What are keyed services in dependency injection?

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**Keyed services (.NET 8)** — register multiple implementations of an interface with keys. **Registration:** ```csharp builder.Services.AddKeyedSingleton("redis"); builder.Services.AddKeyedSingleton("memory"); builder.Services.AddKeyedScoped("sendgrid"); builder.Services.AddKeyedScoped("smtp"); ``` **Injection:** ```csharp public class OrderService( [FromKeyedServices("redis")] ICacheProvider cache) { } // Manual resolution var cache = provider.GetKeyedService("redis"); var required = provider.GetRequiredKeyedService("redis"); ``` **Attribute injection:** ```csharp [ApiController] [Route("api/[controller]")] public class NotificationsController( [FromKeyedServices("email")] INotifier email, [FromKeyedServices("sms")] INotifier sms) : ControllerBase { } ``` **Use cases:** - Multiple payment gateways. - Different storage providers. - Strategy pattern implementation.

C#: What are keyed services in dependency injection?