostedServiceExecutor>();