PatternKit.Examples 0.113.22

PatternKit

Fluent Design Patterns for Modern .NET
Fluent APIs and incremental source generators for classic patterns in modern .NET.

Overview

PatternKit provides fluent runtime helpers and Roslyn incremental source generators for GoF-style design patterns in .NET. Instead of repeating handwritten pattern scaffolding, it favors:

• Fluent builders and DSLs (chainable, declarative, composable).
• Source generators that emit deterministic code at compile time.
• AOT-friendly generated code where pattern shape is known at design time.
• Strong typing with validation diagnostics from analyzers and generator tests.
• Testable, deterministic APIs that pair naturally with BDD and xUnit/NUnit/MSTest.

The goal is to make patterns clear to read, straightforward to test, and inexpensive to maintain.

Quick Start

Install via NuGet:

dotnet add package PatternKit --version <latest>

Use a pattern immediately—here’s a simple Strategy:

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(5));   // positive
Console.WriteLine(classify.Execute(-3));  // negative
Console.WriteLine(classify.Execute(0));   // zero

Or a TryStrategy for first-match-wins pipelines:

var parse = 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 (parse.Execute("123", out var n))
    Console.WriteLine(n); // 123

A forkable, lookahead ReplayableSequence (Iterator+):

using PatternKit.Behavioral.Iterator;

var seq = ReplayableSequence<int>.From(Enumerable.Range(1, 5));
var c = seq.GetCursor();

// Look ahead without consuming
var first = c.Lookahead(0).OrDefault(); // 1
var third = c.Lookahead(2).OrDefault(); // 3

// Consume immutably (returns next cursor)
if (c.TryNext(out var v1, out var c2) && c2.TryNext(out var v2, out var c3))
{
    // v1 = 1, v2 = 2; c3 now points at 3
}

// Fork speculative branch
var fork = c3.Fork();
if (fork.TryNext(out var v3, out fork) && fork.TryNext(out var v4, out fork))
{
    // fork saw 3,4 while c3 is still at 3
}

// LINQ over a cursor (non-destructive to original cursor)
var evens = c3.Where(x => x % 2 == 0).ToList(); // [2,4] using a snapshot enumeration

WindowSequence (sliding / striding windows)

using PatternKit.Behavioral.Iterator;

// Full sliding windows (size 3, stride 1)
var slides = Enumerable.Range(1,7)
    .Windows(size:3)
    .Select(w => string.Join(',', w.ToArray()))
    .ToList(); // ["1,2,3","2,3,4","3,4,5","4,5,6","5,6,7"]

// Stride 2 (skip one between window starts)
var stride = Enumerable.Range(1,9)
    .Windows(size:4, stride:2)
    .Select(w => string.Join('-', w.ToArray()))
    .ToList(); // ["1-2-3-4","3-4-5-6","5-6-7-8"]

// Include trailing partial
var partial = new[]{1,2,3,4,5}
    .Windows(size:3, stride:3, includePartial:true)
    .Select(w => (Vals: w.ToArray(), w.IsPartial))
    .ToList(); // [ ([1,2,3], false), ([4,5], true) ]

// Reuse buffer (zero alloc per full window) – copy if you persist
var reused = Enumerable.Range(1,6)
    .Windows(size:3, reuseBuffer:true)
    .Select(w => w.ToArray()) // snapshot copy each window
    .ToList();

Pattern Quick Reference

Tiny, copy‑paste friendly snippets for the most common patterns. Each builds an immutable, hot‑path friendly artifact.

ActionChain (middleware style rule pack)

using PatternKit.Behavioral.Chain;

var log = new List<string>();
var chain = ActionChain<HttpRequest>.Create()
    .When((in r) => r.Path.StartsWith("/admin") && !r.Headers.ContainsKey("Authorization"))
    .ThenStop(r => log.Add("deny"))
    .When((in r) => r.Headers.ContainsKey("X-Request-Id"))
    .ThenContinue(r => log.Add($"req={r.Headers["X-Request-Id"]}"))
    .Finally((in r, next) => { log.Add($"{r.Method} {r.Path}"); next(in r); })
    .Build();

chain.Execute(new("GET","/health", new Dictionary<string,string>()));

ResultChain (first-match value producer with fallback)

using PatternKit.Behavioral.Chain;

public readonly record struct Request(string Method, string Path);
public readonly record struct Response(int Status, string Body);

var router = ResultChain<Request, Response>.Create()
    .When(static (in r) => r.Method == "GET" && r.Path == "/health")
        .Then(r => new Response(200, "OK"))
    .When(static (in r) => r.Method == "GET" && r.Path.StartsWith("/users/"))
        .Then(r => new Response(200, $"user:{r.Path[7..]}"))
    // default / not found tail (only runs if no earlier handler produced)
    .Finally(static (in _, out Response? res, _) => { res = new Response(404, "not found"); return true; })
    .Build();

router.Execute(new Request("GET", "/health"), out var ok);   // ok.Status = 200
router.Execute(new Request("GET", "/users/alice"), out var u); // 200, Body = user:alice
router.Execute(new Request("GET", "/missing"), out var nf);    // 404, Body = not found

Strategy (first matching branch)

using PatternKit.Behavioral.Strategy;
var classify = Strategy<int,string>.Create()
    .When(i => i > 0).Then(_ => "positive")
    .When(i => i < 0).Then(_ => "negative")
    .Default(_ => "zero")
    .Build();
var result = classify.Execute(-5); // negative

TryStrategy (first success wins parsing)

var parse = TryStrategy<string,int>.Create()
    .Always((in string s, out int v) => int.TryParse(s, out v))
    .Finally((in string _, out int v) => { v = 0; return true; })
    .Build();
parse.Execute("42", out var n); // n=42

ActionStrategy (multi-fire side‑effects)

using PatternKit.Behavioral.Strategy;
var audit = new List<string>();
var strat = ActionStrategy<int>.Create()
    .When(i => i % 2 == 0).Then(i => audit.Add($"even:{i}"))
    .When(i => i > 10).Then(i => audit.Add($"big:{i}"))
    .Build();
strat.Execute(12); // adds even:12, big:12

AsyncStrategy (await external work)

var asyncStrat = AsyncStrategy<string,string>.Create()
    .When(s => s.StartsWith("http"))
    .Then(async s => await Task.FromResult("url"))
    .Default(async s => await Task.FromResult("text"))
    .Build();
var kind = await asyncStrat.Execute("http://localhost");

BranchBuilder (first-match router)

using PatternKit.Creational.Builder;

// Define delegate shapes (predicates take `in` param for struct-friendliness)
public delegate bool IntPred(in int x);
public delegate string IntHandler(in int x);

var router = BranchBuilder<IntPred, IntHandler>.Create()
    .Add(static (in int v) => v < 0,   static (in int v) => "neg")
    .Add(static (in int v) => v > 0,   static (in int v) => "pos")
    .Default(static (in int _) => "zero")
    .Build(
        fallbackDefault: static (in int _) => "zero",
        projector: static (preds, handlers, hasDefault, def) =>
        {
            // Project into a single dispatch function
            return (Func<int, string>)(x =>
            {
                for (int i = 0; i < preds.Length; i++)
                    if (preds[i](in x))
                        return handlers[i](in x);
                return def(in x);
            });
        });

var a = router(-5); // "neg"
var b = router(10); // "pos"
var c = router(0);  // "zero"

ChainBuilder (collect → project)

using PatternKit.Creational.Builder;

var log = new List<string>();
var pipeline = ChainBuilder<Action<string>>.Create()
    .Add(static s => log.Add($"A:{s}"))
    .AddIf(true, static s => log.Add($"B:{s}"))
    .Add(static s => log.Add($"C:{s}"))
    .Build(actions => (Action<string>)(msg =>
    {
        foreach (var a in actions) a(msg);
    }));

pipeline("run");
// log => ["A:run", "B:run", "C:run"]

Composer (functional state accumulation)

using PatternKit.Creational.Builder;

public readonly record struct PersonState(string? Name, int Age);
public sealed record Person(string Name, int Age);

var person = Composer<PersonState, Person>
    .New(static () => default)
    .With(static s => s with { Name = "Ada" })
    .With(static s => s with { Age = 30 })
    .Require(static s => string.IsNullOrWhiteSpace(s.Name) ? "Name required" : null)
    .Build(static s => new Person(s.Name!, s.Age));

MutableBuilder (imperative mutations + validation)

using PatternKit.Creational.Builder;

public sealed class Options 
{
    public string? Host { get; set; } 
    public int Port { get; set; }
}

var opts = MutableBuilder<Options>.New(static () => new Options())
    .With(static o => o.Host = "localhost")
    .With(static o => o.Port = 8080)
    .RequireNotEmpty(static o => o.Host, nameof(Options.Host))
    .RequireRange(static o => o.Port, 1, 65535, nameof(Options.Port))
    .Build();

Prototype (clone + mutate)

using PatternKit.Creational.Prototype;

public sealed class User { public string Role { get; set; } = "user"; public bool Active { get; set; } = true; }

// Single prototype
var proto = Prototype<User>.Create(
        source: new User { Role = "user", Active = true },
        cloner: static (in User u) => new User { Role = u.Role, Active = u.Active })
    .With(static u => u.Active = false) // default mutation applied to every clone
    .Build();

var admin = proto.Create(u => u.Role = "admin"); // clone + extra mutation

// Registry of prototypes
var registry = Prototype<string, User>.Create()
    .Map("basic", new User { Role = "user", Active = true }, static (in User u) => new User { Role = u.Role, Active = u.Active })
    .Map("admin", new User { Role = "admin", Active = true }, static (in User u) => new User { Role = u.Role, Active = u.Active })
    .Mutate("admin", static u => u.Active = true) // append mutation to admin family
    .Default(new User { Role = "guest", Active = false }, static (in User u) => new User { Role = u.Role, Active = u.Active })
    .Build();

var guest = registry.Create("missing-key"); // falls back to default (guest)

Decorator (fluent wrapping & extension)

using PatternKit.Structural.Decorator;

// Add logging to any operation
var calculator = Decorator<int, int>.Create(static x => x * x)
    .Around((x, next) => {
        Console.WriteLine($"Input: {x}");
        var result = next(x);
        Console.WriteLine($"Output: {result}");
        return result;
    })
    .Build();

var squared = calculator.Execute(7); // Logs: Input: 7, Output: 49

// Add caching
var cache = new Dictionary<int, int>();
var cachedOp = Decorator<int, int>.Create(x => ExpensiveComputation(x))
    .Around((x, next) => {
        if (cache.TryGetValue(x, out var cached))
            return cached;
        var result = next(x);
        cache[x] = result;
        return result;
    })
    .Build();

// Chain multiple decorators: validation + transformation
var validated = Decorator<int, int>.Create(static x => 100 / x)
    .Before(static x => x == 0 ? throw new ArgumentException("Cannot be zero") : x)
    .After(static (input, result) => result + input)
    .Build();

var output = validated.Execute(5); // (100 / 5) + 5 = 25

Facade (unified subsystem interface)

using PatternKit.Structural.Facade;

// Simplify complex e-commerce operations
public record OrderRequest(string ProductId, int Quantity, string CustomerEmail, decimal Price);
public record OrderResult(bool Success, string? OrderId = null, string? ErrorMessage = null);

var orderFacade = Facade<OrderRequest, OrderResult>.Create()
    .Operation("place-order", (in OrderRequest req) => {
        // Coordinate inventory, payment, shipping, notifications
        var reservationId = inventoryService.Reserve(req.ProductId, req.Quantity);
        var txId = paymentService.Charge(req.Price * req.Quantity);
        var shipmentId = shippingService.Schedule(req.CustomerEmail);
        notificationService.SendConfirmation(req.CustomerEmail);
        
        return new OrderResult(true, OrderId: Guid.NewGuid().ToString());
    })
    .Operation("cancel-order", (in OrderRequest req) => {
        inventoryService.Release(req.ProductId);
        paymentService.Refund(req.ProductId);
        return new OrderResult(true);
    })
    .Default((in OrderRequest _) => new OrderResult(false, ErrorMessage: "Unknown operation"))
    .Build();

// Simple client code - complex subsystem coordination hidden
var result = orderFacade.Execute("place-order", orderRequest);

// Case-insensitive operations
var apiFacade = Facade<string, string>.Create()
    .OperationIgnoreCase("Status", (in string _) => "System OK")
    .OperationIgnoreCase("Version", (in string _) => "v2.0")
    .Build();

var status = apiFacade.Execute("STATUS", ""); // Works with any casing

Proxy (access control & lazy initialization)

using PatternKit.Structural.Proxy;

// Virtual Proxy - lazy initialization
var dbProxy = Proxy<string, string>.Create()
    .VirtualProxy(() => {
        var db = new ExpensiveDatabase("connection-string");
        return sql => db.Query(sql);
    })
    .Build();
// Database not created until first Execute call
var result = dbProxy.Execute("SELECT * FROM Users");

// Protection Proxy - access control
var deleteProxy = Proxy<User, bool>.Create(user => DeleteUser(user))
    .ProtectionProxy(user => user.IsAdmin)
    .Build();
deleteProxy.Execute(regularUser); // Throws UnauthorizedAccessException

// Caching Proxy - memoization
var cachedCalc = Proxy<int, int>.Create(x => ExpensiveFibonacci(x))
    .CachingProxy()
    .Build();
cachedCalc.Execute(100); // Calculates
cachedCalc.Execute(100); // Returns cached result

// Logging Proxy - audit trail
var loggedOp = Proxy<Payment, bool>.Create(p => ProcessPayment(p))
    .LoggingProxy(msg => logger.Log(msg))
    .Build();

// Custom Interception - retry logic
var retryProxy = Proxy<string, string>.Create(CallUnreliableService)
    .Intercept((input, next) => {
        for (int i = 0; i < 3; i++) {
            try { return next(input); }
            catch (Exception) when (i < 2) { Thread.Sleep(1000); }
        }
        throw new Exception("Max retries exceeded");
    })
    .Build();

// Remote Proxy - combine caching + logging
var remoteProxy = Proxy<int, string>.Create(id => CallRemoteApi(id))
    .Intercept((id, next) => {
        logger.Log($"Request for ID: {id}");
        var result = next(id);
        logger.Log("Response received");
        return result;
    })
    .Build();
var cachedRemoteProxy = Proxy<int, string>.Create(id => remoteProxy.Execute(id))
    .CachingProxy()
    .Build();

Patterns Table

PatternKit currently tracks 88 production-readiness patterns. Each catalog pattern is represented in tests, documentation, real-world examples, IoC integration, and the BenchmarkDotNet coverage matrix.

Benchmark Snapshot

BenchmarkDotNet guidance is documented in docs/guides/benchmarks.md, and the full published result matrix is in docs/guides/benchmark-results.md. This snapshot was captured on Windows 11, Intel Core i9-14900K, .NET SDK 10.0.108, .NET 10.0.8, BenchmarkDotNet 0.15.8, using the current-tfm job.

Run the benchmarks on target hardware before making final route decisions:

dotnet run -c Release --framework net10.0 --project benchmarks/PatternKit.Benchmarks -- --filter *LeaderElection* --artifacts artifacts/benchmarks --join