Dependency injection in ASP.NET Core

By Steve Smith, Scott Addie, and Luke Latham

ASP.NET Core supports the dependency injection (DI) software design pattern, which is a technique for achieving Inversion of Control (IoC) between classes and their dependencies.

For more information specific to dependency injection within MVC controllers, see Dependency injection into controllers in ASP.NET Core.

View or download sample code (how to download)

Overview of dependency injection

A dependency is any object that another object requires. Examine the following MyDependency class with a WriteMessage method that other classes in an app depend upon:

public class MyDependency
{
    public MyDependency()
    {
    }

    public Task WriteMessage(string message)
    {
        Console.WriteLine(
            $"MyDependency.WriteMessage called. Message: {message}");

        return Task.FromResult(0);
    }
}

An instance of the MyDependency class can be created to make the WriteMessage method available to a class. The MyDependency class is a dependency of the IndexModel class:

public class IndexModel : PageModel
{
MyDependency _dependency = new MyDependency();

public async Task OnGetAsync()
{
await _myDependency.WriteMessage(
"IndexModel.OnGetAsync created this message.");
}
}

An instance of the MyDependency class can be created to make the WriteMessage method available to a class. The MyDependency class is a dependency of the HomeController class:

public class HomeController : Controller
{
MyDependency _dependency = new MyDependency();

public async Task<IActionResult> Index()
{
await _myDependency.WriteMessage(
"HomeController.Index created this message.");

return View();
}
}

The class creates and directly depends on the MyDependency instance. Code dependencies (such as the previous example) are problematic and should be avoided for the following reasons:

  • To replace MyDependency with a different implementation, the class must be modified.
  • If MyDependency has dependencies, they must be configured by the class. In a large project with multiple classes depending on MyDependency, the configuration code becomes scattered across the app.
  • This implementation is difficult to unit test. The app should use a mock or stub MyDependency class, which isn't possible with this approach.

Dependency injection addresses these problems through:

  • The use of an interface to abstract the dependency implementation.
  • Registration of the dependency in a service container. ASP.NET Core provides a built-in service container, IServiceProvider. Services are registered in the app's Startup.ConfigureServices method.
  • Injection of the service into the constructor of the class where it's used. The framework takes on the responsibility of creating an instance of the dependency and disposing of it when it's no longer needed.

In the sample app, the IMyDependency interface defines a method that the service provides to the app:

public interface IMyDependency
{
    Task WriteMessage(string message);
}
public interface IMyDependency
{
    Task WriteMessage(string message);
}

This interface is implemented by a concrete type, MyDependency:

public class MyDependency : IMyDependency
{
    private readonly ILogger<MyDependency> _logger;

    public MyDependency(ILogger<MyDependency> logger)
    {
        _logger = logger;
    }

    public Task WriteMessage(string message)
    {
        _logger.LogInformation(
            "MyDependency.WriteMessage called. Message: {MESSAGE}", 
            message);

        return Task.FromResult(0);
    }
}
public class MyDependency : IMyDependency
{
    private readonly ILogger<MyDependency> _logger;

    public MyDependency(ILogger<MyDependency> logger)
    {
        _logger = logger;
    }

    public Task WriteMessage(string message)
    {
        _logger.LogInformation(
            "MyDependency.WriteMessage called. Message: {MESSAGE}", 
            message);

        return Task.FromResult(0);
    }
}

MyDependency requests an ILogger<TCategoryName> in its constructor. It's not unusual to use dependency injection in a chained fashion. Each requested dependency in turn requests its own dependencies. The container resolves the dependencies in the graph and returns the fully resolved service. The collective set of dependencies that must be resolved is typically referred to as a dependency tree, dependency graph, or object graph.

IMyDependency and ILogger<TCategoryName> must be registered in the service container. IMyDependency is registered in Startup.ConfigureServices. ILogger<TCategoryName> is registered by the logging abstractions infrastructure, so it's a framework-provided service registered by default by the framework.

In the sample app, the IMyDependency service is registered with the concrete type MyDependency. The registration scopes the service lifetime to the lifetime of a single request. Service lifetimes are described later in this topic.

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

    services.AddScoped<IMyDependency, MyDependency>();
    services.AddTransient<IOperationTransient, Operation>();
    services.AddScoped<IOperationScoped, Operation>();
    services.AddSingleton<IOperationSingleton, Operation>();
    services.AddSingleton<IOperationSingletonInstance>(new Operation(Guid.Empty));

    // OperationService depends on each of the other Operation types.
    services.AddTransient<OperationService, OperationService>();
}
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    services.AddScoped<IMyDependency, MyDependency>();
    services.AddTransient<IOperationTransient, Operation>();
    services.AddScoped<IOperationScoped, Operation>();
    services.AddSingleton<IOperationSingleton, Operation>();
    services.AddSingleton<IOperationSingletonInstance>(new Operation(Guid.Empty));

    // OperationService depends on each of the other Operation types.
    services.AddTransient<OperationService, OperationService>();
}

Note

Each services.Add{SERVICE_NAME} extension method adds (and potentially configures) services. For example, services.AddMvc() adds the services Razor Pages and MVC require. We recommended that apps follow this convention. Place extension methods in the Microsoft.Extensions.DependencyInjection namespace to encapsulate groups of service registrations.

If the service's constructor requires a primitive, such as a string, the primitive can be injected by using configuration or the options pattern:

public class MyDependency : IMyDependency
{
    public MyDependency(IConfiguration config)
    {
        var myStringValue = config["MyStringKey"];

        // Use myStringValue
    }

    ...
}

An instance of the service is requested via the constructor of a class where the service is used and assigned to a private field. The field is used to access the service as necessary throughout the class.

In the sample app, the IMyDependency instance is requested and used to call the service's WriteMessage method:

public class IndexModel : PageModel
{
    private readonly IMyDependency _myDependency;

    public IndexModel(
        IMyDependency myDependency, 
        OperationService operationService,
        IOperationTransient transientOperation,
        IOperationScoped scopedOperation,
        IOperationSingleton singletonOperation,
        IOperationSingletonInstance singletonInstanceOperation)
    {
        _myDependency = myDependency;
        OperationService = operationService;
        TransientOperation = transientOperation;
        ScopedOperation = scopedOperation;
        SingletonOperation = singletonOperation;
        SingletonInstanceOperation = singletonInstanceOperation;
    }

    public OperationService OperationService { get; }
    public IOperationTransient TransientOperation { get; }
    public IOperationScoped ScopedOperation { get; }
    public IOperationSingleton SingletonOperation { get; }
    public IOperationSingletonInstance SingletonInstanceOperation { get; }

    public async Task OnGetAsync()
    {
        await _myDependency.WriteMessage(
            "IndexModel.OnGetAsync created this message.");
    }
}
public class MyDependencyController : Controller
{
    private readonly IMyDependency _myDependency;

    public MyDependencyController(IMyDependency myDependency)
    {
        _myDependency = myDependency;
    }

    // GET: /mydependency/
    public async Task<IActionResult> Index()
    {
        await _myDependency.WriteMessage(
            "MyDependencyController.Index created this message.");

        return View();
    }
}

Framework-provided services

The Startup.ConfigureServices method is responsible for defining the services the app uses, including platform features, such as Entity Framework Core and ASP.NET Core MVC. Initially, the IServiceCollection provided to ConfigureServices has the following services defined (depending on how the host was configured):

Service Type Lifetime
Microsoft.AspNetCore.Hosting.Builder.IApplicationBuilderFactory Transient
Microsoft.AspNetCore.Hosting.IApplicationLifetime Singleton
Microsoft.AspNetCore.Hosting.IHostingEnvironment Singleton
Microsoft.AspNetCore.Hosting.IStartup Singleton
Microsoft.AspNetCore.Hosting.IStartupFilter Transient
Microsoft.AspNetCore.Hosting.Server.IServer Singleton
Microsoft.AspNetCore.Http.IHttpContextFactory Transient
Microsoft.Extensions.Logging.ILogger<T> Singleton
Microsoft.Extensions.Logging.ILoggerFactory Singleton
Microsoft.Extensions.ObjectPool.ObjectPoolProvider Singleton
Microsoft.Extensions.Options.IConfigureOptions<T> Transient
Microsoft.Extensions.Options.IOptions<T> Singleton
System.Diagnostics.DiagnosticSource Singleton
System.Diagnostics.DiagnosticListener Singleton

When a service collection extension method is available to register a service (and its dependent services, if required), the convention is to use a single Add{SERVICE_NAME} extension method to register all of the services required by that service. The following code is an example of how to add additional services to the container using the extension methods AddDbContext, AddIdentity, and AddMvc:

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));

    services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<ApplicationDbContext>()
        .AddDefaultTokenProviders();

    services.AddMvc();
}

For more information, see the ServiceCollection Class in the API documentation.

Service lifetimes

Choose an appropriate lifetime for each registered service. ASP.NET Core services can be configured with the following lifetimes:

Transient

Transient lifetime services are created each time they're requested. This lifetime works best for lightweight, stateless services.

Scoped

Scoped lifetime services are created once per request.

Warning

When using a scoped service in a middleware, inject the service into the Invoke or InvokeAsync method. Don't inject via constructor injection because it forces the service to behave like a singleton. For more information, see ASP.NET Core Middleware.

Singleton

Singleton lifetime services are created the first time they're requested (or when ConfigureServices is run and an instance is specified with the service registration). Every subsequent request uses the same instance. If the app requires singleton behavior, allowing the service container to manage the service's lifetime is recommended. Don't implement the singleton design pattern and provide user code to manage the object's lifetime in the class.

Warning

It's dangerous to resolve a scoped service from a singleton. It may cause the service to have incorrect state when processing subsequent requests.

Constructor injection behavior

Services can be resolved by two mechanisms:

  • IServiceProvider
  • ActivatorUtilities – Permits object creation without service registration in the dependency injection container. ActivatorUtilities is used with user-facing abstractions, such as Tag Helpers, MVC controllers, SignalR Hubs, and model binders.

Constructors can accept arguments that aren't provided by dependency injection, but the arguments must assign default values.

When services are resolved by IServiceProvider or ActivatorUtilities, constructor injection requires a public constructor.

When services are resolved by ActivatorUtilities, constructor injection requires that only one applicable constructor exist. Constructor overloads are supported, but only one overload can exist whose arguments can all be fulfilled by dependency injection.

Entity Framework contexts

Entity Framework contexts should be added to the service container using the scoped lifetime. This is handled automatically with a call to the AddDbContext method when registering the database context. Services that use the database context should also use the scoped lifetime.

Lifetime and registration options

To demonstrate the difference between the lifetime and registration options, consider the following interfaces that represent tasks as an operation with a unique identifier, OperationId. Depending on how the lifetime of an operations service is configured for the following interfaces, the container provides either the same or a different instance of the service when requested by a class:

public interface IOperation
{
    Guid OperationId { get; }
}

public interface IOperationTransient : IOperation
{
}

public interface IOperationScoped : IOperation
{
}

public interface IOperationSingleton : IOperation
{
}

public interface IOperationSingletonInstance : IOperation
{
}
public interface IOperation
{
    Guid OperationId { get; }
}

public interface IOperationTransient : IOperation
{
}

public interface IOperationScoped : IOperation
{
}

public interface IOperationSingleton : IOperation
{
}

public interface IOperationSingletonInstance : IOperation
{
}

The interfaces are implemented in the Operation class. The Operation constructor generates a GUID if one isn't supplied:

public class Operation : IOperationTransient, 
    IOperationScoped, 
    IOperationSingleton, 
    IOperationSingletonInstance
{
    public Operation() : this(Guid.NewGuid())
    {
    }

    public Operation(Guid id)
    {
        OperationId = id;
    }

    public Guid OperationId { get; private set; }
}
public class Operation : IOperationTransient, 
    IOperationScoped, 
    IOperationSingleton, 
    IOperationSingletonInstance
{
    public Operation() : this(Guid.NewGuid())
    {
    }

    public Operation(Guid id)
    {
        OperationId = id;
    }

    public Guid OperationId { get; private set; }
}

An OperationService is registered that depends on each of the other Operation types. When OperationService is requested via dependency injection, it receives either a new instance of each service or an existing instance based on the lifetime of the dependent service.

  • If transient services are created when requested, the OperationsId of the IOperationTransient service is different than the OperationsId of the OperationService. OperationService receives a new instance of the IOperationTransient class. The new instance yields a different OperationsId.
  • If scoped services are created per request, the OperationsId of the IOperationScoped service is the same as that of OperationService within a request. Across requests, both services share a different OperationsId value.
  • If singleton and singleton-instance services are created once and used across all requests and all services, the OperationsId is constant across all service requests.
public class OperationService
{
    public OperationService(
        IOperationTransient transientOperation,
        IOperationScoped scopedOperation,
        IOperationSingleton singletonOperation,
        IOperationSingletonInstance instanceOperation)
    {
        TransientOperation = transientOperation;
        ScopedOperation = scopedOperation;
        SingletonOperation = singletonOperation;
        SingletonInstanceOperation = instanceOperation;
    }

    public IOperationTransient TransientOperation { get; }
    public IOperationScoped ScopedOperation { get; }
    public IOperationSingleton SingletonOperation { get; }
    public IOperationSingletonInstance SingletonInstanceOperation { get; }
}
public class OperationService
{
    public OperationService(IOperationTransient transientOperation,
        IOperationScoped scopedOperation,
        IOperationSingleton singletonOperation,
        IOperationSingletonInstance instanceOperation)
    {
        TransientOperation = transientOperation;
        ScopedOperation = scopedOperation;
        SingletonOperation = singletonOperation;
        SingletonInstanceOperation = instanceOperation;
    }

    public IOperationTransient TransientOperation { get; }
    public IOperationScoped ScopedOperation { get; }
    public IOperationSingleton SingletonOperation { get; }
    public IOperationSingletonInstance SingletonInstanceOperation { get; }
}

In Startup.ConfigureServices, each type is added to the container according to its named lifetime:

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<CookiePolicyOptions>(options =>
    {
        options.CheckConsentNeeded = context => true;
        options.MinimumSameSitePolicy = SameSiteMode.None;
    });

    services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);

    services.AddScoped<IMyDependency, MyDependency>();
    services.AddTransient<IOperationTransient, Operation>();
    services.AddScoped<IOperationScoped, Operation>();
    services.AddSingleton<IOperationSingleton, Operation>();
    services.AddSingleton<IOperationSingletonInstance>(new Operation(Guid.Empty));

    // OperationService depends on each of the other Operation types.
    services.AddTransient<OperationService, OperationService>();
}
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc();

    services.AddScoped<IMyDependency, MyDependency>();
    services.AddTransient<IOperationTransient, Operation>();
    services.AddScoped<IOperationScoped, Operation>();
    services.AddSingleton<IOperationSingleton, Operation>();
    services.AddSingleton<IOperationSingletonInstance>(new Operation(Guid.Empty));

    // OperationService depends on each of the other Operation types.
    services.AddTransient<OperationService, OperationService>();
}

The IOperationSingletonInstance service is using a specific instance with a known ID of Guid.Empty. It's clear when this type is in use (its GUID is all zeroes).

The sample app demonstrates object lifetimes within and between individual requests. The sample app's IndexModel requests each kind of IOperation type and the OperationService. The page then displays all of the page model class's and service's OperationId values through property assignments:

public class IndexModel : PageModel
{
    private readonly IMyDependency _myDependency;

    public IndexModel(
        IMyDependency myDependency, 
        OperationService operationService,
        IOperationTransient transientOperation,
        IOperationScoped scopedOperation,
        IOperationSingleton singletonOperation,
        IOperationSingletonInstance singletonInstanceOperation)
    {
        _myDependency = myDependency;
        OperationService = operationService;
        TransientOperation = transientOperation;
        ScopedOperation = scopedOperation;
        SingletonOperation = singletonOperation;
        SingletonInstanceOperation = singletonInstanceOperation;
    }

    public OperationService OperationService { get; }
    public IOperationTransient TransientOperation { get; }
    public IOperationScoped ScopedOperation { get; }
    public IOperationSingleton SingletonOperation { get; }
    public IOperationSingletonInstance SingletonInstanceOperation { get; }

    public async Task OnGetAsync()
    {
        await _myDependency.WriteMessage(
            "IndexModel.OnGetAsync created this message.");
    }
}

The sample app demonstrates object lifetimes within and between individual requests. The sample app includes an OperationsController that requests each kind of IOperation type and the OperationService. The Index action sets the services into the ViewBag for display of the service's OperationId values:

public class OperationsController : Controller
{
    private readonly OperationService _operationService;
    private readonly IOperationTransient _transientOperation;
    private readonly IOperationScoped _scopedOperation;
    private readonly IOperationSingleton _singletonOperation;
    private readonly IOperationSingletonInstance _singletonInstanceOperation;

    public OperationsController(
        OperationService operationService,
        IOperationTransient transientOperation,
        IOperationScoped scopedOperation,
        IOperationSingleton singletonOperation,
        IOperationSingletonInstance singletonInstanceOperation)
    {
        _operationService = operationService;
        _transientOperation = transientOperation;
        _scopedOperation = scopedOperation;
        _singletonOperation = singletonOperation;
        _singletonInstanceOperation = singletonInstanceOperation;
    }

    public IActionResult Index()
    {
        // Viewbag contains controller-requested services.
        ViewBag.Transient = _transientOperation;
        ViewBag.Scoped = _scopedOperation;
        ViewBag.Singleton = _singletonOperation;
        ViewBag.SingletonInstance = _singletonInstanceOperation;
        
        // Operation service has its own requested services.
        ViewBag.Service = _operationService;

        return View();
    }
}

Two following output shows the results of two requests:

First request:

Controller operations:

Transient: d233e165-f417-469b-a866-1cf1935d2518
Scoped: 5d997e2d-55f5-4a64-8388-51c4e3a1ad19
Singleton: 01271bc1-9e31-48e7-8f7c-7261b040ded9
Instance: 00000000-0000-0000-0000-000000000000

OperationService operations:

Transient: c6b049eb-1318-4e31-90f1-eb2dd849ff64
Scoped: 5d997e2d-55f5-4a64-8388-51c4e3a1ad19
Singleton: 01271bc1-9e31-48e7-8f7c-7261b040ded9
Instance: 00000000-0000-0000-0000-000000000000

Second request:

Controller operations:

Transient: b63bd538-0a37-4ff1-90ba-081c5138dda0
Scoped: 31e820c5-4834-4d22-83fc-a60118acb9f4
Singleton: 01271bc1-9e31-48e7-8f7c-7261b040ded9
Instance: 00000000-0000-0000-0000-000000000000

OperationService operations:

Transient: c4cbacb8-36a2-436d-81c8-8c1b78808aaf
Scoped: 31e820c5-4834-4d22-83fc-a60118acb9f4
Singleton: 01271bc1-9e31-48e7-8f7c-7261b040ded9
Instance: 00000000-0000-0000-0000-000000000000

Observe which of the OperationId values vary within a request and between requests:

  • Transient objects are always different. Note that the transient OperationId value for both the first and second requests are different for both OperationService operations and across requests. A new instance is provided to each service and request.
  • Scoped objects are the same within a request but different across requests.
  • Singleton objects are the same for every object and every request regardless of whether an Operation instance is provided in ConfigureServices.

Call services from main

Create an IServiceScope with IServiceScopeFactory.CreateScope to resolve a scoped service within the app's scope. This approach is useful to access a scoped service at startup to run initialization tasks. The following example shows how to obtain a context for the MyScopedService in Program.Main:

public static void Main(string[] args)
{
    var host = CreateWebHostBuilder(args).Build();

    using (var serviceScope = host.Services.CreateScope())
    {
        var services = serviceScope.ServiceProvider;

        try
        {
            var serviceContext = services.GetRequiredService<MyScopedService>();
            // Use the context here
        }
        catch (Exception ex)
        {
            var logger = services.GetRequiredService<ILogger<Program>>();
            logger.LogError(ex, "An error occurred.");
        }
    }

    host.Run();
}

Scope validation

When the app is running in the Development environment, the default service provider performs checks to verify that:

  • Scoped services aren't directly or indirectly resolved from the root service provider.
  • Scoped services aren't directly or indirectly injected into singletons.

The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app/server's lifetime when the provider starts with the app and is disposed when the app shuts down.

Scoped services are disposed by the container that created them. If a scoped service is created in the root container, the service's lifetime is effectively promoted to singleton because it's only disposed by the root container when app/server is shut down. Validating service scopes catches these situations when BuildServiceProvider is called.

For more information, see ASP.NET Core Web Host.

Request Services

The services available within an ASP.NET Core request from HttpContext are exposed through the HttpContext.RequestServices collection.

Request Services represent the services configured and requested as part of the app. When the objects specify dependencies, these are satisfied by the types found in RequestServices, not ApplicationServices.

Generally, the app shouldn't use these properties directly. Instead, request the types that classes require via class constructors and allow the framework inject the dependencies. This yields classes that are easier to test (see the Test and debug topics).

Note

Prefer requesting dependencies as constructor parameters to accessing the RequestServices collection.

Design services for dependency injection

Best practices are to:

  • Design services to use dependency injection to obtain their dependencies.
  • Avoid stateful, static method calls (a practice known as static cling).
  • Avoid direct instantiation of dependent classes within services. Direct instantiation couples the code to a particular implementation.

By following the SOLID Principles of Object Oriented Design, app classes naturally tend to be small, well-factored, and easily tested.

If a class seems to have too many injected dependencies, this is generally a sign that the class has too many responsibilities and is violating the Single Responsibility Principle (SRP). Attempt to refactor the class by moving some of its responsibilities into a new class. Keep in mind that Razor Pages page model classes and MVC controller classes should focus on UI concerns. Business rules and data access implementation details should be kept in classes appropriate to these separate concerns.

Disposal of services

The container calls Dispose for the IDisposable types it creates. If an instance is added to the container by user code, it isn't disposed automatically.

// Services that implement IDisposable:
public class Service1 : IDisposable {}
public class Service2 : IDisposable {}
public class Service3 : IDisposable {}

public interface ISomeService {}
public class SomeServiceImplementation : ISomeService, IDisposable {}

public void ConfigureServices(IServiceCollection services)
{
    // The container creates the following instances and disposes them automatically:
    services.AddScoped<Service1>();
    services.AddSingleton<Service2>();
    services.AddSingleton<ISomeService>(sp => new SomeServiceImplementation());

    // The container doesn't create the following instances, so it doesn't dispose of
    // the instances automatically:
    services.AddSingleton<Service3>(new Service3());
    services.AddSingleton(new Service3());
}

Note

In ASP.NET Core 1.0, the container calls dispose on all IDisposable objects, including those it didn't create.

Default service container replacement

The built-in service container is meant to serve the basic needs of the framework and most consumer apps built on it. However, developers can replace the built-in container with their preferred container. The Startup.ConfigureServices method typically returns void. If the method's signature is changed to return an IServiceProvider, a different container can be configured and returned. There are many IoC containers available for .NET. In the following example, the Autofac container is used:

  1. Install the appropriate container package(s):

  2. Configure the container in Startup.ConfigureServices and return an IServiceProvider:

    public IServiceProvider ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
        // Add other framework services
    
        // Add Autofac
        var containerBuilder = new ContainerBuilder();
        containerBuilder.RegisterModule<DefaultModule>();
        containerBuilder.Populate(services);
        var container = containerBuilder.Build();
        return new AutofacServiceProvider(container);
    }
    

    To use a 3rd party container, Startup.ConfigureServices must return IServiceProvider.

  3. Configure Autofac in DefaultModule:

    public class DefaultModule : Module
    {
        protected override void Load(ContainerBuilder builder)
        {
            builder.RegisterType<CharacterRepository>().As<ICharacterRepository>();
        }
    }
    

At runtime, Autofac is used to resolve types and inject dependencies. To learn more about using Autofac with ASP.NET Core, see the Autofac documentation.

Thread safety

Singleton services need to be thread safe. If a singleton service has a dependency on a transient service, the transient service may also need to be thread safe depending how it's used by the singleton.

The factory method of single service, such as the second argument to AddSingleton<TService>(IServiceCollection, Func<IServiceProvider,TService>), doesn't need to be thread-safe. Like a type (static) constructor, it's guaranteed to be called once by a single thread.

Recommendations

When working with dependency injection, keep the following recommendations in mind:

  • Avoid storing data and configuration directly in the service container. For example, a user's shopping cart shouldn't typically be added to the service container. Configuration should use the options pattern. Similarly, avoid "data holder" objects that only exist to allow access to some other object. It's better to request the actual item via dependency injection, if possible.

  • Avoid static access to services (for example, statically-typing IApplicationBuilder.ApplicationServices for use elsewhere).

  • Avoid using the service locator pattern (for example, IServiceProvider.GetService).

  • Avoid static access to HttpContext (for example, IHttpContextAccessor.HttpContext).

Like all sets of recommendations, you may encounter situations where ignoring a recommendation is required. Exceptions are rare—mostly special cases within the framework itself.

Dependency injection is an alternative to static/global object access patterns. You may not be able to realize the benefits of dependency injection if you mix it with static object access.

Additional resources