ZeroAlloc.TestHelpers

Prove "zero allocation" in a unit test: exact per-call allocation budgets that fail CI when a hot path regresses, plus snapshot testing for source generators, shipped as source with no runtime dependency.

Why ZeroAlloc.TestHelpers

• A claim becomes a test. AllocationGate runs a call as many times as you ask and fails if it allocates more bytes per call than you allow. A BenchmarkDotNet [MemoryDiagnoser] run shows a number; this gate fails CI when the number changes.
• Exact counts. Synchronous gates count with GC.GetAllocatedBytesForCurrentThread after a forced GC and a warmup, and round the average up, so a recurring allocation never reads as 0 B.
• Steady state, not first runs. Each gate measures windows of calls until two in a row agree and keeps the least, so a one-off allocation is not counted. A window that is not yet within budget does not end the gate until tiered compilation has settled, so unoptimised tier-0 code, which can box where the optimised code does not, never fails a budget. A passing gate stops after two windows. A measurement that cannot settle within 20 s throws instead of returning a figure that may be tier-0.
• Async calls too. The Async gates await every call and count allocations on every thread, for code that really yields on I/O.
• Source-only, nothing to ship. The helpers compile into your test project as internal classes: no runtime DLL, nothing on your public API, and AllocationGate compiles into a NativeAOT app; CI publishes one with it.

Install

dotnet add package ZeroAlloc.TestHelpers

The package is a development dependency, so it stays private to the project that references it.

Quick start

using Xunit;
using ZeroAlloc.TestHelpers;

public class FormattingTests
{
    private readonly char[] _buffer = new char[16];

    [Fact]
    public void TryFormat_into_a_buffer_allocates_nothing() =>
        AllocationGate.AssertBudget(0, 1_000, () => 12_345.TryFormat(_buffer, out _), "TryFormat");

    [Fact]
    public void ToString_allocates_exactly_one_string() =>
        Assert.Equal(32, AllocationGate.MeasureBytesPerCall(1_000, () => 12_345.ToString()));
}

Put 12_345.ToString() under the zero budget and the test fails with the exact cost:

AllocationGate: ToString allocated 32000 B total over 1000 iterations (32 B/call avg, the least of 59 windows), budget is 0 B/call (0 B total).

What you get

• Budgets and measurements for Action and ValueTask<T> calls: AssertBudget, MeasureBytesPerCall and their ValueTask variants. AllocationGate
• Relative gates: AssertNoMoreThan fails when a candidate path allocates more than a baseline measured in the same run.
• Async gates with a documented method and tolerance, for calls that complete on another thread. Calls that complete asynchronously
• TextSnapshot for string snapshots, with no Roslyn reference. Snapshot testing
• GeneratorSnapshot for source-generator output, reading and writing the Verify.SourceGenerators file format, so existing *.verified.cs files keep working. It is compiled only into projects that reference Roslyn. Installation and packaging

Learn more

• Documentation: docs/
• Related packages: ZeroAlloc.Analyzers finds allocations at compile time; this package proves they are gone. The ZeroAlloc libraries gate their hot paths with it
• Contributing: issues and pull requests are welcome on GitHub
• License: MIT