A Test Strategy for an API

# The test steps of the job from the Build and Test in CI lesson; checkout, SDK setup,
# locked restore, the Release build and the TRX upload stay exactly as shown there.
- name: Unit tests
  run: dotnet test --project tests/Shop.Domain.UnitTests -c Release --no-build --results-directory TestResults --report-xunit-trx
- name: Integration tests
  run: dotnet test --project tests/Shop.Api.IntegrationTests -c Release --no-build --results-directory TestResults --report-xunit-trx

using System.Globalization;
using Microsoft.Extensions.Time.Testing;
using Xunit;

public sealed class OrderNumbers(TimeProvider time)
{
    private int _sequence;

    // SO-20260302-0001: the date part comes from the injected clock, not DateTime.Now.
    public string Next() =>
        $"SO-{time.GetUtcNow():yyyyMMdd}-{Interlocked.Increment(ref _sequence).ToString("D4", CultureInfo.InvariantCulture)}";
}

public sealed class OrderNumberTests
{
    // Flaky version, kept as a warning:
    //   static readonly OrderNumbers Shared = new(TimeProvider.System);
    //   Assert.Equal($"SO-{DateTime.Now:yyyyMMdd}-0001", Shared.Next());
    // It fails when another test used Shared first, when the run crosses midnight,
    // and on agents whose local date differs from UTC.

    [Fact]
    public void First_number_of_the_day_is_0001()
    {
        var clock = new FakeTimeProvider(new DateTimeOffset(2026, 3, 2, 23, 59, 59, TimeSpan.Zero));
        var numbers = new OrderNumbers(clock);

        Assert.Equal("SO-20260302-0001", numbers.Next());
        clock.Advance(TimeSpan.FromSeconds(2));
        Assert.Equal("SO-20260303-0002", numbers.Next());
    }
}

using System.Text.Json;
using System.Text.Json.Nodes;
using Microsoft.AspNetCore.Mvc.Testing;
using Xunit;

public sealed class OpenApiContractTests(WebApplicationFactory<Program> factory) : IClassFixture<WebApplicationFactory<Program>>
{
    // Committed file; regenerate on purpose with UPDATE_OPENAPI_SNAPSHOT=1 dotnet test.
    private static readonly string SnapshotPath =
        Path.Combine(AppContext.BaseDirectory, "..", "..", "..", "openapi", "v1.json");

    [Fact]
    public async Task Published_contract_matches_the_committed_snapshot()
    {
        using var client = factory.CreateClient();
        var actual = JsonNode.Parse(await client.GetStringAsync("/openapi/v1.json", TestContext.Current.CancellationToken))!;
        var pretty = actual.ToJsonString(new JsonSerializerOptions { WriteIndented = true });

        if (Environment.GetEnvironmentVariable("UPDATE_OPENAPI_SNAPSHOT") == "1")
        {
            await File.WriteAllTextAsync(SnapshotPath, pretty, TestContext.Current.CancellationToken);
            return;
        }

        var expected = JsonNode.Parse(await File.ReadAllTextAsync(SnapshotPath, TestContext.Current.CancellationToken));
        Assert.True(JsonNode.DeepEquals(expected, actual),
            "The OpenAPI document changed. If the change is intended, regenerate the snapshot and review the diff in the pull request.");
    }
}

// Stand-in so this file compiles on its own. Delete it in a real test project: in .NET 10 the API's
// generated Program is already public, and a Program declared here would shadow it (CS0436).
public partial class Program;