ASP.NET Core Blazor dependency injection

By Rainer Stropek and Mike Rousos

This article explains how Blazor apps can inject services into components.

Dependency injection (DI) is a technique for accessing services configured in a central location:

• Framework-registered services can be injected directly into Razor components.
• Blazor apps define and register custom services and make them available throughout the app via DI.

Note

We recommend reading Dependency injection in ASP.NET Core before reading this topic.

Default services

The services shown in the following table are commonly used in Blazor apps.

Service	Lifetime	Description
HttpClient	Scoped	Provides methods for sending HTTP requests and receiving HTTP responses from a resource identified by a URI. Client-side, an instance of HttpClient is registered by the app in the Program file and uses the browser for handling the HTTP traffic in the background. Server-side, an HttpClient isn't configured as a service by default. In server-side code, provide an HttpClient. For more information, see Call a web API from an ASP.NET Core Blazor app. An HttpClient is registered as a scoped service, not singleton. For more information, see the Service lifetime section.
IJSRuntime	Client-side: Singleton Server-side: Scoped The Blazor framework registers IJSRuntime in the app's service container.	Represents an instance of a JavaScript runtime where JavaScript calls are dispatched. For more information, see Call JavaScript functions from .NET methods in ASP.NET Core Blazor. When seeking to inject the service into a singleton service on the server, take either of the following approaches: Change the service registration to scoped to match IJSRuntime's registration, which is appropriate if the service deals with user-specific state. Pass the IJSRuntime into the singleton service's implementation as an argument of its method calls instead of injecting it into the singleton.
NavigationManager	Client-side: Singleton Server-side: Scoped The Blazor framework registers NavigationManager in the app's service container.	Contains helpers for working with URIs and navigation state. For more information, see URI and navigation state helpers.

Additional services registered by the Blazor framework are described in the documentation where they're used to describe Blazor features, such as configuration and logging.

A custom service provider doesn't automatically provide the default services listed in the table. If you use a custom service provider and require any of the services shown in the table, add the required services to the new service provider.

Add client-side services

Configure services for the app's service collection in the Program file. In the following example, the ExampleDependency implementation is registered for IExampleDependency:

var builder = WebAssemblyHostBuilder.CreateDefault(args);
...
builder.Services.AddSingleton<IExampleDependency, ExampleDependency>();
...

await builder.Build().RunAsync();

After the host is built, services are available from the root DI scope before any components are rendered. This can be useful for running initialization logic before rendering content:

var builder = WebAssemblyHostBuilder.CreateDefault(args);
...
builder.Services.AddSingleton<WeatherService>();
...

var host = builder.Build();

var weatherService = host.Services.GetRequiredService<WeatherService>();
await weatherService.InitializeWeatherAsync();

await host.RunAsync();

The host provides a central configuration instance for the app. Building on the preceding example, the weather service's URL is passed from a default configuration source (for example, appsettings.json) to InitializeWeatherAsync:

var builder = WebAssemblyHostBuilder.CreateDefault(args);
...
builder.Services.AddSingleton<WeatherService>();
...

var host = builder.Build();

var weatherService = host.Services.GetRequiredService<WeatherService>();
await weatherService.InitializeWeatherAsync(
    host.Configuration["WeatherServiceUrl"]);

await host.RunAsync();

Add server-side services

After creating a new app, examine part of the Program file:

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddRazorComponents()
    .AddInteractiveServerComponents();

The builder variable represents a WebApplicationBuilder with an IServiceCollection, which is a list of service descriptor objects. Services are added by providing service descriptors to the service collection. The following example demonstrates the concept with the IDataAccess interface and its concrete implementation DataAccess:

builder.Services.AddSingleton<IDataAccess, DataAccess>();

Register common services

If one or more common services are required client- and server-side, you can place the common service registrations in a method client-side and call the method to register the services in both projects.

First, factor common service registrations into a separate method. For example, create a ConfigureCommonServices method client-side:

public static void ConfigureCommonServices(IServiceCollection services)
{
    services.Add...;
}

For the client-side Program file, call ConfigureCommonServices to register the common services:

var builder = WebAssemblyHostBuilder.CreateDefault(args);

...

ConfigureCommonServices(builder.Services);

In the server-side Program file, call ConfigureCommonServices to register the common services:

var builder = WebApplication.CreateBuilder(args);

...

Client.Program.ConfigureCommonServices(builder.Services);

For an example of this approach, see ASP.NET Core Blazor WebAssembly additional security scenarios.

Client-side services that fail during prerendering

This section only applies to WebAssembly components in Blazor Web Apps.

Blazor Web Apps normally prerender client-side WebAssembly components. If an app is run with a required service only registered in the .Client project, executing the app results in a runtime error similar to the following when a component attempts to use the required service during prerendering:

InvalidOperationException: Cannot provide a value for {PROPERTY} on type '{ASSEMBLY}}.Client.Pages.{COMPONENT NAME}'. There is no registered service of type '{SERVICE}'.

Use either of the following approaches to resolve this problem:

• Register the service in the main project to make it available during component prerendering.
• If prerendering isn't required for the component, disable prerendering by following the guidance in ASP.NET Core Blazor render modes. If you adopt this approach, you don't need to register the service in the main project.

For more information, see Client-side services fail to resolve during prerendering.

Service lifetime

Services can be configured with the lifetimes shown in the following table.

Lifetime	Description
Scoped	Client-side doesn't currently have a concept of DI scopes. Scoped-registered services behave like Singleton services. Server-side development supports the Scoped lifetime across HTTP requests but not across SignalR connection/circuit messages among components that are loaded on the client. The Razor Pages or MVC portion of the app treats scoped services normally and recreates the services on each HTTP request when navigating among pages or views or from a page or view to a component. Scoped services aren't reconstructed when navigating among components on the client, where the communication to the server takes place over the SignalR connection of the user's circuit, not via HTTP requests. In the following component scenarios on the client, scoped services are reconstructed because a new circuit is created for the user: The user closes the browser's window. The user opens a new window and navigates back to the app. The user closes a tab of the app in a browser window. The user opens a new tab and navigates back to the app. The user selects the browser's reload/refresh button. For more information on preserving user state in server-side apps, see ASP.NET Core Blazor state management.
Singleton	DI creates a single instance of the service. All components requiring a Singleton service receive the same instance of the service.
Transient	Whenever a component obtains an instance of a Transient service from the service container, it receives a new instance of the service.

The DI system is based on the DI system in ASP.NET Core. For more information, see Dependency injection in ASP.NET Core.

Request a service in a component

After services are added to the service collection, inject one or more services into components with the @inject Razor directive, which has two parameters:

• Type: The type of the service to inject.
• Property: The name of the property receiving the injected app service. The property doesn't require manual creation. The compiler creates the property.

For more information, see Dependency injection into views in ASP.NET Core.

Use multiple @inject statements to inject different services.

The following example demonstrates shows how to use the @inject directive. The service implementing Services.NavigationManager is injected into the component's property Navigation. Note how the code is only using the NavigationManager abstraction.

PropertyInjection.razor:

@page "/property-injection"
@inject NavigationManager Navigation

<button @onclick="@(() => Navigation.NavigateTo("/counter"))">
    Take me to the Counter component
</button>

Internally, the generated property (Navigation) uses the [Inject] attribute. Typically, this attribute isn't used directly. If a base class is required for components and injected properties are also required for the base class, manually add the [Inject] attribute:

using Microsoft.AspNetCore.Components;

public class ComponentBase : IComponent
{
    [Inject]
    protected NavigationManager Navigation { get; set; } = default!;

    ...
}

Note

Since injected services are expected to be available, the default literal with the null-forgiving operator (default!) is assigned in .NET 6 or later. For more information, see Nullable reference types (NRTs) and .NET compiler null-state static analysis.

In components derived from a base class, the @inject directive isn't required. The InjectAttribute of the base class is sufficient. The component only requires the @inherits directive. In the following example, any injected services of CustomComponentBase are available to the Demo component:

@page "/demo"
@inherits CustomComponentBase

Use DI in services

Complex services might require additional services. In the following example, DataAccess requires the HttpClient default service. @inject (or the [Inject] attribute) isn't available for use in services. Constructor injection must be used instead. Required services are added by adding parameters to the service's constructor. When DI creates the service, it recognizes the services it requires in the constructor and provides them accordingly. In the following example, the constructor receives an HttpClient via DI. HttpClient is a default service.

using System.Net.Http;

public class DataAccess : IDataAccess
{
    public DataAccess(HttpClient http)
    {
        ...
    }

    ...
}

Constructor injection is supported with primary constructors in C# 12 (.NET 8) or later:

using System.Net.Http;

public class DataAccess(HttpClient http) : IDataAccess
{
    ...
}

Prerequisites for constructor injection:

• One constructor must exist whose arguments can all be fulfilled by DI. Additional parameters not covered by DI are allowed if they specify default values.
• The applicable constructor must be public.
• One applicable constructor must exist. In case of an ambiguity, DI throws an exception.

Inject keyed services into components

Blazor supports injecting keyed services using the [Inject] attribute. Keys allow for scoping of registration and consumption of services when using dependency injection. Use the InjectAttribute.Key property to specify the key for the service to inject:

[Inject(Key = "my-service")]
public IMyService MyService { get; set; }

Utility base component classes to manage a DI scope

In non-Blazor ASP.NET Core apps, scoped and transient services are typically scoped to the current request. After the request completes, scoped and transient services are disposed by the DI system.

In interactive server-side Blazor apps, the DI scope lasts for the duration of the circuit (the SignalR connection between the client and server), which can result in scoped and disposable transient services living much longer than the lifetime of a single component. Therefore, don't directly inject a scoped service into a component if you intend the service lifetime to match the lifetime of the component. Transient services injected into a component that don't implement IDisposable are garbage collected when the component is disposed. However, injected transient services that implement IDisposable are maintained by the DI container for the lifetime of the circuit, which prevents service garbage collection when the component is disposed and results in a memory leak. An alternative approach for scoped services based on the OwningComponentBase type is described later in this section, and disposable transient services shouldn't be used at all. For more information, see Design for solving transient disposables on Blazor Server (dotnet/aspnetcore #26676).

Even in client-side Blazor apps that don't operate over a circuit, services registered with a scoped lifetime are treated as singletons, so they live longer than scoped services in typical ASP.NET Core apps. Client-side disposable transient services also live longer than the components where they're injected because the DI container, which holds references to disposable services, persists for the lifetime of the app, preventing garbage collection on the services. Although long-lived disposable transient services are of greater concern on the server, they should be avoided as client service registrations as well. Use of the OwningComponentBase type is also recommended for client-side scoped services to control service lifetime, and disposable transient services shouldn't be used at all.

An approach that limits a service lifetime is use of the OwningComponentBase type. OwningComponentBase is an abstract type derived from ComponentBase that creates a DI scope corresponding to the lifetime of the component. Using this scope, a component can inject services with a scoped lifetime and have them live as long as the component. When the component is destroyed, services from the component's scoped service provider are disposed as well. This can be useful for services reused within a component but not shared across components.

Two versions of OwningComponentBase type are available and described in the next two sections:

• OwningComponentBase
• OwningComponentBase<TService>

OwningComponentBase

OwningComponentBase is an abstract, disposable child of the ComponentBase type with a protected ScopedServices property of type IServiceProvider. The provider can be used to resolve services that are scoped to the lifetime of the component.

DI services injected into the component using @inject or the [Inject] attribute aren't created in the component's scope. To use the component's scope, services must be resolved using ScopedServices with either GetRequiredService or GetService. Any services resolved using the ScopedServices provider have their dependencies provided in the component's scope.

The following example demonstrates the difference between injecting a scoped service directly and resolving a service using ScopedServices on the server. The following interface and implementation for a time travel class include a DT property to hold a DateTime value. The implementation calls DateTime.Now to set DT when the TimeTravel class is instantiated.

ITimeTravel.cs:

public interface ITimeTravel
{
    public DateTime DT { get; set; }
}

TimeTravel.cs:

public class TimeTravel : ITimeTravel
{
    public DateTime DT { get; set; } = DateTime.Now;
}

The service is registered as scoped in the server-side Program file. Server-side, scoped services have a lifetime equal to the duration of the circuit.

In the Program file:

builder.Services.AddScoped<ITimeTravel, TimeTravel>();

In the following TimeTravel component:

• The time travel service is directly injected with @inject as TimeTravel1.
• The service is also resolved separately with ScopedServices and GetRequiredService as TimeTravel2.

TimeTravel.razor:

@page "/time-travel"
@inject ITimeTravel TimeTravel1
@inherits OwningComponentBase

<h1><code>OwningComponentBase</code> Example</h1>

<ul>
    <li>TimeTravel1.DT: @TimeTravel1?.DT</li>
    <li>TimeTravel2.DT: @TimeTravel2?.DT</li>
</ul>

@code {
    private ITimeTravel TimeTravel2 { get; set; } = default!;

    protected override void OnInitialized()
    {
        TimeTravel2 = ScopedServices.GetRequiredService<ITimeTravel>();
    }
}

Initially navigating to the TimeTravel component, the time travel service is instantiated twice when the component loads, and TimeTravel1 and TimeTravel2 have the same initial value:

TimeTravel1.DT: 8/31/2022 2:54:45 PM
TimeTravel2.DT: 8/31/2022 2:54:45 PM

When navigating away from the TimeTravel component to another component and back to the TimeTravel component:

• TimeTravel1 is provided the same service instance that was created when the component first loaded, so the value of DT remains the same.
• TimeTravel2 obtains a new ITimeTravel service instance in TimeTravel2 with a new DT value.

TimeTravel1.DT: 8/31/2022 2:54:45 PM
TimeTravel2.DT: 8/31/2022 2:54:48 PM

TimeTravel1 is tied to the user's circuit, which remains intact and isn't disposed until the underlying circuit is deconstructed. For example, the service is disposed if the circuit is disconnected for the disconnected circuit retention period.

In spite of the scoped service registration in the Program file and the longevity of the user's circuit, TimeTravel2 receives a new ITimeTravel service instance each time the component is initialized.

OwningComponentBase<TService>

OwningComponentBase<TService> derives from OwningComponentBase and adds a Service property that returns an instance of T from the scoped DI provider. This type is a convenient way to access scoped services without using an instance of IServiceProvider when there's one primary service the app requires from the DI container using the component's scope. The ScopedServices property is available, so the app can get services of other types, if necessary.

@page "/users"
@attribute [Authorize]
@inherits OwningComponentBase<AppDbContext>

<h1>Users (@Service.Users.Count())</h1>

<ul>
    @foreach (var user in Service.Users)
    {
        <li>@user.UserName</li>
    }
</ul>

Detect client-side transient disposables

Custom code can be added to a client-side Blazor app to detect disposable transient services in an app that should use OwningComponentBase. This approach is useful if you're concerned that code added to the app in the future consumes one or more transient disposable services, including services added by libraries. Demonstration code is available in the Blazor samples GitHub repository (how to download).

Inspect the following in .NET 6 or later versions of the BlazorSample_WebAssembly sample:

• DetectIncorrectUsagesOfTransientDisposables.cs
• Services/TransientDisposableService.cs
• In Program.cs:
  • The app's Services namespace is provided at the top of the file (using BlazorSample.Services;).
  • DetectIncorrectUsageOfTransients is called immediately after the builder is assigned from WebAssemblyHostBuilder.CreateDefault.
  • The TransientDisposableService is registered (builder.Services.AddTransient<TransientDisposableService>();).
  • EnableTransientDisposableDetection is called on the built host in the processing pipeline of the app (host.EnableTransientDisposableDetection();).
• The app registers the TransientDisposableService service without throwing an exception. However, attempting to resolve the service in TransientService.razor throws an InvalidOperationException when the framework attempts to construct an instance of TransientDisposableService.

Detect server-side transient disposables

Custom code can be added to a server-side Blazor app to detect server-side disposable transient services in an app that should use OwningComponentBase. This approach is useful if you're concerned that code added to the app in the future consumes one or more transient disposable services, including services added by libraries. Demonstration code is available in the Blazor samples GitHub repository (how to download).

Inspect the following in .NET 8 or later versions of the BlazorSample_BlazorWebApp sample:

• DetectIncorrectUsagesOfTransientDisposables.cs
• Services/TransitiveTransientDisposableDependency.cs:
• In Program.cs:
  • The app's Services namespace is provided at the top of the file (using BlazorSample.Services;).
  • DetectIncorrectUsageOfTransients is called on the host builder (builder.DetectIncorrectUsageOfTransients();).
  • The TransientDependency service is registered (builder.Services.AddTransient<TransientDependency>();).
  • The TransitiveTransientDisposableDependency is registered for ITransitiveTransientDisposableDependency (builder.Services.AddTransient<ITransitiveTransientDisposableDependency, TransitiveTransientDisposableDependency>();).
• The app registers the TransientDependency service without throwing an exception. However, attempting to resolve the service in TransientService.razor throws an InvalidOperationException when the framework attempts to construct an instance of TransientDependency.

Transient service registrations for IHttpClientFactory/HttpClient handlers

Transient service registrations for IHttpClientFactory/HttpClient handlers are recommended. If the app contains IHttpClientFactory/HttpClient handlers and uses the IRemoteAuthenticationBuilder<TRemoteAuthenticationState,TAccount> to add support for authentication, the following transient disposables for client-side authentication are also discovered, which is expected and can be ignored:

• BaseAddressAuthorizationMessageHandler
• AuthorizationMessageHandler

Other instances of IHttpClientFactory/HttpClient are also discovered. These instances can also be ignored.

The Blazor sample apps in the Blazor samples GitHub repository (how to download) demonstrate the code to detect transient disposables. However, the code is deactivated because the sample apps include IHttpClientFactory/HttpClient handlers.

To activate the demonstration code and witness its operation:

• Uncomment the transient disposable lines in Program.cs.

• Remove the conditional check in NavLink.razor that prevents the TransientService component from displaying in the app's navigation sidebar:

  - else if (name != "TransientService")
  + else
  

• Run the sample app and navigate to the TransientService component at /transient-service.

Use of an Entity Framework Core (EF Core) DbContext from DI

For more information, see ASP.NET Core Blazor with Entity Framework Core (EF Core).

Access server-side Blazor services from a different DI scope

Circuit activity handlers provide an approach for accessing scoped Blazor services from other non-Blazor dependency injection (DI) scopes, such as scopes created using IHttpClientFactory.

Prior to the release of ASP.NET Core in .NET 8, accessing circuit-scoped services from other dependency injection scopes required using a custom base component type. With circuit activity handlers, a custom base component type isn't required, as the following example demonstrates:

public class CircuitServicesAccessor
{
    static readonly AsyncLocal<IServiceProvider> blazorServices = new();

    public IServiceProvider? Services
    {
        get => blazorServices.Value;
        set => blazorServices.Value = value;
    }
}

public class ServicesAccessorCircuitHandler(
    IServiceProvider services, CircuitServicesAccessor servicesAccessor) 
    : CircuitHandler
{
    public override Func<CircuitInboundActivityContext, Task> CreateInboundActivityHandler(
        Func<CircuitInboundActivityContext, Task> next) => 
            async context =>
            {
                servicesAccessor.Services = services;
                await next(context);
                servicesAccessor.Services = null;
            };
}

public static class CircuitServicesServiceCollectionExtensions
{
    public static IServiceCollection AddCircuitServicesAccessor(
        this IServiceCollection services)
    {
        services.AddScoped<CircuitServicesAccessor>();
        services.AddScoped<CircuitHandler, ServicesAccessorCircuitHandler>();

        return services;
    }
}

Access the circuit-scoped services by injecting the CircuitServicesAccessor where it's needed.

For an example that shows how to access the AuthenticationStateProvider from a DelegatingHandler set up using IHttpClientFactory, see Server-side ASP.NET Core Blazor additional security scenarios.

Additional resources

• Service injection via a top-level imports file (_Imports.razor) in Blazor Web Apps
• Dependency injection in ASP.NET Core
• IDisposable guidance for Transient and shared instances
• Dependency injection into views in ASP.NET Core
• Primary constructors (C# Guide)