Builder Pattern in C# – Real-Time Example (Invoice Creation)

What is Builder Pattern?

The Builder Pattern is a creational design pattern that builds complex objects step by step.

It separates object construction from its representation.

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Why Use Builder Pattern?

• Handle complex object creation
• Avoid constructors with too many parameters
• Improve readability and maintainability
• Create different representations of same object

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Real-Time Scenario

In an Invoice System, an invoice may contain:

• Customer details
• Items
• Tax
• Discount
• Date

👉 Instead of one huge constructor, we build it step by step

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Implementation

Step 1: Product (Invoice)

public class Invoice
{
    public string Customer { get; set; }
    public decimal Amount { get; set; }
    public decimal Tax { get; set; }
    public decimal Discount { get; set; }
}

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Step 2: Builder

public class InvoiceBuilder
{
    private readonly Invoice _invoice = new();

    public InvoiceBuilder SetCustomer(string customer)
    {
        _invoice.Customer = customer;
        return this;
    }

    public InvoiceBuilder SetAmount(decimal amount)
    {
        _invoice.Amount = amount;
        return this;
    }

    public InvoiceBuilder SetTax(decimal tax)
    {
        _invoice.Tax = tax;
        return this;
    }

    public InvoiceBuilder SetDiscount(decimal discount)
    {
        _invoice.Discount = discount;
        return this;
    }

    public Invoice Build()
    {
        return _invoice;
    }
}

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Usage Example

var invoice = new InvoiceBuilder()
    .SetCustomer("Sai")
    .SetAmount(1000)
    .SetTax(180)
    .SetDiscount(50)
    .Build();

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Key Concept

Instead of:

new Invoice("Sai", 1000, 180, 50); // ❌ confusing

We do:

new InvoiceBuilder()
    .SetCustomer("Sai")
    .SetAmount(1000)
    .Build();

👉 Clear and readable

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Advantages

• Improves readability
• Handles complex objects easily
• Flexible object creation
• Avoids constructor overloading

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Disadvantages

• More code (builder class)
• Slight complexity increase
• Not needed for simple objects

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When to Use

• When object has many properties
• When object creation is complex
• When different configurations are needed
• When readability is important

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Real Project Mapping (.NET + Angular)

Feature	Usage
Invoice creation	Builder
API request models	Builder
Complex DTO creation	Builder
Configuration objects	Builder

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Pro Tip (Advanced .NET Usage)

• Combine with Fluent API style (method chaining)
• Use with Immutable objects
• Helpful in test data creation (Test Builders)

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Summary

Builder Pattern helps you:

• Build complex objects step by step
• Improve readability and maintainability
• Avoid constructor complexity

👉 Perfect for invoice systems, DTOs, complex object creation