Factory Method Pattern
Overview
Export jobs share a workflow, but CSV and JSON jobs need different writers. Repeating the whole workflow merely to change its writer makes maintenance harder.
Factory Method lets a subclass override the method that creates a product. The base workflow uses the product through its common interface.
UML
ExportJob.Run() obtains an IWriter through Create(). CsvJob and JsonJob override Create() while sharing the base Run() implementation.
History
GoF cataloged Factory Method as a creational pattern in Design Patterns in 1994. It delegates a concrete creation choice to subclasses.1
Why use it?
A new writer can reuse the workflow. The creation method identifies where the concrete product is chosen.
A static function that returns an object is often called a factory function. The specific mechanism here is overriding a creation method through inheritance; an already-injected writer may make this hierarchy unnecessary.
Example
CsvJob creates the CSV writer, and the shared ExportJob.Run() uses it.
using System;
using System.Collections.Generic;
using System.Linq;
Console.WriteLine(new CsvJob().Run());
interface IWriter { string Write(string value); }
sealed class CsvWriter : IWriter
{
public string Write(string value) => "\"" + value.Replace("\"", "\"\"") + "\"";
}
sealed class JsonWriter : IWriter
{
public string Write(string value) => System.Text.Json.JsonSerializer.Serialize(new { value });
}
abstract class ExportJob
{
protected abstract IWriter Create();
public string Run() => Create().Write("hello");
}
sealed class CsvJob : ExportJob
{
protected override IWriter Create() => new CsvWriter();
}
sealed class JsonJob : ExportJob
{
protected override IWriter Create() => new JsonWriter();
}
"hello"Run() never constructs CsvWriter directly. Choosing a job implementation is separate from the shared workflow.
An extended example
Add a JSON job and run both through ExportJob. The JSON serializer handles string escaping rather than assembling JSON by concatenation.
using System;
using System.Collections.Generic;
using System.Linq;
foreach (ExportJob job in new ExportJob[] { new CsvJob(), new JsonJob() })
Console.WriteLine(job.Run());
interface IWriter { string Write(string value); }
sealed class CsvWriter : IWriter
{
public string Write(string value) => "\"" + value.Replace("\"", "\"\"") + "\"";
}
sealed class JsonWriter : IWriter
{
public string Write(string value) => System.Text.Json.JsonSerializer.Serialize(new { value });
}
abstract class ExportJob
{
protected abstract IWriter Create();
public string Run() => Create().Write("hello");
}
sealed class CsvJob : ExportJob
{
protected override IWriter Create() => new CsvWriter();
}
sealed class JsonJob : ExportJob
{
protected override IWriter Create() => new JsonWriter();
}
"hello"
{"value":"hello"}Run() stays the same while Create() returns different products. A Factory Method can also be one of the customizable steps inside a Template Method.
Related reading
Each example is a separate .NET 10 console program.
Footnotes
- Gamma, Helm, Johnson, Vlissides. Design Patterns (1994). Publisher record. Creational patterns, Factory Method. β©