ASP.NET Core Blazor dependency injection

By Rainer Stropek

Blazor supports dependency injection (DI). Apps can use built-in services by injecting them into components. Apps can also define and register custom services and make them available throughout the app via DI.

DI is a technique for accessing services configured in a central location. This can be useful in Blazor apps to:

  • Share a single instance of a service class across many components, known as a singleton service.
  • Decouple components from concrete service classes by using reference abstractions. For example, consider an interface IDataAccess for accessing data in the app. The interface is implemented by a concrete DataAccess class and registered as a service in the app's service container. When a component uses DI to receive an IDataAccess implementation, the component isn't coupled to the concrete type. The implementation can be swapped, perhaps for a mock implementation in unit tests.

Default services

Default services are automatically added to the app's service collection.

Service Lifetime Description
HttpClient Singleton Provides methods for sending HTTP requests and receiving HTTP responses from a resource identified by a URI. Note that this instance of HttpClient uses the browser for handling the HTTP traffic in the background. HttpClient.BaseAddress is automatically set to the base URI prefix of the app. For more information, see Call a web API from ASP.NET Core Blazor.
IJSRuntime Singleton Represents an instance of a JavaScript runtime where JavaScript calls are dispatched. For more information, see ASP.NET Core Blazor JavaScript interop.
IUriHelper Singleton Contains helpers for working with URIs and navigation state. For more information, see URI and navigation state helpers.

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 services to an app

After creating a new app, examine the Startup.ConfigureServices method:

public void ConfigureServices(IServiceCollection services)
{
    // Add custom services here
}

The ConfigureServices method is passed an IServiceCollection, which is a list of service descriptor objects (ServiceDescriptor). 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:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<IDataAccess, DataAccess>();
}

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

Lifetime Description
Scoped Blazor client-side doesn't currently have a concept of DI scopes. Scoped-registered services behave like Singleton services. However, the server-side hosting model supports the Scoped lifetime. In a Razor component, a scoped service registration is scoped to the connection. For this reason, using scoped services is preferred for services that should be scoped to the current user, even if the current intent is to run client-side in the browser.
Singleton DI creates a single instance of the service. All components requiring a Singleton service receive an instance of the same 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 the services into the components using the @inject Razor directive. @inject 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 shows how to use @inject. The service implementing Services.IDataAccess is injected into the component's property DataRepository. Note how the code is only using the IDataAccess abstraction:

@page "/customer-list"
@using Services
@inject IDataAccess DataRepository

@if (Customers != null)
{
    <ul>
        @foreach (var customer in Customers)
        {
            <li>@customer.FirstName @customer.LastName</li>
        }
    </ul>
}

@code {
    private IReadOnlyList<Customer> Customers;

    protected override async Task OnInitAsync()
    {
        // The property DataRepository received an implementation
        // of IDataAccess through dependency injection. Use 
        // DataRepository to obtain data from the server.
        Customers = await DataRepository.GetAllCustomersAsync();
    }
}

Internally, the generated property (DataRepository) is decorated with the InjectAttribute 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 InjectAttribute:

public class ComponentBase : IComponent
{
    // DI works even if using the InjectAttribute in a component's base class.
    [Inject]
    protected IDataAccess DataRepository { get; set; }
    ...
}

In components derived from the base class, the @inject directive isn't required. The InjectAttribute of the base class is sufficient:

@page "/demo"
@inherits ComponentBase

<h1>Demo Component</h1>

Use DI in services

Complex services might require additional services. In the prior example, DataAccess might require the HttpClient default service. @inject (or the InjectAttribute) 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.

public class DataAccess : IDataAccess
{
    // The constructor receives an HttpClient via dependency
    // injection. HttpClient is a default service.
    public DataAccess(HttpClient client)
    {
        ...
    }
}

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.

Additional resources