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title author description monikerRange ms.author ms.custom ms.date uid
Dependency injection in ASP.NET Core
guardrex
Learn how ASP.NET Core implements dependency injection and how to use it.
>= aspnetcore-2.1
riande
mvc
08/14/2019
fundamentals/dependency-injection

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 xref:mvc/controllers/dependency-injection.

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 _dependency.WriteMessage(
            "IndexModel.OnGetAsync created this message.");
    }
}

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 or base class to abstract the dependency implementation.
  • Registration of the dependency in a service container. ASP.NET Core provides a built-in service container, xref:System.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:

[!code-csharp]

This interface is implemented by a concrete type, MyDependency:

[!code-csharp]

MyDependency requests an xref:Microsoft.Extensions.Logging.ILogger`1 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.

The container resolves ILogger<TCategoryName> by taking advantage of (generic) open types, eliminating the need to register every (generic) constructed type:

services.AddSingleton(typeof(ILogger<T>), typeof(Logger<T>));

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.

[!code-csharp]

[!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 built in type, such as a string, the type 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:

[!code-csharp]

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
xref:Microsoft.AspNetCore.Hosting.Builder.IApplicationBuilderFactory?displayProperty=fullName Transient
xref:Microsoft.AspNetCore.Hosting.IApplicationLifetime?displayProperty=fullName Singleton
xref:Microsoft.AspNetCore.Hosting.IHostingEnvironment?displayProperty=fullName Singleton
xref:Microsoft.AspNetCore.Hosting.IStartup?displayProperty=fullName Singleton
xref:Microsoft.AspNetCore.Hosting.IStartupFilter?displayProperty=fullName Transient
xref:Microsoft.AspNetCore.Hosting.Server.IServer?displayProperty=fullName Singleton
xref:Microsoft.AspNetCore.Http.IHttpContextFactory?displayProperty=fullName Transient
xref:Microsoft.Extensions.Logging.ILogger`1?displayProperty=fullName Singleton
xref:Microsoft.Extensions.Logging.ILoggerFactory?displayProperty=fullName Singleton
xref:Microsoft.Extensions.ObjectPool.ObjectPoolProvider?displayProperty=fullName Singleton
xref:Microsoft.Extensions.Options.IConfigureOptions`1?displayProperty=fullName Transient
xref:Microsoft.Extensions.Options.IOptions`1?displayProperty=fullName Singleton
xref:System.Diagnostics.DiagnosticSource?displayProperty=fullName Singleton
xref:System.Diagnostics.DiagnosticListener?displayProperty=fullName 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<TContext>, xref:Microsoft.Extensions.DependencyInjection.IdentityServiceCollectionExtensions.AddIdentityCore*, and xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.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 xref:Microsoft.Extensions.DependencyInjection.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 (xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddTransient*) are created each time they're requested from the service container. This lifetime works best for lightweight, stateless services.

Scoped

Scoped lifetime services (xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddScoped*) are created once per client request (connection).

[!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 xref:fundamentals/middleware/write#per-request-middleware-dependencies.

Singleton

Singleton lifetime services (xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionServiceExtensions.AddSingleton*) are created the first time they're requested (or when Startup.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.

Service registration methods

Each service registration extension method offers overloads that are useful in specific scenarios.

Method Automatic
object
disposal
Multiple
implementations
Pass args
Add{LIFETIME}<{SERVICE}, {IMPLEMENTATION}>()
Example:
services.AddScoped<IMyDep, MyDep>();
Yes Yes No
Add{LIFETIME}<{SERVICE}>(sp => new {IMPLEMENTATION})
Examples:
services.AddScoped<IMyDep>(sp => new MyDep());
services.AddScoped<IMyDep>(sp => new MyDep("A string!"));
Yes Yes Yes
Add{LIFETIME}<{IMPLEMENTATION}>()
Example:
services.AddScoped<MyDep>();
Yes No No
Add{LIFETIME}<{SERVICE}>(new {IMPLEMENTATION})
Examples:
services.AddScoped<IMyDep>(new MyDep());
services.AddScoped<IMyDep>(new MyDep("A string!"));
No Yes Yes
Add{LIFETIME}(new {IMPLEMENTATION})
Examples:
services.AddScoped(new MyDep());
services.AddScoped(new MyDep("A string!"));
No No Yes

For more information on type disposal, see the Disposal of services section. A common scenario for multiple implementations is mocking types for testing.

TryAdd{LIFETIME} methods only register the service if there isn't already an implementation registered.

In the following example, the first line registers MyDependency for IMyDependency. The second line has no effect because IMyDependency already has a registered implementation:

services.AddSingleton<IMyDependency, MyDependency>();
// The following line has no effect:
services.TryAddSingleton<IMyDependency, DifferentDependency>();

For more information, see:

  • xref:Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAdd*
  • xref:Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAddTransient*
  • xref:Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAddScoped*
  • xref:Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAddSingleton*

TryAddEnumerable(ServiceDescriptor) methods only register the service if there isn't already an implementation of the same type. Multiple services are resolved via IEnumerable<{SERVICE}>. When registering services, the developer only wants to add an instance if one of the same type hasn't already been added. Generally, this method is used by library authors to avoid registering two copies of an instance in the container.

In the following example, the first line registers MyDep for IMyDep1. The second line registers MyDep for IMyDep2. The third line has no effect because IMyDep1 already has a registered implementation of MyDep:

public interface IMyDep1 {}
public interface IMyDep2 {}

public class MyDep : IMyDep1, IMyDep2 {}

services.TryAddEnumerable(ServiceDescriptor.Singleton<IMyDep1, MyDep>());
services.TryAddEnumerable(ServiceDescriptor.Singleton<IMyDep2, MyDep>());
// Two registrations of MyDep for IMyDep1 is avoided by the following line:
services.TryAddEnumerable(ServiceDescriptor.Singleton<IMyDep1, MyDep>());

Constructor injection behavior

Services can be resolved by two mechanisms:

  • xref:System.IServiceProvider
  • xref:Microsoft.Extensions.DependencyInjection.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, 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 exists. 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 are usually added to the service container using the scoped lifetime because web app database operations are normally scoped to the client request. The default lifetime is scoped if a lifetime isn't specified by an AddDbContext<TContext> overload when registering the database context. Services of a given lifetime shouldn't use a database context with a shorter lifetime than the service.

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:

[!code-csharp]

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

[!code-csharp]

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.

  • When transient services are created when requested from the container, the OperationId of the IOperationTransient service is different than the OperationId of the OperationService. OperationService receives a new instance of the IOperationTransient class. The new instance yields a different OperationId.
  • When scoped services are created per client request, the OperationId of the IOperationScoped service is the same as that of OperationService within a client request. Across client requests, both services share a different OperationId value.
  • When singleton and singleton-instance services are created once and used across all client requests and all services, the OperationId is constant across all service requests.

[!code-csharp]

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

[!code-csharp]

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:

[!code-csharp]

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

Call services from main

Create an xref:Microsoft.Extensions.DependencyInjection.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 xref:Microsoft.Extensions.DependencyInjection.ServiceCollectionContainerBuilderExtensions.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 xref:fundamentals/host/web-host#scope-validation.

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.

[!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.
  • Avoid direct instantiation of dependent classes within services. Direct instantiation couples the code to a particular implementation.
  • Make app classes 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 xref:System.IDisposable.Dispose* for the xref:System.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());
}

Default service container replacement

The built-in service container is meant to serve the needs of the framework and most consumer apps. We recommend using the built-in container unless you need a specific feature that it doesn't support. Some of the features supported in 3rd party containers not found in the built-in container:

  • Property injection
  • Injection based on name
  • Child containers
  • Custom lifetime management
  • Func<T> support for lazy initialization

See the Dependency Injection readme.md file for a list of some of the containers that support adapters.

The following sample replaces the built-in container with Autofac:

  • Install the appropriate container package(s):

  • 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.

  • 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

Create thread-safe singleton services. If a singleton service has a dependency on a transient service, the transient service may also require thread safety 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

  • async/await and Task based service resolution is not supported. C# does not support asynchronous constructors; therefore, the recommended pattern is to use asynchronous methods after synchronously resolving the service.

  • 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 DI.

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

  • Avoid using the service locator pattern. For example, don't invoke xref:System.IServiceProvider.GetService* to obtain a service instance when you can use DI instead:

    Incorrect:

    public class MyClass()
    {
        public void MyMethod()
        {
            var optionsMonitor = 
                _services.GetService<IOptionsMonitor<MyOptions>>();
            var option = optionsMonitor.CurrentValue.Option;
    
            ...
        }
    }

    Correct:

    public class MyClass
    {
        private readonly IOptionsMonitor<MyOptions> _optionsMonitor;
    
        public MyClass(IOptionsMonitor<MyOptions> optionsMonitor)
        {
            _optionsMonitor = optionsMonitor;
        }
    
        public void MyMethod()
        {
            var option = _optionsMonitor.CurrentValue.Option;
    
            ...
        }
    }
  • Another service locator variation to avoid is injecting a factory that resolves dependencies at runtime. Both of these practices mix Inversion of Control strategies.

  • 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.

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

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