PatternKit Documentation

Fluent Design Patterns for Modern .NET
Elegant, declarative, allocation-light implementations of classic patterns—optimized for .NET 9.

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🌟 Overview

PatternKit is a library that reimagines classic Gang of Four design patterns for the modern .NET ecosystem.

Instead of heavyweight base classes and verbose boilerplate, PatternKit favors:

• Fluent builders and chainable DSLs
• Source generators to remove reflection and runtime cost
• Zero-allocation handlers with in parameters for hot paths
• Strong typing and discoverable APIs
• Readable BDD-style tests using TinyBDD

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🚀 Getting Started

Install the NuGet package:

dotnet add package PatternKit

Example: Strategy<TIn, TOut>

using PatternKit.Behavioral.Strategy;

var classify = Strategy<int, string>.Create()
    .When(i => i > 0).Then(i => "positive")
    .When(i => i < 0).Then(i => "negative")
    .Default(_ => "zero")
    .Build();

Console.WriteLine(classify.Execute(42));  // positive
Console.WriteLine(classify.Execute(-7));  // negative
Console.WriteLine(classify.Execute(0));   // zero

Example: TryStrategy<TIn, TOut>

var parser = TryStrategy<string, int>.Create()
    .Always((in string s, out int r) => int.TryParse(s, out r))
    .Finally((in string _, out int r) => { r = 0; return true; })
    .Build();

if (parser.Execute("123", out var value))
    Console.WriteLine(value); // 123

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📚 Available Patterns

PatternKit will grow to cover Creational, Structural, and Behavioral patterns with fluent, discoverable APIs:

Category	Patterns
Creational	Factory • Composer • ChainBuilder • BranchBuilder • MutableBuilder • Prototype • Singleton
Structural	Adapter • Bridge • Composite • Decorator • Facade • Flyweight • Proxy
Behavioral	Strategy • TryStrategy • ActionStrategy • ActionChain • ResultChain • Command • ReplayableSequence • WindowSequence • Mediator • Memento • Observer • AsyncObserver • Visitor • State • Template Method

Each pattern will ship with:

• Fluent API: readable and composable (.When(...), .Then(...), .Finally(...))
• Strongly-typed handlers using in parameters
• Performance focus — no hidden allocations or reflection
• XML docs and examples available in API reference

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🧪 Testing Philosophy

All patterns are tested with TinyBDD, enabling Gherkin-like, human-readable scenarios:

[Feature("Strategy")]
public class StrategyTests : TinyBddXunitBase
{
    [Scenario("Positive/negative classification")]
    [Fact]
    public async Task ClassificationWorks()
    {
        await Given("a strategy with three branches", BuildStrategy)
            .When("executing with 5", s => s.Execute(5))
            .Then("result should be 'positive'", r => r == "positive")
            .AssertPassed();
    }
}