Use dependency injection to manage dependencies and improve testability in .NET Core and C# applications. Dependency injection is a design pattern that allows objects to receive their dependencies from an external source rather than creating them themselves. This makes it easier to manage dependencies and improves the testability of the application.

In .NET Core, dependency injection is built into the framework and can be used to inject dependencies into controllers, services, and other components. By using dependency injection, you can easily swap out dependencies for mock objects during testing, making it easier to write unit tests for your application.

For example, consider a controller that depends on a service to retrieve data from a database. Without dependency injection, the controller would need to create an instance of the service itself, making it difficult to test. With dependency injection, the service can be injected into the controller, making it easier to swap out the real service for a mock service during testing.

// Without dependency injection
public class MyController : Controller
{
    private readonly MyService _service;

    public MyController()
    {
        _service = new MyService();
    }

    public IActionResult Index()
    {
        var data = _service.GetData();
        return View(data);
    }
}

// With dependency injection
public class MyController : Controller
{
    private readonly IMyService _service;

    public MyController(IMyService service)
    {
        _service = service;
    }

    public IActionResult Index()
    {
        var data = _service.GetData();
        return View(data);
    }
}