Seed extensions

Influence generated values with seeded Create, CreateMany, and Freeze overloads from AutoFixture.SeedExtensions.

The AutoFixture.SeedExtensions package adds seed-aware Create, CreateMany, and Freeze overloads. Seeds are generic (Create<T>(T seed)), not string-only — string seeds are the most common case because AutoFixture turns them into readable prefixes by default.

Prerequisites

Install

<PackageReference Include="AutoFixture.SeedExtensions" Version="5.0.0-rc.1" />

Example scenario

You are testing code that displays generated labels. Random strings are hard to read in failure output — you want values that start with a known prefix but stay anonymous. Or you customize numeric creation with FromSeed and need to pass an explicit seed into Create.

Create with a seed

var fixture = new Fixture();

var result = fixture.Create("SeedPrefix");

Assert.StartsWith("SeedPrefix", result);

CreateMany with a seed

Without a count, AutoFixture uses RepeatCount:

var results = fixture.CreateMany("Item").ToList();

Assert.Equal(fixture.RepeatCount, results.Count);
Assert.All(results, item => Assert.StartsWith("Item", item));

Pass an explicit count when you need a fixed length:

var results = fixture.CreateMany("Item", 3).ToList();

Assert.Equal(3, results.Count);
Assert.All(results, item => Assert.StartsWith("Item", item));

Freeze with a seed

Freeze(seed) creates a seeded value and freezes it so later requests for that type reuse the same instance:

var frozen = fixture.Freeze("Frozen");

var again = fixture.Create("Something else");

Assert.Equal(frozen, again);

The seed shapes the first value. After that, Freeze pins that instance — a different seed on a later Create does not create a new string. To inject a known value without seeding, use Inject instead.

Seeds beyond strings

With Customizations FromSeed, an explicit seed drives non-string types too:

fixture.Customize<int>(composer => composer.FromSeed(seed => seed * 10));

var value = fixture.Create(5);

Assert.Equal(50, value);

Bare Create<int>() still uses default(int) as the seed (0). Seeded overloads only matter when generators honor the seed — by default that is mainly strings; for other types you customize with FromSeed.

How it works

  • Create(seed) — one specimen, potentially influenced by the seed
  • CreateMany(seed) — seeded sequence of length RepeatCount
  • CreateMany(seed, count) — seeded sequence of fixed length
  • Freeze(seed) — create from seed, then reuse that instance
  • Seeds are T values; string seeds become readable prefixes by default

See also Customizations for FromSeed and Register, Freeze, and Inject for unseeded Freeze/Inject.

Next steps

API

Idioms

Overview