Why should you care?

ShopNest services need fakes in tests and swappable payment clients without editing constructors by hand everywhere.

See it live — copy this example

Paste into a C# console or class library project and run dotnet run.

public interface IClock { DateTime UtcNow { get; } }
public sealed class SystemClock : IClock { public DateTime UtcNow => DateTime.UtcNow; }
public sealed class OrderNumberService
{
    private readonly IClock _clock;
    public OrderNumberService(IClock clock) => _clock = clock;
    public string Next() => $"SN-{_clock.UtcNow:yyyyMMdd}-1";
}

var svc = new OrderNumberService(new SystemClock());
Console.WriteLine(svc.Next());

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• OrderNumberService asks for IClock.
• Tests pass a fixed clock.
• In ASP.NET Core the container wires this automatically.

Practice next

1. Print an order number.
2. Inject a fake clock returning a fixed date.
3. Prefer constructor injection.
4. Add IIdGenerator dependency.
5. Register in a Host ApplicationBuilder.

Remember

Inject dependencies. Compose at the root. Enables testing.

ShopNest testable services

Handlers take interfaces via DI.

Outcome: Unit tests run without SQL or PSP calls.