What is AddKeyedTransient?

AddKeyedTransient is a dependency injection (DI) registration method in ASP.NET Core that lets you register multiple implementations of the same service type, each identified by a key. It was introduced in .NET 8 as part of the new keyed services feature.

Unlike AddTransient, which registers a single implementation (or multiple without an easy way to distinguish them), AddKeyedTransient allows you to resolve a specific implementation using a unique key such as a string, enum, or other object.

Syntax

builder.Services.AddKeyedTransient<IMessageService, EmailService>("email");
builder.Services.AddKeyedTransient<IMessageService, SmsService>("sms");

Resolving a Keyed Service

You can inject a specific implementation using the [FromKeyedServices] attribute.

public class NotificationController : ControllerBase
{
    private readonly IMessageService _messageService;

    public NotificationController(
        [FromKeyedServices("email")] IMessageService messageService)
    {
        _messageService = messageService;
    }
}

Example

Interface

public interface IMessageService
{
    void Send(string message);
}

Implementations

public class EmailService : IMessageService
{
    public void Send(string message)
    {
        Console.WriteLine($"Email: {message}");
    }
}

public class SmsService : IMessageService
{
    public void Send(string message)
    {
        Console.WriteLine($"SMS: {message}");
    }
}

Registration

builder.Services.AddKeyedTransient<IMessageService, EmailService>("email");
builder.Services.AddKeyedTransient<IMessageService, SmsService>("sms");

Injection

public class NotificationService
{
    private readonly IMessageService _emailService;
    private readonly IMessageService _smsService;

    public NotificationService(
        [FromKeyedServices("email")] IMessageService emailService,
        [FromKeyedServices("sms")] IMessageService smsService)
    {
        _emailService = emailService;
        _smsService = smsService;
    }

    public void Notify()
    {
        _emailService.Send("Welcome!");
        _smsService.Send("OTP: 123456");
    }
}

Lifetime

AddKeyedTransient creates a new instance every time the keyed service is requested, just like AddTransient.

When to Use `AddKeyedTransient`

Register multiple implementations of the same interface.
Select an implementation based on a known key.
Replace manual service factories or large switch statements.
Support scenarios like different payment gateways, notification providers, or storage services.

Comparison

Method	Lifetime	Supports Keys	Use Case
`AddTransient`	New instance every request	No	Single implementation
`AddKeyedTransient`	New instance every request	Yes	Multiple implementations with keys
`AddScoped`	One instance per request	No	Request-scoped services
`AddKeyedScoped`	One instance per request	Yes	Keyed request-scoped services
`AddSingleton`	One instance for application lifetime	No	Shared services
`AddKeyedSingleton`	One shared instance per key	Yes	Shared keyed services

interview