What is dependency injection in ASP.NET Core?

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**Dependency Injection (DI)** — technique for achieving loose coupling. **Built-in container in ASP.NET Core:** ```csharp // Program.cs var builder = WebApplication.CreateBuilder(args); // Register services builder.Services.AddScoped(); builder.Services.AddSingleton(); builder.Services.AddTransient(); var app = builder.Build(); ``` **Lifetime:** - **Singleton** — one instance for entire app lifetime. - **Scoped** — one instance per HTTP request. - **Transient** — new instance every time. **Constructor injection:** ```csharp public class OrderService { private readonly IOrderRepository _repo; public OrderService(IOrderRepository repo) { _repo = repo; } } ``` **Benefits:** - Loose coupling. - Easier testing (mock dependencies). - Single Responsibility Principle.

C#: What is dependency injection in ASP.NET Core?