Mastering Asynchronous Programming with C# async/await - Part 6: Advanced Topics

Part 6: Advanced Topics

Now that we’ve mastered the basics and patterns, let’s go further into advanced async scenarios you’ll meet when optimizing throughput or designing libraries.

ValueTask vs Task

Task is a reference type allocated on the heap. In ultra‑hot paths where many calls complete synchronously, ValueTask can save allocations.

public async ValueTask<int> GetNumberAsync(bool cached)
{
    if (cached) return 42;        // completes synchronously (no allocation)
    await Task.Delay(1000);       // completes asynchronously
    return 42;
}

Guidelines

• Prefer Task by default (simpler, safer).
• Consider ValueTask when profiling shows allocation pressure and many sync completions.
• Don’t await a ValueTask more than once and don’t store it; convert to Task via .AsTask() if you must pass it around.

Custom Awaiters (rare, but educational)

Anything with a GetAwaiter() that returns an awaiter implementing INotifyCompletion (or ICriticalNotifyCompletion) is awaitable.

public sealed class DelayAwaitable
{
    private readonly int _ms;
    public DelayAwaitable(int ms) => _ms = ms;
    public TaskAwaiter GetAwaiter() => Task.Delay(_ms).GetAwaiter();
}

// usage
await new DelayAwaitable(250);
Console.WriteLine("custom awaiter resumed");

When would you do this?

• Domain‑specific scheduling/timing semantics.
• Interop layers.
  Most apps never need it; understanding it helps you reason about await.

Async Coordination Primitives

Limit concurrency with SemaphoreSlim

var gate = new SemaphoreSlim(3); // allow 3 concurrent ops

async Task ProcessAsync(string item)
{
    await gate.WaitAsync();
    try
    {
        await DoWorkAsync(item);
    }
    finally
    {
        gate.Release();
    }
}

Use this to throttle IO‑heavy fan‑out work (APIs, disk, DB) and avoid overloading dependencies.

Pipelines with Channel<T>

System.Threading.Channels enables high‑throughput producer/consumer pipelines.

var channel = Channel.CreateUnbounded<string>();

// producer
_ = Task.Run(async () =>
{
    foreach (var url in urls)
        await channel.Writer.WriteAsync(url);
    channel.Writer.Complete();
});

// consumer(s)
await foreach (var url in channel.Reader.ReadAllAsync())
{
    var html = await client.GetStringAsync(url);
    Console.WriteLine($"{url} -> {html.Length} bytes");
}

Channels support backpressure (bounded channels), multiple consumers, and graceful completion.

Performance Tuning Checklist

• Avoid unnecessary Task.Run in ASP.NET Core — IO work should be awaited; Task.Run is for CPU‑bound work that you explicitly want off the request thread.
• Batch awaits with Task.WhenAll to reduce continuation overhead when independent operations can run together.
• Use ConfigureAwait(false) in libraries/background services to skip context capture. (In ASP.NET Core there’s no synchronization context, but it still avoids overhead.)
• Minimize layers of trivial async wrappers; if you just return a task, you may not need async/await at that layer.
• Pool and reuse expensive objects correctly (e.g., reuse HttpClient via IHttpClientFactory).
• Measure first with a profiler (allocs, context switches, contention) before micro‑optimizing.

Debuggable, Defensive Async

• Prefer cancellable APIs: accept a CancellationToken.
• Use timeouts where appropriate to avoid stuck requests.
• Add structured logging around awaits (operation name, correlation IDs).
• In fire‑and‑forget scenarios, capture and log exceptions inside the launched task.

Key Takeaways

• ValueTask can reduce allocations in hot paths; default to Task otherwise.
• Custom awaiters are a niche but deepen understanding of await.
• SemaphoreSlim and Channel<T> are your go‑to tools for concurrency control and pipelines.
• Tune with evidence: profile, batch awaits, and avoid unnecessary context capture.

👉 In Part 7, we’ll wrap up with testing and debugging async code: async unit tests, mocking, Visual Studio’s async tools, and tracing async flows.

Series Navigation

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