C# Test Data Generator: Bogus Faker With StrictMode and xUnit

Call StrictMode(true) before any RuleFor. A forgotten property then throws a ValidationException at generate time. Validate() returns a bool for unit assertions; AssertConfigurationIsValid() throws on the same gaps when you want an exception rather than a bool. Use Ignore for intentionally unconfigured properties (Bogus docs, retrieved 2026-08-01).

Add new RuleFor rules last. Inserting a rule mid-chain reshuffles the internal random stream for every property that follows, so the same UseSeed produces a different CustomerName sequence. The sample below asserts that before/after difference.

Verified against Bogus 35.6.5 docs retrieved 2026-08-01. Copy into your test tree and adjust domain vocabulary as needed.

// NuGet: Bogus 35.6.5 (nuget.org, retrieved 2025-10-25); xUnit [Fact]
using System.Collections.Generic;
using Bogus;
using Xunit;

public enum Channel { Web, Ios, PartnerApi }

public class Subscription
{
    public int Id { get; set; }
    public string CustomerName { get; set; }
    public Channel Channel { get; set; }
    public int MonthlySeats { get; set; }
}

public class SubscriptionFakerTests
{
    // StrictMode(true) before any RuleFor: unfinished rules fail loudly.
    private static Faker<Subscription> Builder() =>
        new Faker<Subscription>()
            .StrictMode(true)
            .RuleFor(s => s.Id, f => f.IndexFaker)
            .RuleFor(s => s.CustomerName, f => f.Name.LastName())
            .RuleFor(s => s.Channel, f => f.PickRandom<Channel>())
            .RuleFor(s => s.MonthlySeats, f => f.Random.Number(1, 250));

    [Fact]
    public void SameLocalSeedProducesTheSameSubscriptions()
    {
        List<Subscription> first = Builder().UseSeed(4242).Generate(20);
        List<Subscription> second = Builder().UseSeed(4242).Generate(20);

        Assert.Equal(
            first.ConvertAll(s => s.CustomerName),
            second.ConvertAll(s => s.CustomerName));
    }

    [Fact]
    public void EveryPropertyHasARule()
    {
        // Validate() returns bool; AssertConfigurationIsValid() throws on gaps.
        Assert.True(Builder().Validate());
        Builder().AssertConfigurationIsValid();
    }

    [Fact]
    public void InsertingARuleMidChainReshufflesDownstreamValues()
    {
        var threeRules = new Faker<Subscription>()
            .StrictMode(true)
            .RuleFor(s => s.Id, f => f.IndexFaker)
            .RuleFor(s => s.CustomerName, f => f.Name.LastName())
            .RuleFor(s => s.Channel, f => f.PickRandom<Channel>())
            .RuleFor(s => s.MonthlySeats, f => f.Random.Number(1, 250))
            .UseSeed(4242)
            .Generate(5);

        // New RuleFor inserted before CustomerName: same seed, different names.
        var fourRulesInserted = new Faker<Subscription>()
            .StrictMode(true)
            .RuleFor(s => s.Id, f => f.IndexFaker)
            .RuleFor(s => s.MonthlySeats, f => f.Random.Number(1, 250)) // inserted mid-chain
            .RuleFor(s => s.CustomerName, f => f.Name.LastName())
            .RuleFor(s => s.Channel, f => f.PickRandom<Channel>())
            .UseSeed(4242)
            .Generate(5);

        Assert.NotEqual(
            threeRules.ConvertAll(s => s.CustomerName),
            fourRulesInserted.ConvertAll(s => s.CustomerName));
    }
}