ZeroAlloc.AsyncEvents

Zero-allocation async event handler structs for .NET. Lock-free registration, ValueTask invocation, ArrayPool parallel dispatch.

Key Characteristics

• Lock-free registration — CAS-loop register/unregister, no locks
• ValueTask throughout — no Task allocations on hot paths
• ArrayPool parallel dispatch — rented array for fan-out, returned immediately after WhenAll
• Multi-target — netstandard2.0, netstandard2.1, net8.0, net10.0
• AOT compatible

Performance

10 handlers registered, invoked once. Compared against EventHandler<T> (sync multicast, baseline) and naive async (Func<string, Task> + Task.WhenAll). BenchmarkDotNet v0.14.0, .NET 9, X64 RyuJIT AVX2.

Method	Mean	Error	StdDev	Ratio	Allocated
Sync_MulticastDelegate_10Handlers	22.96 ns	0.926 ns	2.597 ns	1.01	—
NaiveAsync_TaskWhenAll_10Handlers	203.48 ns	4.111 ns	7.307 ns	8.96x	280 B
ZeroAlloc_Parallel_10Handlers	70.10 ns	1.446 ns	4.103 ns	3.09x	136 B
ZeroAlloc_Sequential_10Handlers	16.10 ns	0.655 ns	1.857 ns	0.71x	—

ZeroAlloc sequential mode is 30% faster than a sync multicast delegate with zero allocations. ZeroAlloc parallel mode is 3× faster than naive async with 51% less memory.

Installation

dotnet add package ZeroAlloc.AsyncEvents

Quick Start

// Declare the backing field
private AsyncEventHandler<OrderPlacedArgs> _orderPlaced = new(InvokeMode.Parallel);

// Expose as a C# event — or use [AsyncEvent] to let the source generator do this
public event AsyncEvent<OrderPlacedArgs> OrderPlaced
{
    add    => _orderPlaced.Register(value);
    remove => _orderPlaced.Unregister(value);
}

// Invoke
await _orderPlaced.InvokeAsync(new OrderPlacedArgs(orderId), cancellationToken);

Source Generator

Annotate fields with [AsyncEvent] on a partial class and the generator writes the event property for you:

public partial class OrderService
{
    [AsyncEvent(InvokeMode.Parallel)]
    private AsyncEventHandler<OrderPlacedArgs> _orderPlaced = new(InvokeMode.Parallel);
}

Generates:

public event AsyncEvent<OrderPlacedArgs> OrderPlaced
{
    add    => _orderPlaced.Register(value);
    remove => _orderPlaced.Unregister(value);
}

Apply [AsyncEvent] to the class instead to cover all AsyncEventHandler<TArgs> fields at once. See Source Generator for details.

Async INotify* Interfaces

Async INotify* interfaces and event args are provided by ZeroAlloc.Notify, which builds on this package.

Design Philosophy

AsyncEventHandler<TArgs> is a struct wrapping a State reference — copy semantics are intentional and match how event fields work in C#. Use it as a private field and expose Register/Unregister methods, or use the +=/-= operators directly.

CancellationToken is threaded through every call site — handlers opt into cooperative cancellation at the delegate boundary. Sequential mode respects cancellation between handler invocations; parallel mode checks before dispatch.

See docs/ for full documentation.