.NET dependency injection

Artikkel
07/18/2024

.NET supports the dependency injection (DI) software design pattern, which is a technique for achieving Inversion of Control (IoC) between classes and their dependencies. Dependency injection in .NET is a built-in part of the framework, along with configuration, logging, and the options pattern.

A dependency is an object that another object depends on. Examine the following MessageWriter class with a Write method that other classes depend on:

public class MessageWriter
{
    public void Write(string message)
    {
        Console.WriteLine($"MessageWriter.Write(message: \"{message}\")");
    }
}

A class can create an instance of the MessageWriter class to make use of its Write method. In the following example, the MessageWriter class is a dependency of the Worker class:

public class Worker : BackgroundService
{
    private readonly MessageWriter _messageWriter = new();

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            _messageWriter.Write($"Worker running at: {DateTimeOffset.Now}");
            await Task.Delay(1_000, stoppingToken);
        }
    }
}

The class creates and directly depends on the MessageWriter class. Hard-coded dependencies, such as in the previous example, are problematic and should be avoided for the following reasons:

To replace MessageWriter with a different implementation, the Worker class must be modified.
If MessageWriter has dependencies, they must also be configured by the Worker class. In a large project with multiple classes depending on MessageWriter, the configuration code becomes scattered across the app.
This implementation is difficult to unit test. The app should use a mock or stub MessageWriter 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. .NET provides a built-in service container, IServiceProvider. Services are typically registered at the app's start-up and appended to an IServiceCollection. Once all services are added, you use BuildServiceProvider to create the service container.
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.

As an example, the IMessageWriter interface defines the Write method:

namespace DependencyInjection.Example;

public interface IMessageWriter
{
    void Write(string message);
}

This interface is implemented by a concrete type, MessageWriter:

namespace DependencyInjection.Example;

public class MessageWriter : IMessageWriter
{
    public void Write(string message)
    {
        Console.WriteLine($"MessageWriter.Write(message: \"{message}\")");
    }
}

The sample code registers the IMessageWriter service with the concrete type MessageWriter. The AddSingleton method registers the service with a singleton lifetime, the lifetime of the app. Service lifetimes are described later in this article.

using DependencyInjection.Example;

HostApplicationBuilder builder = Host.CreateApplicationBuilder(args);

builder.Services.AddHostedService<Worker>();
builder.Services.AddSingleton<IMessageWriter, MessageWriter>();

using IHost host = builder.Build();

host.Run();

In the preceding code, the sample app:

Creates a host app builder instance.
Configures the services by registering:
- The Worker as a hosted service. For more information, see Worker Services in .NET.
- The IMessageWriter interface as a singleton service with a corresponding implementation of the MessageWriter class.
Builds the host and runs it.

The host contains the dependency injection service provider. It also contains all the other relevant services required to automatically instantiate the Worker and provide the corresponding IMessageWriter implementation as an argument.

namespace DependencyInjection.Example;

public sealed class Worker(IMessageWriter messageWriter) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            messageWriter.Write($"Worker running at: {DateTimeOffset.Now}");
            await Task.Delay(1_000, stoppingToken);
        }
    }
}

By using the DI pattern, the worker service:

Doesn't use the concrete type MessageWriter, only the IMessageWriter interface that implements it. That makes it easy to change the implementation that the worker service uses without modifying the worker service.
Doesn't create an instance of MessageWriter. The instance is created by the DI container.

The implementation of the IMessageWriter interface can be improved by using the built-in logging API:

namespace DependencyInjection.Example;

public class LoggingMessageWriter(
    ILogger<LoggingMessageWriter> logger) : IMessageWriter
{
    public void Write(string message) =>
        logger.LogInformation("Info: {Msg}", message);
}

The updated AddSingleton method registers the new IMessageWriter implementation:

builder.Services.AddSingleton<IMessageWriter, LoggingMessageWriter>();

The HostApplicationBuilder (builder) type is part of the Microsoft.Extensions.Hosting NuGet package.

LoggingMessageWriter depends on ILogger<TCategoryName>, which it requests in the constructor. ILogger<TCategoryName> is a framework-provided service.

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.

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

With dependency injection terminology, a service:

Is typically an object that provides a service to other objects, such as the IMessageWriter service.
Is not related to a web service, although the service may use a web service.

The framework provides a robust logging system. The IMessageWriter implementations shown in the preceding examples were written to demonstrate basic DI, not to implement logging. Most apps shouldn't need to write loggers. The following code demonstrates using the default logging, which only requires the Worker to be registered as a hosted service AddHostedService:

public sealed class Worker(ILogger<Worker> logger) : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            logger.LogInformation("Worker running at: {time}", DateTimeOffset.Now);

            await Task.Delay(1_000, stoppingToken);
        }
    }
}

Using the preceding code, there is no need to update Program.cs, because logging is provided by the framework.

Multiple constructor discovery rules

When a type defines more than one constructor, the service provider has logic for determining which constructor to use. The constructor with the most parameters where the types are DI-resolvable is selected. Consider the following C# example service:

public class ExampleService
{
    public ExampleService()
    {
    }

    public ExampleService(ILogger<ExampleService> logger)
    {
        // omitted for brevity
    }

    public ExampleService(FooService fooService, BarService barService)
    {
        // omitted for brevity
    }
}

In the preceding code, assume that logging has been added and is resolvable from the service provider but the FooService and BarService types are not. The constructor with the ILogger<ExampleService> parameter is used to resolve the ExampleService instance. Even though there's a constructor that defines more parameters, the FooService and BarService types are not DI-resolvable.

If there's ambiguity when discovering constructors, an exception is thrown. Consider the following C# example service:

public class ExampleService
{
    public ExampleService()
    {
    }

    public ExampleService(ILogger<ExampleService> logger)
    {
        // omitted for brevity
    }

    public ExampleService(IOptions<ExampleOptions> options)
    {
        // omitted for brevity
    }
}

Warning

The ExampleService code with ambiguous DI-resolvable type parameters would throw an exception. Do not do this—it's intended to show what is meant by "ambiguous DI-resolvable types".

In the preceding example, there are three constructors. The first constructor is parameterless and requires no services from the service provider. Assume that both logging and options have been added to the DI container and are DI-resolvable services. When the DI container attempts to resolve the ExampleService type, it will throw an exception, as the two constructors are ambiguous.

You can avoid ambiguity by defining a constructor that accepts both DI-resolvable types instead:

public class ExampleService
{
    public ExampleService()
    {
    }

    public ExampleService(
        ILogger<ExampleService> logger,
        IOptions<ExampleOptions> options)
    {
        // omitted for brevity
    }
}

Register groups of services with extension methods

Microsoft Extensions uses a convention for registering a group of related services. The convention is to use a single Add{GROUP_NAME} extension method to register all of the services required by a framework feature. For example, the AddOptions extension method registers all of the services required for using options.

Framework-provided services

When using any of the available host or app builder patterns, defaults are applied and services are registered by the framework. Consider some of the most popular host and app builder patterns:

After creating a builder from any of these APIs, the IServiceCollection has services defined by the framework, depending on how the host was configured. For apps based on the .NET templates, the framework could register hundreds of services.

The following table lists a small sample of these framework-registered services:

Service Type	Lifetime
Microsoft.Extensions.DependencyInjection.IServiceScopeFactory	Singleton
IHostApplicationLifetime	Singleton
Microsoft.Extensions.Logging.ILogger<TCategoryName>	Singleton
Microsoft.Extensions.Logging.ILoggerFactory	Singleton
Microsoft.Extensions.ObjectPool.ObjectPoolProvider	Singleton
Microsoft.Extensions.Options.IConfigureOptions<TOptions>	Transient
Microsoft.Extensions.Options.IOptions<TOptions>	Singleton
System.Diagnostics.DiagnosticListener	Singleton
System.Diagnostics.DiagnosticSource	Singleton

Service lifetimes

Services can be registered with one of the following lifetimes:

Transient
Scoped
Singleton

The following sections describe each of the preceding lifetimes. Choose an appropriate lifetime for each registered service.

Transient

Transient lifetime services are created each time they're requested from the service container. To register a service as transient, call AddTransient.

In apps that process requests, transient services are disposed at the end of the request. This lifetime incurs per/request allocations, as services are resolved and constructed every time. For more information, see Dependency Injection Guidelines: IDisposable guidance for transient and shared instances.

Scoped

For web applications, a scoped lifetime indicates that services are created once per client request (connection). Register scoped services with AddScoped.

In apps that process requests, scoped services are disposed at the end of the request.

When using Entity Framework Core, the AddDbContext extension method registers DbContext types with a scoped lifetime by default.

Note

Do not resolve a scoped service from a singleton and be careful not to do so indirectly, for example, through a transient service. It may cause the service to have incorrect state when processing subsequent requests. It's fine to:

Resolve a singleton service from a scoped or transient service.
Resolve a scoped service from another scoped or transient service.

By default, in the development environment, resolving a service from another service with a longer lifetime throws an exception. For more information, see Scope validation.

Singleton

Singleton lifetime services are created either:

The first time they're requested.
By the developer, when providing an implementation instance directly to the container. This approach is rarely needed.

Every subsequent request of the service implementation from the dependency injection container uses the same instance. If the app requires singleton behavior, allow the service container to manage the service's lifetime. Don't implement the singleton design pattern and provide code to dispose of the singleton. Services should never be disposed by code that resolved the service from the container. If a type or factory is registered as a singleton, the container disposes the singleton automatically.

Register singleton services with AddSingleton. Singleton services must be thread safe and are often used in stateless services.

In apps that process requests, singleton services are disposed when the ServiceProvider is disposed on application shutdown. Because memory is not released until the app is shut down, consider memory use with a singleton service.

Service registration methods

The framework provides service registration extension methods that are useful in specific scenarios:

Method	Automatic object disposal	Multiple implementations	Pass args
`Add{LIFETIME}<{SERVICE}, {IMPLEMENTATION}>()` Example: `services.AddSingleton<IMyDep, MyDep>();`	Yes	Yes	No
`Add{LIFETIME}<{SERVICE}>(sp => new {IMPLEMENTATION})` Examples: `services.AddSingleton<IMyDep>(sp => new MyDep());` `services.AddSingleton<IMyDep>(sp => new MyDep(99));`	Yes	Yes	Yes
`Add{LIFETIME}<{IMPLEMENTATION}>()` Example: `services.AddSingleton<MyDep>();`	Yes	No	No
`AddSingleton<{SERVICE}>(new {IMPLEMENTATION})` Examples: `services.AddSingleton<IMyDep>(new MyDep());` `services.AddSingleton<IMyDep>(new MyDep(99));`	No	Yes	Yes
`AddSingleton(new {IMPLEMENTATION})` Examples: `services.AddSingleton(new MyDep());` `services.AddSingleton(new MyDep(99));`	No	No	Yes

For more information on type disposal, see the Disposal of services section.

Registering a service with only an implementation type is equivalent to registering that service with the same implementation and service type. For example, consider the following code:

services.AddSingleton<ExampleService>();

This is equivalent to registering the service with both the service and implementation of the same types:

services.AddSingleton<ExampleService, ExampleService>();

This equivalency is why multiple implementations of a service can't be registered using the methods that don't take an explicit service type. These methods can register multiple instances of a service, but they will all have the same implementation type.

Any of the above service registration methods can be used to register multiple service instances of the same service type. In the following example, AddSingleton is called twice with IMessageWriter as the service type. The second call to AddSingleton overrides the previous one when resolved as IMessageWriter and adds to the previous one when multiple services are resolved via IEnumerable<IMessageWriter>. Services appear in the order they were registered when resolved via IEnumerable<{SERVICE}>.

using ConsoleDI.IEnumerableExample;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

HostApplicationBuilder builder = Host.CreateApplicationBuilder(args);

builder.Services.AddSingleton<IMessageWriter, ConsoleMessageWriter>();
builder.Services.AddSingleton<IMessageWriter, LoggingMessageWriter>();
builder.Services.AddSingleton<ExampleService>();

using IHost host = builder.Build();

_ = host.Services.GetService<ExampleService>();

await host.RunAsync();

The preceding sample source code registers two implementations of the IMessageWriter.

using System.Diagnostics;

namespace ConsoleDI.IEnumerableExample;

public sealed class ExampleService
{
    public ExampleService(
        IMessageWriter messageWriter,
        IEnumerable<IMessageWriter> messageWriters)
    {
        Trace.Assert(messageWriter is LoggingMessageWriter);

        var dependencyArray = messageWriters.ToArray();
        Trace.Assert(dependencyArray[0] is ConsoleMessageWriter);
        Trace.Assert(dependencyArray[1] is LoggingMessageWriter);
    }
}

The ExampleService defines two constructor parameters; a single IMessageWriter, and an IEnumerable<IMessageWriter>. The single IMessageWriter is the last implementation to have been registered, whereas the IEnumerable<IMessageWriter> represents all registered implementations.

The framework also provides TryAdd{LIFETIME} extension methods, which register the service only if there isn't already an implementation registered.

In the following example, the call to AddSingleton registers ConsoleMessageWriter as an implementation for IMessageWriter. The call to TryAddSingleton has no effect because IMessageWriter already has a registered implementation:

services.AddSingleton<IMessageWriter, ConsoleMessageWriter>();
services.TryAddSingleton<IMessageWriter, LoggingMessageWriter>();

The TryAddSingleton has no effect, as it was already added and the "try" will fail. The ExampleService would assert the following:

public class ExampleService
{
    public ExampleService(
        IMessageWriter messageWriter,
        IEnumerable<IMessageWriter> messageWriters)
    {
        Trace.Assert(messageWriter is ConsoleMessageWriter);
        Trace.Assert(messageWriters.Single() is ConsoleMessageWriter);
    }
}

For more information, see:

The TryAddEnumerable(ServiceDescriptor) methods register the service only if there isn't already an implementation of the same type. Multiple services are resolved via IEnumerable<{SERVICE}>. When registering services, add an instance if one of the same types hasn't already been added. Library authors use TryAddEnumerable to avoid registering multiple copies of an implementation in the container.

In the following example, the first call to TryAddEnumerable registers MessageWriter as an implementation for IMessageWriter1. The second call registers MessageWriter for IMessageWriter2. The third call has no effect because IMessageWriter1 already has a registered implementation of MessageWriter:

public interface IMessageWriter1 { }
public interface IMessageWriter2 { }

public class MessageWriter : IMessageWriter1, IMessageWriter2
{
}

services.TryAddEnumerable(ServiceDescriptor.Singleton<IMessageWriter1, MessageWriter>());
services.TryAddEnumerable(ServiceDescriptor.Singleton<IMessageWriter2, MessageWriter>());
services.TryAddEnumerable(ServiceDescriptor.Singleton<IMessageWriter1, MessageWriter>());

Service registration is generally order-independent except when registering multiple implementations of the same type.

IServiceCollection is a collection of ServiceDescriptor objects. The following example shows how to register a service by creating and adding a ServiceDescriptor:

string secretKey = Configuration["SecretKey"];
var descriptor = new ServiceDescriptor(
    typeof(IMessageWriter),
    _ => new DefaultMessageWriter(secretKey),
    ServiceLifetime.Transient);

services.Add(descriptor);

The built-in Add{LIFETIME} methods use the same approach. For example, see the AddScoped source code.

Constructor injection behavior

Services can be resolved by using:

IServiceProvider
ActivatorUtilities:
- Creates objects that aren't registered in the container.
- Used with some framework features.

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.

Scope validation

When the app runs in the Development environment and calls CreateApplicationBuilder to build the host, the default service provider performs checks to verify that:

Scoped services aren't resolved from the root service provider.
Scoped services aren't injected into singletons.

The root service provider is created when BuildServiceProvider is called. The root service provider's lifetime corresponds to the app'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 the app shuts down. Validating service scopes catches these situations when BuildServiceProvider is called.

Scope scenarios

The IServiceScopeFactory is always registered as a singleton, but the IServiceProvider can vary based on the lifetime of the containing class. For example, if you resolve services from a scope, and any of those services take an IServiceProvider, it'll be a scoped instance.

To achieve scoping services within implementations of IHostedService, such as the BackgroundService, do not inject the service dependencies via constructor injection. Instead, inject IServiceScopeFactory, create a scope, then resolve dependencies from the scope to use the appropriate service lifetime.

namespace WorkerScope.Example;

public sealed class Worker(
    ILogger<Worker> logger,
    IServiceScopeFactory serviceScopeFactory)
    : BackgroundService
{
    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        while (!stoppingToken.IsCancellationRequested)
        {
            using (IServiceScope scope = serviceScopeFactory.CreateScope())
            {
                try
                {
                    logger.LogInformation(
                        "Starting scoped work, provider hash: {hash}.",
                        scope.ServiceProvider.GetHashCode());

                    var store = scope.ServiceProvider.GetRequiredService<IObjectStore>();
                    var next = await store.GetNextAsync();
                    logger.LogInformation("{next}", next);

                    var processor = scope.ServiceProvider.GetRequiredService<IObjectProcessor>();
                    await processor.ProcessAsync(next);
                    logger.LogInformation("Processing {name}.", next.Name);

                    var relay = scope.ServiceProvider.GetRequiredService<IObjectRelay>();
                    await relay.RelayAsync(next);
                    logger.LogInformation("Processed results have been relayed.");

                    var marked = await store.MarkAsync(next);
                    logger.LogInformation("Marked as processed: {next}", marked);
                }
                finally
                {
                    logger.LogInformation(
                        "Finished scoped work, provider hash: {hash}.{nl}",
                        scope.ServiceProvider.GetHashCode(), Environment.NewLine);
                }
            }
        }
    }
}

In the preceding code, while the app is running, the background service:

Depends on the IServiceScopeFactory.
Creates an IServiceScope for resolving additional services.
Resolves scoped services for consumption.
Works on processing objects and then relaying them, and finally marks them as processed.

From the sample source code, you can see how implementations of IHostedService can benefit from scoped service lifetimes.

Keyed services

Starting with .NET 8, there is support for service registrations and lookups based on a key, meaning it's possible to register multiple services with a different key, and use this key for the lookup.

For example, consider the case where you have different implementations of the interface IMessageWriter: MemoryMessageWriter and QueueMessageWriter.

You can register these services using the overload of the service registration methods (seen earlier) that supports a key as a parameter:

services.AddKeyedSingleton<IMessageWriter, MemoryMessageWriter>("memory");
services.AddKeyedSingleton<IMessageWriter, QueueMessageWriter>("queue");

The key isn't limited to string, it can be any object you want, as long as the type correctly implements Equals.

In the constructor of the class that uses IMessageWriter, you add the FromKeyedServicesAttribute to specify the key of the service to resolve:

public class ExampleService
{
    public ExampleService(
        [FromKeyedServices("queue")] IMessageWriter writer)
    {
        // Omitted for brevity...
    }
}

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