Visitor Pattern in C# – Real-Time Example (Adding Operations Without Modifying Objects)

What is Visitor Pattern?

The Visitor Pattern is a behavioral design pattern that allows you to add new operations to existing object structures without modifying those classes.

👉 Separate:

• Object structure
• Operations/behaviors

---

Why Use Visitor Pattern?

• Add new functionality easily
• Avoid modifying existing classes
• Follow Open/Closed Principle
• Separate business logic from objects
• Improve maintainability

---

Real-Time Scenario

Invoice System:

Need multiple operations:

• PDF Export
• Excel Export
• Tax Calculation
• Report Generation

👉 Instead of modifying Invoice class repeatedly, use Visitors.

---

Real-Time Example – Invoice Export System

---

Step 1: Visitor Interface

public interface IVisitor
{
    void Visit(Invoice invoice);
}

---

Step 2: Element Interface

public interface IElement
{
    void Accept(IVisitor visitor);
}

---

Step 3: Concrete Element

public class Invoice : IElement
{
    public decimal Amount { get; set; }

    public void Accept(IVisitor visitor)
    {
        visitor.Visit(this);
    }
}

---

Step 4: Concrete Visitors

PDF Export Visitor

public class PdfExportVisitor : IVisitor
{
    public void Visit(Invoice invoice)
    {
        Console.WriteLine(
            $"Exporting Invoice {invoice.Amount} to PDF"
        );
    }
}

---

Excel Export Visitor

public class ExcelExportVisitor : IVisitor
{
    public void Visit(Invoice invoice)
    {
        Console.WriteLine(
            $"Exporting Invoice {invoice.Amount} to Excel"
        );
    }
}

---

Usage Example

var invoice = new Invoice
{
    Amount = 5000
};

invoice.Accept(new PdfExportVisitor());

invoice.Accept(new ExcelExportVisitor());

---

Output

Exporting Invoice 5000 to PDF
Exporting Invoice 5000 to Excel

---

Key Concept

Instead of:

invoice.ExportPdf();
invoice.ExportExcel();
invoice.CalculateTax();

We do:

invoice.Accept(visitor);

👉 Operations added externally.

---

Diagram Understanding

Client
   ↓
Visitor
   ↓
Element (Invoice)

👉 Visitor performs operation on object.

---

Advantages

• ✔ Easy to add new operations
• ✔ Follows Open/Closed Principle
• ✔ Separates logic from objects
• ✔ Cleaner business logic organization
• ✔ Good for complex object structures

---

Disadvantages

• ✖ Difficult to add new element types
• ✖ Increased complexity
• ✖ More interfaces/classes required

---

When to Use

Use Visitor Pattern when:

• Operations change frequently
• Object structure remains stable
• Multiple operations needed on same objects
• Logic should remain separate from entities

---

Real Project Mapping (.NET + Angular)

Feature	Usage
PDF export	Visitor
Excel export	Visitor
Tax calculations	Visitor
Analytics/reporting	Visitor
Validation engines	Visitor

---

ASP.NET Core Real Example

Export Service

Invoice
   ↓
PdfVisitor
ExcelVisitor
CsvVisitor

👉 Multiple operations on same entity.

---

Advanced Example – File System

Files/Folders
   ↓
VirusScanVisitor
BackupVisitor
CompressionVisitor

👉 Add new operations without changing file classes.

---

Visitor vs Strategy

Visitor	Strategy
Adds operations to objects	Changes behavior/algorithm
External operation logic	Encapsulated algorithm
Works on object structures	Runtime behavior switching

---

Pro Tip

Visitor Pattern works well with:

• Export systems
• Reporting engines
• Analytics
• Validation frameworks
• File processing systems

---

Summary

Visitor Pattern helps you:

• Add new operations easily
• Keep entities clean
• Separate business logic from objects

👉 Perfect for:

• Export systems
• Reporting
• Analytics
• Validation engines