Why should you care?

ShopNest order validation (stock → fraud → coupon) should be a pipeline you can reorder without nested ifs.

See it live — copy this example

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

public abstract class OrderHandler
{
    protected OrderHandler? Next;
    public OrderHandler SetNext(OrderHandler n) { Next = n; return n; }
    public void Handle(OrderCtx ctx)
    {
        if (!Process(ctx)) return;
        Next?.Handle(ctx);
    }
    protected abstract bool Process(OrderCtx ctx);
}
public sealed record OrderCtx(string Sku, decimal Amount, List<string> Errors);
public sealed class StockHandler : OrderHandler
{
    protected override bool Process(OrderCtx ctx)
    {
        if (ctx.Sku == "BAD") { ctx.Errors.Add("out of stock"); return false; }
        return true;
    }
}
public sealed class FraudHandler : OrderHandler
{
    protected override bool Process(OrderCtx ctx)
    {
        if (ctx.Amount > 100000) { ctx.Errors.Add("fraud hold"); return false; }
        return true;
    }
}

var ctx = new OrderCtx("HD-100", 4999, new());
var stock = new StockHandler();
stock.SetNext(new FraudHandler());
stock.Handle(ctx);
Console.WriteLine(ctx.Errors.Count == 0 ? "ok" : string.Join(",", ctx.Errors));

Run Example »

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

What happened?

• Stock runs first; on failure it stops the chain.
• Fraud runs only if stock passes.
• ASP.NET middleware is the same idea at HTTP level.

Practice next

1. Run happy path and a BAD sku.
2. Reorder handlers.
3. Add CouponHandler.
4. Make Process async Task.
5. Collect warnings without stopping.

Remember

Pipeline of handlers. Stop or forward. Easy to extend.

ShopNest place-order checks

Stock → fraud → coupon chain before payment.

Outcome: New checks plug in as another handler.