Why should you care?

ShopNest UI widgets (cart, coupon, totals) thrash if every control events every other control.

See it live — copy this example

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

public interface ICheckoutMediator
{
    void CouponChanged(string code);
    void NotifyTotals(decimal total);
}
public sealed class CheckoutMediator : ICheckoutMediator
{
    public CartPanel? Cart { get; set; }
    public TotalsPanel? Totals { get; set; }
    public void CouponChanged(string code)
    {
        var total = code == "SAVE10" ? 4500m : 4999m;
        Totals?.Show(total);
    }
    public void NotifyTotals(decimal total) => Totals?.Show(total);
}
public sealed class TotalsPanel { public void Show(decimal t) => Console.WriteLine($"total={t}"); }
public sealed class CartPanel
{
    private readonly ICheckoutMediator _m;
    public CartPanel(ICheckoutMediator m) => _m = m;
    public void ApplyCoupon(string c) => _m.CouponChanged(c);
}

var med = new CheckoutMediator();
med.Totals = new TotalsPanel();
new CartPanel(med).ApplyCoupon("SAVE10");

Run Example »

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

What happened?

• CartPanel never references TotalsPanel.
• Mediator updates totals.
• MediatR is a related in-process messaging style for application commands.

Practice next

1. Apply coupon and see totals update.
2. Add ShippingPanel notified via mediator.
3. Avoid mediator becoming a god object — split by feature.
4. Clear coupon and reset total.
5. Log every mediator call.

Remember

Hub for colleague chatter. Reduces many-to-many links. Keep mediators focused.

ShopNest checkout widgets

Cart/coupon/totals coordinate via mediator.

Outcome: Widgets stay independently testable.