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Deterministic time & ids

DateTime.UtcNow and Guid.NewGuid() make a test unrepeatable — you can’t assert on a value you don’t control. The fix is a two-part discipline: production code depends on abstractions (IDateTimeProvider, IIdGenerator), and tests arrange those abstractions to fixed values through a verb.

Fixed values + two arrange verbs

Section titled “Fixed values + two arrange verbs”

Keep the fixed constants in one place, then wrap the mock setup in Clock and Ids verbs:

public static readonly DateTime FixedUtcNow = new(2026, 1, 15, 9, 30, 0, DateTimeKind.Utc);
public static readonly Guid FixedClientId = Guid.Parse("11111111-1111-1111-1111-111111111111");


public static ITestArrange Clock(this ITestArrange arrange, DateTime? utcNow = null) =>
    arrange.Then(host => host.Execute<Mock<IDateTimeProvider>>(mock =>
        mock.SetupGet(x => x.UtcNow).Returns(utcNow ?? FixedUtcNow)));


public static ITestArrange Ids(this ITestArrange arrange, params Guid[] ids)
{
    var sequence = ids.Length == 0 ? new[] { FixedClientId } : ids;
    return arrange.Then(host => host.Execute<Mock<IIdGenerator>>(mock =>
    {
        var queue = new Queue<Guid>(sequence);
        // Hand out each id in turn; repeat the last once the queue runs dry.
        mock.Setup(x => x.NewId()).Returns(() => queue.Count > 1 ? queue.Dequeue() : queue.Peek());
    }));
}

Ids(...) taking params Guid[] means a test that creates several entities can pin each id in order — .Ids(firstId, secondId) — while the common case, .Ids(), just uses the fixed default.

Using them

Section titled “Using them”

The clock and id are arranged first, so the handler runs against known values and the result is exactly assertable:

await Arrange
    .Clock(ArrangeClient.FixedUtcNow)
    .Ids(ArrangeClient.FixedClientId)
    .CreateClientCommand(out var command, WithName("Acme Corporation"));


var result = await Act.SaveClient(command);


await Inspect
    .SaveResult(result).IsSuccess(ArrangeClient.FixedClientId)   // the id we pinned
    .DbClientById(ArrangeClient.FixedClientId, out var saved, c => Assert.That(c!.CreatedAt, Is.EqualTo(FixedUtcNow)));

Because both the created-at timestamp and the id are values you chose, the assertions are exact equalities — no “roughly now”, no “some Guid”. That determinism is also what makes Verify snapshots stable across runs.

Next

Section titled “Next”