Why should you care?

ShopNest inventory changes should refresh cache listeners and search indexers without hard references everywhere.

See it live — copy this example

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

public sealed class StockSubject
{
    public event Action<string, int>? Changed;
    public void SetQty(string sku, int qty) => Changed?.Invoke(sku, qty);
}
public sealed class CacheObserver
{
    public void OnChanged(string sku, int qty) => Console.WriteLine($"cache bust {sku}={qty}");
}

var stock = new StockSubject();
var cache = new CacheObserver();
stock.Changed += cache.OnChanged;
stock.SetQty("HD-100", 3);

Run Example »

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

What happened?

• StockSubject raises Changed; CacheObserver reacts.
• C# event is the idiomatic Observer.
• Unsubscribe to avoid leaks.

Practice next

1. Raise SetQty and see cache log.
2. Add SearchObserver.
3. Unsubscribe with -= .
4. Filter only HD-* skus in observer.
5. Use EventHandler args record.

Remember

Publish state changes. Observers react. Watch lifetimes.

ShopNest stock listeners

Qty change notifies cache + search.

Outcome: PDP stops showing stale “in stock” quickly.