Builder

Context: Separate the construction of a complex object from its representation so that the same construction process can create different representations.

public class Product
{
    public string PartA { get; set; }
    public string PartB { get; set; }
}

public interface IBuilder
{
    void BuildPartA();
    void BuildPartB();
    Product GetResult();
}

public class ConcreteBuilder : IBuilder
{
    private Product _product = new();
    public void BuildPartA() => _product.PartA = "A";
    public void BuildPartB() => _product.PartB = "B";
    public Product GetResult() => _product;
}

public class Director
{
    public void Construct(IBuilder builder)
    {
        builder.BuildPartA();
        builder.BuildPartB();
    }
}

// Usage
var builder = new ConcreteBuilder();
new Director().Construct(builder);
Product p = builder.GetResult();

Real-world usage example

Building SQL queries: A SqlQueryBuilder allows step‑by‑step construction of SELECT, WHERE, ORDER BY clauses, producing different query strings.

Example: In C#, StringBuilder is a classic builder for constructing strings efficiently. Entity Framework Core’s IQueryable uses fluent builder methods like .Where(), .OrderBy(), .Select() to build queries.

Builder

Real-world usage example

Related sections