Why should you care?

ShopNest IPriceService needs audit logging without editing the core pricing class.

See it live — copy this example

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

public interface IPriceService { decimal PriceOf(string sku); }
public sealed class BasicPriceService : IPriceService
{
    public decimal PriceOf(string sku) => sku == "HD-100" ? 4999m : 999m;
}
public sealed class LoggingPriceDecorator : IPriceService
{
    private readonly IPriceService _inner;
    public LoggingPriceDecorator(IPriceService inner) => _inner = inner;
    public decimal PriceOf(string sku)
    {
        var p = _inner.PriceOf(sku);
        Console.WriteLine($"price {sku}={p}");
        return p;
    }
}

IPriceService prices = new LoggingPriceDecorator(new BasicPriceService());
Console.WriteLine(prices.PriceOf("HD-100"));

Run Example »

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

What happened?

• LoggingPriceDecorator forwards to BasicPriceService then logs.
• You can stack more decorators (cache, auth) in DI.

Practice next

1. Run and see the log line.
2. Add a CachingPriceDecorator outside logging.
3. Register decorators in DI order carefully.
4. Cache results in a dictionary decorator.
5. Measure call counts with a counter decorator.

Remember

Wrap to extend behavior. Same interface throughout. Stack via DI.

ShopNest pricing decorators

Logging + cache wrap core pricing.

Outcome: Core math stays pure and testable.