bogus-data

Overview

Bogus is the canonical .NET test-data generator, ported from faker.js's spirit but typed for the C#-first ecosystem (bogus-readme). It uses typed Faker<T> builders with fluent .RuleFor calls per property - the .NET equivalent of combining Faker's value generation with a factory library's referential integrity.

When to use

• The project is .NET (C# / F# / VB.NET).
• Tests need typed fixtures matching domain entities.
• The team prefers fluent / strongly-typed APIs over factory-as-DSL (FactoryBot-style).
• xUnit / NUnit / MSTest integration is required.

Install

Install-Package Bogus

(Per bogus-readme; via NuGet.)

For .NET CLI:

dotnet add package Bogus

Authoring

Typed faker builder

using Bogus;

public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public DateTime CreatedAt { get; set; }
}

var faker = new Faker<User>()
    .RuleFor(u => u.Id,        f => f.IndexFaker)
    .RuleFor(u => u.Name,      f => f.Name.FullName())
    .RuleFor(u => u.Email,     f => f.Internet.Email())
    .RuleFor(u => u.CreatedAt, f => f.Date.Past());

(Adapted from bogus-readme.)

f is a Faker instance (lowercase) exposing the same data-set modules as the JS / Python ports: f.Name, f.Internet, f.Date, f.Commerce, f.Address, f.Lorem, f.Random, etc.

f.IndexFaker gives a sequential integer per generation - useful for IDs that must be unique within a test.

Strict mode (recommended)

var faker = new Faker<User>()
    .StrictMode(true)
    .RuleFor(u => u.Id,        f => f.IndexFaker)
    .RuleFor(u => u.Name,      f => f.Name.FullName())
    .RuleFor(u => u.Email,     f => f.Internet.Email())
    .RuleFor(u => u.CreatedAt, f => f.Date.Past());

.StrictMode(true) causes Bogus to fail at runtime if any property of T lacks a RuleFor. Recommended - it prevents silent fixture drift when a model adds a new property and the factory isn't updated.

Computed and conditional rules

var faker = new Faker<User>()
    .RuleFor(u => u.FirstName, f => f.Name.FirstName())
    .RuleFor(u => u.LastName,  f => f.Name.LastName())
    .RuleFor(u => u.FullName,  (f, u) => $"{u.FirstName} {u.LastName}")    // computed from already-set fields
    .RuleFor(u => u.IsActive,  f => f.Random.Bool(0.9f));                   // 90% active

The second-argument-form of RuleFor ((f, u) => ...) lets a rule reference already-generated properties on the same instance - useful for derived fields.

Generation

Per bogus-readme:

var oneUser   = faker.Generate();           // single
var hundred   = faker.Generate(100);         // list of 100
var lazyTen   = faker.GenerateLazy(10);      // IEnumerable<User>; deferred

Use GenerateLazy when seeding a database with thousands of rows - it streams instead of materializing.

Seeding

var faker = new Faker<Order>()
    .UseSeed(1338)
    .RuleFor(o => o.Item, f => f.Commerce.Product());

(Per bogus-readme.)

UseSeed(int) makes generation reproducible across runs - same seed produces same data. Mirror the faker-data seeding guidance: seed in test setup so failures reproduce.

For globally seeding the underlying randomizer (affecting any non-Faker<T> direct calls):

Randomizer.Seed = new Random(42);

Locale support

var faker = new Faker<User>("de")
    .RuleFor(u => u.Name, f => f.Name.FullName());

The locale code in the constructor switches data sets. Bogus ships 30+ locales; supported codes match Faker's conventions (en, de, ja, ko, ru, pt_BR, etc.).

Test framework integration

xUnit

public class UserTests
{
    private readonly Faker<User> _userFaker = new Faker<User>("en")
        .UseSeed(42)
        .StrictMode(true)
        .RuleFor(u => u.Name,  f => f.Name.FullName())
        .RuleFor(u => u.Email, f => f.Internet.Email());

    [Fact]
    public void User_HasName()
    {
        var user = _userFaker.Generate();
        Assert.NotEmpty(user.Name);
    }
}

NUnit and MSTest wire identically - Bogus is framework-agnostic.

Anti-patterns

Limitations

• .NET-only. For other languages, use the matching skill: faker-data (Python/JS/Ruby), mimesis-data (Python), factory-bot-data (Ruby).
• PRNG sequence varies across major versions. Pin the package version for deterministic tests.
• No native factory orchestration for graphs. Composing a User with three Orders requires explicit chaining; there's no FactoryBot-style after(:create) block.

References

• bogus-readme - canonical: install, Faker<T>, RuleFor, Generate / GenerateBetween / GenerateLazy, UseSeed.
• faker-data, mimesis-data - Python alternatives.
• factory-bot-data - Ruby alternative.
• synthetic-data-tool-selector - dispatcher.