csharp-concurrency-patterns

SKILL.md

.NET Concurrency: Choosing the Right Tool

When to Use This Skill

Use this skill when:

Deciding how to handle concurrent operations in .NET

Evaluating whether to use async/await, Channels, Akka.NET, or other abstractions

Tempted to use locks, semaphores, or other synchronization primitives

Need to process streams of data with backpressure, batching, or debouncing

Managing state across multiple concurrent entities

Reference Files

advanced-concurrency.md: Akka.NET Streams, Reactive Extensions, Akka.NET Actors (entity-per-actor, state machines, cluster sharding), and async local function patterns

The Philosophy

Start simple, escalate only when needed.
Most concurrency problems can be solved with async/await. Only reach for more sophisticated tools when you have a specific need that async/await can't address cleanly.

Try to avoid shared mutable state. The best way to handle concurrency is to design it away. Immutable data, message passing, and isolated state (like actors) eliminate entire categories of bugs.

Locks should be the exception, not the rule. When you can't avoid shared mutable state:

First choice: Redesign to avoid it (immutability, message passing, actor isolation)

Second choice: Use System.Collections.Concurrent (ConcurrentDictionary, etc.)

Third choice: Use Channel<T> to serialize access through message passing

Last resort: Use lock for simple, short-lived critical sections

Decision Tree

What are you trying to do?
│
├─► Wait for I/O (HTTP, database, file)?
│   └─► Use async/await
│
├─► Process a collection in parallel (CPU-bound)?
│   └─► Use Parallel.ForEachAsync
│
├─► Producer/consumer pattern (work queue)?
│   └─► Use System.Threading.Channels
│
├─► UI event handling (debounce, throttle, combine)?
│   └─► Use Reactive Extensions (Rx)
│
├─► Server-side stream processing (backpressure, batching)?
│   └─► Use Akka.NET Streams
│
├─► State machines with complex transitions?
│   └─► Use Akka.NET Actors (Become pattern)
│
├─► Manage state for many independent entities?
│   └─► Use Akka.NET Actors (entity-per-actor)
│
├─► Coordinate multiple async operations?
│   └─► Use Task.WhenAll / Task.WhenAny
│
└─► None of the above fits?
    └─► Ask yourself: "Do I really need shared mutable state?"
        ├─► Yes → Consider redesigning to avoid it
        └─► Truly unavoidable → Use Channels or Actors to serialize access

Level 1: async/await (Default Choice)

Use for: I/O-bound operations, non-blocking waits, most everyday concurrency.

// Simple async I/O
public async Task<Order> GetOrderAsync(string orderId, CancellationToken ct)
{
    var order = await _database.GetAsync(orderId, ct);
    var customer = await _customerService.GetAsync(order.CustomerId, ct);
    return order with { Customer = customer };
}

// Parallel async operations (when independent)
public async Task<Dashboard> LoadDashboardAsync(string userId, CancellationToken ct)
{
    var ordersTask = _orderService.GetRecentOrdersAsync(userId, ct);
    var notificationsTask = _notificationService.GetUnreadAsync(userId, ct);
    var statsTask = _statsService.GetUserStatsAsync(userId, ct);

    await Task.WhenAll(ordersTask, notificationsTask, statsTask);

    return new Dashboard(
        Orders: await ordersTask,
        Notifications: await notificationsTask,
        Stats: await statsTask);
}

Key principles: Always accept CancellationToken. Use ConfigureAwait(false) in library code. Don't block on async code.

Level 2: Parallel.ForEachAsync (CPU-Bound Parallelism)

Use for: Processing collections in parallel when work is CPU-bound or you need controlled concurrency.

public async Task ProcessOrdersAsync(
    IEnumerable<Order> orders,
    CancellationToken ct)
{
    await Parallel.ForEachAsync(
        orders,
        new ParallelOptions
        {
            MaxDegreeOfParallelism = Environment.ProcessorCount,
            CancellationToken = ct
        },
        async (order, token) =>
        {
            await ProcessOrderAsync(order, token);
        });
}

When NOT to use: Pure I/O operations, when order matters, when you need backpressure.

Level 3: System.Threading.Channels (Producer/Consumer)

Use for: Work queues, producer/consumer patterns, decoupling producers from consumers.

public class OrderProcessor
{
    private readonly Channel<Order> _channel;

    public OrderProcessor()
    {
        _channel = Channel.CreateBounded<Order>(new BoundedChannelOptions(100)
        {
            FullMode = BoundedChannelFullMode.Wait
        });
    }

    // Producer
    public async Task EnqueueOrderAsync(Order order, CancellationToken ct)
    {
        await _channel.Writer.WriteAsync(order, ct);
    }

    // Consumer (run as background task)
    public async Task ProcessOrdersAsync(CancellationToken ct)
    {
        await foreach (var order in _channel.Reader.ReadAllAsync(ct))
        {
            await ProcessOrderAsync(order, ct);
        }
    }

    public void Complete() => _channel.Writer.Complete();
}

Channels are good for: Decoupling speed, buffering with backpressure, fan-out to workers, background queues.

Channels are NOT good for: Complex stream operations (batching, windowing), stateful per-entity processing, sophisticated supervision.

Level 4+: Akka.NET Streams, Reactive Extensions, Actors

For advanced scenarios requiring stream processing, UI event composition, or stateful entity management, see advanced-concurrency.md.

Akka.NET Streams excel at server-side batching, throttling, and backpressure. Reactive Extensions are ideal for UI event composition. Akka.NET Actors handle entity-per-actor patterns, state machines with Become(), and distributed systems via Cluster Sharding.

Anti-Patterns: What to Avoid

Locks for Business Logic

// BAD: Using locks to protect shared state
private readonly object _lock = new();
private Dictionary<string, Order> _orders = new();

public void UpdateOrder(string id, Action<Order> update)
{
    lock (_lock) { if (_orders.TryGetValue(id, out var order)) update(order); }
}

// GOOD: Use an actor or Channel to serialize access

Manual Thread Management

// BAD: Creating threads manually
var thread = new Thread(() => ProcessOrders());
thread.Start();

// GOOD: Use Task.Run or better abstractions
_ = Task.Run(() => ProcessOrdersAsync(cancellationToken));

Blocking in Async Code

// BAD: Blocking on async - deadlock risk!
var result = GetDataAsync().Result;

// GOOD: Async all the way
var result = await GetDataAsync();

Shared Mutable State Without Protection

// BAD: Multiple tasks mutating shared state
var results = new List<Result>();
await Parallel.ForEachAsync(items, async (item, ct) =>
{
    var result = await ProcessAsync(item, ct);
    results.Add(result); // Race condition!
});

// GOOD: Use ConcurrentBag
var results = new ConcurrentBag<Result>();

Quick Reference: Which Tool When?

Need
Tool
Example

Wait for I/O
async/await
HTTP calls, database queries

Parallel CPU work
Parallel.ForEachAsync
Image processing, calculations

Work queue
Channel<T>
Background job processing

UI events with debounce/throttle
Reactive Extensions
Search-as-you-type, auto-save

Server-side batching/throttling
Akka.NET Streams
Event aggregation, rate limiting

State machines
Akka.NET Actors
Payment flows, order lifecycles

Entity state management
Akka.NET Actors
Order management, user sessions

Fire multiple async ops
Task.WhenAll
Loading dashboard data

Race multiple async ops
Task.WhenAny
Timeout with fallback

Periodic work
PeriodicTimer
Health checks, polling

The Escalation Path

async/await (start here)
    │
    ├─► Need parallelism? → Parallel.ForEachAsync
    │
    ├─► Need producer/consumer? → Channel<T>
    │
    ├─► Need UI event composition? → Reactive Extensions
    │
    ├─► Need server-side stream processing? → Akka.NET Streams
    │
    └─► Need state machines or entity management? → Akka.NET Actors

Only escalate when you have a concrete need. Don't reach for actors or streams "just in case".