Why should you care?

ShopNest pricing reports over a tree of bundles/items need new operations without editing every product type monthly.

See it live — copy this example

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

public interface ICatalogVisitor { void Visit(SkuItem item); void Visit(Bundle bundle); }
public interface ICatalogNode { void Accept(ICatalogVisitor v); }
public sealed class SkuItem : ICatalogNode
{
    public string Sku { get; }
    public decimal Price { get; }
    public SkuItem(string sku, decimal price) { Sku = sku; Price = price; }
    public void Accept(ICatalogVisitor v) => v.Visit(this);
}
public sealed class Bundle : ICatalogNode
{
    public List<ICatalogNode> Kids { get; } = new();
    public void Accept(ICatalogVisitor v) { v.Visit(this); foreach (var k in Kids) k.Accept(v); }
}
public sealed class TotalVisitor : ICatalogVisitor
{
    public decimal Total { get; private set; }
    public void Visit(SkuItem item) => Total += item.Price;
    public void Visit(Bundle bundle) { }
}

var kit = new Bundle();
kit.Kids.Add(new SkuItem("HD-100", 4999));
kit.Kids.Add(new SkuItem("CASE", 299));
var v = new TotalVisitor();
kit.Accept(v);
Console.WriteLine(v.Total);

Run Example »

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

What happened?

• Accept calls the right Visit overload.
• TotalVisitor sums SKUs.
• New reports become new visitors if the element set is stable.

Practice next

1. Sum the sample bundle.
2. Add CountVisitor.
3. Know Visitor hurts when element types change often.
4. DiscountVisitor subtracting 10%.
5. Print SKUs with a PrintVisitor.

Remember

Externalize operations. Stable element hierarchy. New ops = new visitors.

ShopNest bundle pricing report

Finance runs TotalVisitor over promo bundles.

Outcome: New report types without editing SkuItem.