IServiceCollection in ASP.NET Core

The IServiceCollection   interface is defined in the Microsoft.Extensions.DependencyInjection   namespace. It’s used within the dependency injection   mechanism for .NET to hold all registered services accessible to a service container that implements IServiceProvider   .

IServiceCollection implements both the ICollection<T>   interface, which grants it methods to manipulate generic collections, and, the IEnumerable   interface, which allows code to iterate over its contents. An instance of a class that implements the IServiceCollection interface provides its services to objects that have access to the instance. As an example, we can take a look at the Services   property of the WebApplicationBuilder   class:

public Microsoft.Extensions.DependencyInjection.IServiceCollection Services { get; }

In a default ASP.NET (v8.0) app, an instance of the WebApplicationBuilder is created in the Program.cs class:

var builder = WebApplication.CreateBuilder(args);

Services can then be added to the builder.Services collection via its numerous extension methods   .

In the example below, we add a database context service via the AddDbContext method of the EntityFrameworkServiceCollectionExtensions class  

// builder.Services is the IServiceCollection of the application builder
builder.Services.AddDbContext<TodoContext>(opt => opt.UseInMemoryDatabase("todo"));

We can then access the database context service via a controller:

[Route("api/[controller]")]
public class TodoController : ControllerBase {
    private readonly TodoContext _context;

    public TodoController(TodoContext context)
    {
        _context = context;
    }

    [HttpGet]
    public async Task<ActionResult<IEnumerable<Todo>>> GetTodos()
    {
        return await _context.Todos.ToListAsync();
    }
}