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What is the Builder pattern?

SOLID & Design Patterns Patterns Intermediate 2–3 Yrs3–5 Yrs Architecture 1 min read

Quick Answer

Builder separates the construction of a complex object from its representation, building it step by step — usually with a fluent API: new ReportBuilder().WithTitle(...).AddSection(...).Build(). It solves the “telescoping constructor” problem (too many constructor overloads / optional parameters).

Builder is for constructing a complicated object step by step, usually through a fluent chain — .WithTitle().AddSection().Build(). It fixes the problem where a class has so many optional bits that you either have ten constructor overloads or a constructor with fifteen parameters. .NET is full of them — StringBuilder, WebApplicationBuilder, EF's ModelBuilder. I also use it a lot in tests as a test-data builder, so a test reads new OrderBuilder().WithTwoLines().Build() and isn't tied to the exact constructor. For a small object I wouldn't bother — an initializer does the job.

  • Good for objects with many optional parts, required validation at the end, or an immutable result assembled from mutable steps.
  • Familiar examples: StringBuilder, HostBuilder/WebApplicationBuilder, EF Core's ModelBuilder, LINQ query building.
  • Test data builders — new CustomerBuilder().AsVip().Build() — keep tests readable and resilient to constructor changes.
  • For a simple object, an object initializer or a record with with is enough — don't add a builder for three properties.
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