dotnet-unit-testing

Unit Test Generator

Overview

Unit tests for Clean Architecture handlers:

• xUnit - Test framework
• NSubstitute - Mocking library
• FluentAssertions - Readable assertions
• AAA pattern - Arrange, Act, Assert

Quick Reference

Test Type	Purpose	Example
Success test	Verify happy path	Should_ReturnSuccess_When_ValidRequest
Failure test	Verify error handling	Should_ReturnFailure_When_NotFound
Validation test	Verify input validation	Should_ReturnValidationError_When_EmptyName
Behavior test	Verify side effects	Should_CallRepository_When_ValidRequest

---

Test Project Structure

tests/
└── {name}.Application.UnitTests/
    ├── {Feature}/
    │   ├── Create{Entity}/
    │   │   ├── Create{Entity}CommandHandlerTests.cs
    │   │   └── Create{Entity}CommandValidatorTests.cs
    │   └── Get{Entity}ById/
    │       └── Get{Entity}ByIdQueryHandlerTests.cs
    ├── Abstractions/
    │   └── BaseTest.cs
    └── {name}.Application.UnitTests.csproj

---

Template: Test Project File

<!-- tests/{name}.Application.UnitTests/{name}.Application.UnitTests.csproj -->
<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <IsPackable>false</IsPackable>
    <IsTestProject>true</IsTestProject>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="FluentAssertions" Version="6.12.0" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.8.0" />
    <PackageReference Include="NSubstitute" Version="5.1.0" />
    <PackageReference Include="NSubstitute.Analyzers.CSharp" Version="1.0.16">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
    <PackageReference Include="xunit" Version="2.6.2" />
    <PackageReference Include="xunit.runner.visualstudio" Version="2.5.4">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
    <PackageReference Include="coverlet.collector" Version="6.0.0">
      <PrivateAssets>all</PrivateAssets>
      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
    </PackageReference>
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\..\src\{name}.application\{name}.application.csproj" />
    <ProjectReference Include="..\..\src\{name}.domain\{name}.domain.csproj" />
  </ItemGroup>

</Project>

---

Template: Base Test Class

// tests/{name}.Application.UnitTests/Abstractions/BaseTest.cs
using NSubstitute;
using {name}.domain.abstractions;

namespace {name}.Application.UnitTests.Abstractions;

public abstract class BaseTest
{
    protected static CancellationToken CancellationToken => CancellationToken.None;

    /// <summary>
    /// Creates a mock that returns the provided result
    /// </summary>
    protected static T CreateMock<T>() where T : class
    {
        return Substitute.For<T>();
    }

    /// <summary>
    /// Helper to create a successful Result
    /// </summary>
    protected static Result<T> SuccessResult<T>(T value)
    {
        return Result.Success(value);
    }

    /// <summary>
    /// Helper to create a failed Result
    /// </summary>
    protected static Result<T> FailureResult<T>(Error error)
    {
        return Result.Failure<T>(error);
    }
}

---

Template: Command Handler Tests

// tests/{name}.Application.UnitTests/{Feature}/Create{Entity}/Create{Entity}CommandHandlerTests.cs
using FluentAssertions;
using NSubstitute;
using {name}.application.{feature}.Create{Entity};
using {name}.domain.{aggregate};
using {name}.domain.abstractions;
using {name}.Application.UnitTests.Abstractions;

namespace {name}.Application.UnitTests.{Feature}.Create{Entity};

public sealed class Create{Entity}CommandHandlerTests : BaseTest
{
    private readonly I{Entity}Repository _{entity}Repository;
    private readonly IUnitOfWork _unitOfWork;
    private readonly Create{Entity}CommandHandler _handler;

    public Create{Entity}CommandHandlerTests()
    {
        // Arrange - Setup mocks (runs before each test)
        _{entity}Repository = CreateMock<I{Entity}Repository>();
        _unitOfWork = CreateMock<IUnitOfWork>();

        _handler = new Create{Entity}CommandHandler(
            _{entity}Repository,
            _unitOfWork);
    }

    // ═══════════════════════════════════════════════════════════════
    // SUCCESS TESTS
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public async Task Handle_Should_ReturnSuccess_When_ValidRequest()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Test Entity",
            Description: "Test Description",
            OrganizationId: Guid.NewGuid());

        _{entity}Repository
            .GetByNameAsync(command.Name, CancellationToken)
            .Returns((Domain.{Aggregate}.{Entity}?)null);

        // Act
        var result = await _handler.Handle(command, CancellationToken);

        // Assert
        result.IsSuccess.Should().BeTrue();
        result.Value.Should().NotBeEmpty();
    }

    [Fact]
    public async Task Handle_Should_AddEntity_When_ValidRequest()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Test Entity",
            Description: "Test Description",
            OrganizationId: Guid.NewGuid());

        _{entity}Repository
            .GetByNameAsync(command.Name, CancellationToken)
            .Returns((Domain.{Aggregate}.{Entity}?)null);

        // Act
        await _handler.Handle(command, CancellationToken);

        // Assert
        _{entity}Repository
            .Received(1)
            .Add(Arg.Is<Domain.{Aggregate}.{Entity}>(e =>
                e.Name == command.Name &&
                e.OrganizationId == command.OrganizationId));
    }

    [Fact]
    public async Task Handle_Should_CallSaveChanges_When_ValidRequest()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Test Entity",
            Description: "Test Description",
            OrganizationId: Guid.NewGuid());

        _{entity}Repository
            .GetByNameAsync(command.Name, CancellationToken)
            .Returns((Domain.{Aggregate}.{Entity}?)null);

        // Act
        await _handler.Handle(command, CancellationToken);

        // Assert
        await _unitOfWork
            .Received(1)
            .SaveChangesAsync(CancellationToken);
    }

    // ═══════════════════════════════════════════════════════════════
    // FAILURE TESTS
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public async Task Handle_Should_ReturnFailure_When_NameAlreadyExists()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Existing Entity",
            Description: "Test Description",
            OrganizationId: Guid.NewGuid());

        var existing{Entity} = CreateTest{Entity}(command.Name);

        _{entity}Repository
            .GetByNameAsync(command.Name, CancellationToken)
            .Returns(existing{Entity});

        // Act
        var result = await _handler.Handle(command, CancellationToken);

        // Assert
        result.IsFailure.Should().BeTrue();
        result.Error.Should().Be({Entity}Errors.NameAlreadyExists);
    }

    [Fact]
    public async Task Handle_Should_NotAddEntity_When_NameAlreadyExists()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Existing Entity",
            Description: "Test Description",
            OrganizationId: Guid.NewGuid());

        var existing{Entity} = CreateTest{Entity}(command.Name);

        _{entity}Repository
            .GetByNameAsync(command.Name, CancellationToken)
            .Returns(existing{Entity});

        // Act
        await _handler.Handle(command, CancellationToken);

        // Assert
        _{entity}Repository
            .DidNotReceive()
            .Add(Arg.Any<Domain.{Aggregate}.{Entity}>());
    }

    [Fact]
    public async Task Handle_Should_NotCallSaveChanges_When_NameAlreadyExists()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Existing Entity",
            Description: "Test Description",
            OrganizationId: Guid.NewGuid());

        var existing{Entity} = CreateTest{Entity}(command.Name);

        _{entity}Repository
            .GetByNameAsync(command.Name, CancellationToken)
            .Returns(existing{Entity});

        // Act
        await _handler.Handle(command, CancellationToken);

        // Assert
        await _unitOfWork
            .DidNotReceive()
            .SaveChangesAsync(Arg.Any<CancellationToken>());
    }

    // ═══════════════════════════════════════════════════════════════
    // HELPER METHODS
    // ═══════════════════════════════════════════════════════════════

    private static Domain.{Aggregate}.{Entity} CreateTest{Entity}(string name)
    {
        // Use reflection or internal factory for testing
        // This assumes the entity has a factory method
        var result = Domain.{Aggregate}.{Entity}.Create(
            name,
            "Description",
            Guid.NewGuid());

        return result.Value;
    }
}

---

Template: Query Handler Tests

// tests/{name}.Application.UnitTests/{Feature}/Get{Entity}ById/Get{Entity}ByIdQueryHandlerTests.cs
using FluentAssertions;
using NSubstitute;
using {name}.application.{feature}.Get{Entity}ById;
using {name}.application.abstractions.data;
using {name}.Application.UnitTests.Abstractions;

namespace {name}.Application.UnitTests.{Feature}.Get{Entity}ById;

public sealed class Get{Entity}ByIdQueryHandlerTests : BaseTest
{
    private readonly ISqlConnectionFactory _sqlConnectionFactory;
    private readonly Get{Entity}ByIdQueryHandler _handler;

    public Get{Entity}ByIdQueryHandlerTests()
    {
        _sqlConnectionFactory = CreateMock<ISqlConnectionFactory>();
        _handler = new Get{Entity}ByIdQueryHandler(_sqlConnectionFactory);
    }

    [Fact]
    public async Task Handle_Should_ReturnSuccess_When_EntityExists()
    {
        // Arrange
        var entityId = Guid.NewGuid();
        var query = new Get{Entity}ByIdQuery(entityId);

        var expected = new {Entity}Response
        {
            Id = entityId,
            Name = "Test Entity",
            Description = "Description"
        };

        // Setup Dapper mock (simplified - in practice, mock IDbConnection)
        SetupConnectionToReturn(expected);

        // Act
        var result = await _handler.Handle(query, CancellationToken);

        // Assert
        result.IsSuccess.Should().BeTrue();
        result.Value.Id.Should().Be(entityId);
    }

    [Fact]
    public async Task Handle_Should_ReturnFailure_When_EntityNotFound()
    {
        // Arrange
        var entityId = Guid.NewGuid();
        var query = new Get{Entity}ByIdQuery(entityId);

        SetupConnectionToReturn(null);

        // Act
        var result = await _handler.Handle(query, CancellationToken);

        // Assert
        result.IsFailure.Should().BeTrue();
        result.Error.Should().Be({Entity}Errors.NotFound);
    }

    private void SetupConnectionToReturn({Entity}Response? response)
    {
        // In practice, you'd use a library like Dapper.Contrib.Tests
        // or create a test double for IDbConnection
        // This is a simplified example
    }
}

---

Template: Validator Tests

// tests/{name}.Application.UnitTests/{Feature}/Create{Entity}/Create{Entity}CommandValidatorTests.cs
using FluentAssertions;
using FluentValidation.TestHelper;
using {name}.application.{feature}.Create{Entity};
using {name}.Application.UnitTests.Abstractions;

namespace {name}.Application.UnitTests.{Feature}.Create{Entity};

public sealed class Create{Entity}CommandValidatorTests : BaseTest
{
    private readonly Create{Entity}CommandValidator _validator;

    public Create{Entity}CommandValidatorTests()
    {
        _validator = new Create{Entity}CommandValidator();
    }

    // ═══════════════════════════════════════════════════════════════
    // NAME VALIDATION
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public void Validate_Should_HaveError_When_NameIsEmpty()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: string.Empty,
            Description: "Valid description",
            OrganizationId: Guid.NewGuid());

        // Act
        var result = _validator.TestValidate(command);

        // Assert
        result.ShouldHaveValidationErrorFor(x => x.Name)
            .WithErrorMessage("{Entity} name is required");
    }

    [Fact]
    public void Validate_Should_HaveError_When_NameTooLong()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: new string('a', 101),  // Exceeds 100 char limit
            Description: "Valid description",
            OrganizationId: Guid.NewGuid());

        // Act
        var result = _validator.TestValidate(command);

        // Assert
        result.ShouldHaveValidationErrorFor(x => x.Name)
            .WithErrorMessage("{Entity} name must not exceed 100 characters");
    }

    [Theory]
    [InlineData("A")]
    [InlineData("Valid Name")]
    [InlineData("Name with 100 characters padded................................")]
    public void Validate_Should_NotHaveError_When_NameIsValid(string name)
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: name,
            Description: "Valid description",
            OrganizationId: Guid.NewGuid());

        // Act
        var result = _validator.TestValidate(command);

        // Assert
        result.ShouldNotHaveValidationErrorFor(x => x.Name);
    }

    // ═══════════════════════════════════════════════════════════════
    // ORGANIZATION ID VALIDATION
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public void Validate_Should_HaveError_When_OrganizationIdIsEmpty()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Valid Name",
            Description: "Valid description",
            OrganizationId: Guid.Empty);

        // Act
        var result = _validator.TestValidate(command);

        // Assert
        result.ShouldHaveValidationErrorFor(x => x.OrganizationId);
    }

    [Fact]
    public void Validate_Should_NotHaveError_When_OrganizationIdIsValid()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Valid Name",
            Description: "Valid description",
            OrganizationId: Guid.NewGuid());

        // Act
        var result = _validator.TestValidate(command);

        // Assert
        result.ShouldNotHaveValidationErrorFor(x => x.OrganizationId);
    }

    // ═══════════════════════════════════════════════════════════════
    // FULL VALIDATION
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public void Validate_Should_BeValid_When_AllFieldsAreValid()
    {
        // Arrange
        var command = new Create{Entity}Command(
            Name: "Valid Name",
            Description: "Valid description",
            OrganizationId: Guid.NewGuid());

        // Act
        var result = _validator.TestValidate(command);

        // Assert
        result.ShouldNotHaveAnyValidationErrors();
    }
}

---

Template: Domain Entity Tests

// tests/{name}.Domain.UnitTests/{Aggregate}/{Entity}Tests.cs
using FluentAssertions;
using {name}.domain.{aggregate};
using {name}.domain.{aggregate}.events;

namespace {name}.Domain.UnitTests.{Aggregate};

public sealed class {Entity}Tests
{
    // ═══════════════════════════════════════════════════════════════
    // CREATE TESTS
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public void Create_Should_ReturnSuccess_When_ValidParameters()
    {
        // Arrange
        var name = "Test Entity";
        var description = "Test Description";
        var organizationId = Guid.NewGuid();

        // Act
        var result = {Entity}.Create(name, description, organizationId);

        // Assert
        result.IsSuccess.Should().BeTrue();
        result.Value.Name.Should().Be(name);
        result.Value.OrganizationId.Should().Be(organizationId);
        result.Value.IsActive.Should().BeTrue();
    }

    [Fact]
    public void Create_Should_ReturnFailure_When_NameIsEmpty()
    {
        // Arrange
        var name = string.Empty;
        var description = "Test Description";
        var organizationId = Guid.NewGuid();

        // Act
        var result = {Entity}.Create(name, description, organizationId);

        // Assert
        result.IsFailure.Should().BeTrue();
        result.Error.Should().Be({Entity}Errors.NameRequired);
    }

    [Fact]
    public void Create_Should_RaiseDomainEvent_When_Success()
    {
        // Arrange
        var name = "Test Entity";
        var description = "Test Description";
        var organizationId = Guid.NewGuid();

        // Act
        var result = {Entity}.Create(name, description, organizationId);

        // Assert
        result.Value.GetDomainEvents()
            .Should().ContainSingle()
            .Which.Should().BeOfType<{Entity}CreatedDomainEvent>();
    }

    // ═══════════════════════════════════════════════════════════════
    // UPDATE TESTS
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public void UpdateName_Should_ReturnSuccess_When_ValidName()
    {
        // Arrange
        var entity = Create{Entity}();
        var newName = "Updated Name";

        // Act
        var result = entity.UpdateName(newName);

        // Assert
        result.IsSuccess.Should().BeTrue();
        entity.Name.Should().Be(newName);
    }

    [Fact]
    public void UpdateName_Should_ReturnFailure_When_EmptyName()
    {
        // Arrange
        var entity = Create{Entity}();

        // Act
        var result = entity.UpdateName(string.Empty);

        // Assert
        result.IsFailure.Should().BeTrue();
        result.Error.Should().Be({Entity}Errors.NameRequired);
    }

    // ═══════════════════════════════════════════════════════════════
    // DEACTIVATE TESTS
    // ═══════════════════════════════════════════════════════════════

    [Fact]
    public void Deactivate_Should_SetIsActiveToFalse()
    {
        // Arrange
        var entity = Create{Entity}();
        entity.IsActive.Should().BeTrue();

        // Act
        entity.Deactivate();

        // Assert
        entity.IsActive.Should().BeFalse();
    }

    [Fact]
    public void Deactivate_Should_RaiseDomainEvent()
    {
        // Arrange
        var entity = Create{Entity}();
        entity.ClearDomainEvents();  // Clear create event

        // Act
        entity.Deactivate();

        // Assert
        entity.GetDomainEvents()
            .Should().ContainSingle()
            .Which.Should().BeOfType<{Entity}DeactivatedDomainEvent>();
    }

    // ═══════════════════════════════════════════════════════════════
    // HELPER METHODS
    // ═══════════════════════════════════════════════════════════════

    private static {Entity} Create{Entity}()
    {
        var result = {Entity}.Create(
            "Test Entity",
            "Test Description",
            Guid.NewGuid());

        return result.Value;
    }
}

---

Template: Test Data Builders

// tests/{name}.Application.UnitTests/TestData/{Entity}Builder.cs
using {name}.application.{feature}.Create{Entity};

namespace {name}.Application.UnitTests.TestData;

public sealed class {Entity}CommandBuilder
{
    private string _name = "Default Name";
    private string _description = "Default Description";
    private Guid _organizationId = Guid.NewGuid();

    public {Entity}CommandBuilder WithName(string name)
    {
        _name = name;
        return this;
    }

    public {Entity}CommandBuilder WithDescription(string description)
    {
        _description = description;
        return this;
    }

    public {Entity}CommandBuilder WithOrganizationId(Guid organizationId)
    {
        _organizationId = organizationId;
        return this;
    }

    public Create{Entity}Command Build()
    {
        return new Create{Entity}Command(_name, _description, _organizationId);
    }
}

// Usage in tests:
// var command = new {Entity}CommandBuilder()
//     .WithName("Custom Name")
//     .Build();

---

NSubstitute Quick Reference

// Create mock
var repository = Substitute.For<IRepository>();

// Setup return value
repository.GetByIdAsync(Arg.Any<Guid>(), Arg.Any<CancellationToken>())
    .Returns(entity);

// Setup return null
repository.GetByIdAsync(entityId, CancellationToken)
    .Returns((Entity?)null);

// Verify method was called
repository.Received(1).Add(Arg.Any<Entity>());

// Verify method was NOT called
repository.DidNotReceive().Add(Arg.Any<Entity>());

// Verify with argument matching
repository.Received().Add(Arg.Is<Entity>(e => e.Name == "Test"));

// Verify call order (advanced)
Received.InOrder(() =>
{
    repository.Add(Arg.Any<Entity>());
    unitOfWork.SaveChangesAsync(CancellationToken);
});

// Setup to throw exception
repository.GetByIdAsync(Arg.Any<Guid>(), Arg.Any<CancellationToken>())
    .ThrowsAsync(new Exception("Database error"));

---

FluentAssertions Quick Reference

// Basic assertions
result.Should().BeTrue();
result.Should().BeFalse();
result.Should().BeNull();
result.Should().NotBeNull();

// Equality
result.Should().Be(expected);
result.Should().NotBe(unexpected);
result.Should().BeEquivalentTo(expected);

// Collections
list.Should().BeEmpty();
list.Should().NotBeEmpty();
list.Should().HaveCount(3);
list.Should().Contain(item);
list.Should().ContainSingle();
list.Should().ContainSingle().Which.Should().BeOfType<MyType>();

// Types
result.Should().BeOfType<MyType>();
result.Should().BeAssignableTo<IMyInterface>();

// Strings
name.Should().StartWith("Test");
name.Should().Contain("Entity");
name.Should().BeNullOrEmpty();

// Exceptions
action.Should().Throw<InvalidOperationException>()
    .WithMessage("*not found*");

action.Should().NotThrow();

// Result pattern
result.IsSuccess.Should().BeTrue();
result.Error.Should().Be(ExpectedError);

---

Critical Rules

1. One assert concept per test - Focus on single behavior
2. Descriptive test names - Should_{ExpectedBehavior}_When_{Condition}
3. Arrange-Act-Assert - Clear structure in every test
4. Mock only dependencies - Don't mock the SUT
5. Test behavior, not implementation - Focus on outcomes
6. Use Theory for data-driven tests - Avoid duplicate test logic
7. Test edge cases - Empty, null, boundaries
8. Fast tests - No I/O, no database
9. Independent tests - No shared state
10. Meaningful assertions - Test what matters

---

Coverage Discipline: State, Lifecycles, and Boundaries

These four rules catch the class of bug where a handler silently ignores a value of a discriminator field that another handler in the system is producing. Testing the handler against every status value in isolation is not enough — you have to round-trip through the producer.

1. Discriminator-field matrix is mandatory, not optional

For every handler that branches on a status/kind/enum-as-string/flag field, the test suite must parametrize over every value declared in the canonical constants class (e.g. OrderStatuses.All). Adding a new value to that class is a coverage event: every consumer must declare its expected behavior for the new value, or the suite stops being a spec.

[Theory]
[InlineData("pending",   false)]
[InlineData("shipped",   true)]
[InlineData("delivered", true)]   // <-- often the missed row
[InlineData("cancelled", false)]
public async Task Handle_RespectsStatus(string status, bool shouldSend)
{
    await SeedAsync(NewOrder(status, jobs: [new OrderEmailJob { Sent = false }]));
    var result = await _sut.Handle(new SendDueOrderEmailsCommand(), CancellationToken.None);
    result.Sent.ShouldBe(shouldSend ? 1 : 0);
}

Anti-pattern: writing one Should_NotSend_When_NonShipped fact and one [InlineData] per "not the happy path" value. The table format above forces every row to be a positive assertion of intended behavior; the negative form lets you forget rows.

2. Producer + consumer in the SAME test

When handler A writes a discriminator field and handler B reads it, at least one test must exercise A then B sequentially through the actual store. Do not mutate the field directly in the Arrange step — that bypasses the production code path that does the writing and hides the very bug we are trying to catch.

[Fact]
public async Task SendDueEmails_FiresOnDeliveredOrders_AfterPromoterRuns()
{
    // Arrange: a Shipped order with an unsent job
    var order = NewOrder(OrderStatuses.Shipped, jobs: [new OrderEmailJob { Sent = false }]);
    await SeedAsync(order);

    // Act 1: the OTHER handler runs first and promotes Status to Delivered
    var promoter = new PromoteShippedToDeliveredCommandHandler(_context);
    await promoter.Handle(new PromoteShippedToDeliveredCommand(), CancellationToken.None);

    // Act 2: now the handler under test runs
    var result = await _sut.Handle(new SendDueOrderEmailsCommand(), CancellationToken.None);

    // Assert: business-level question — should a Delivered order still flush its email queue?
    // Answer this against the spec, not against the current Where(...) clause.
    result.Sent.ShouldBe(1);
}

Why this is non-negotiable: the most pernicious bugs in CRUD systems live in the implicit coupling between two handlers nobody wrote together. Direct mutation in the Arrange step (order.Status = "delivered") passes that coupling silently. Round-tripping through the producer makes it real.

When to write this test: any time you find another handler in the codebase that writes to the same field your .Where(...) reads from. Grep for assignments to the field; for each writing handler, add a producer+consumer test.

3. Inject a clock; ban direct DateTime.UtcNow in handlers

Direct DateTime.UtcNow (or DateTime.Now) calls in handlers, services, and validators are banned. Inject TimeProvider (.NET 8+); in tests use Microsoft.Extensions.Time.Testing.FakeTimeProvider so a single test can advance time across a state transition.

public class SendDueOrderEmailsCommandHandler
{
    private readonly TimeProvider _clock;
    // ctor injects TimeProvider

    public async Task<...> Handle(...)
    {
        var utcNow = _clock.GetUtcNow().UtcDateTime;
        // ...
    }
}

// In tests:
private readonly FakeTimeProvider _clock = new(startDateTime: new DateTimeOffset(2026, 5, 18, 12, 0, 0, TimeSpan.Zero));
// Construct the SUT with _clock instead of TimeProvider.System.

[Fact]
public async Task Job_Fires_AfterWindowElapses()
{
    await SeedDueJobAt(_clock.GetUtcNow().UtcDateTime.AddMinutes(60));

    _clock.Advance(TimeSpan.FromMinutes(59));
    (await _sut.Handle(...)).Sent.ShouldBe(0);  // not yet

    _clock.Advance(TimeSpan.FromMinutes(2));
    (await _sut.Handle(...)).Sent.ShouldBe(1);  // crossed the threshold
}

Why mandatory: time-driven state transitions are the second most common source of silent prod bugs after #1 above. They are simply untestable while production reads the system clock directly. Add the TimeProvider ctor parameter the first time you write a handler that reads "now" — retrofitting later is expensive.

4. Threshold triplet tests

Any behavior change at a threshold (event-end + offset, grace-window edge, registration cutoff, capacity limit, retry-after delay) requires three explicit tests: just before the threshold, exactly at it, just after.

[Theory]
[InlineData(59,  false)]   // just before
[InlineData(60,  true)]    // exactly at
[InlineData(61,  true)]    // just after
public async Task Job_FiresOnceWindowReached(int minutesElapsed, bool shouldFire)
{
    await SeedDueJobAt(_clock.GetUtcNow().UtcDateTime.AddMinutes(60));
    _clock.Advance(TimeSpan.FromMinutes(minutesElapsed));
    (await _sut.Handle(...)).Sent.ShouldBe(shouldFire ? 1 : 0);
}

Three lines of [InlineData] are cheaper than the off-by-one bug they prevent.

Rationalization Table — STOP if you catch yourself thinking any of these

Excuse	Reality
"The other statuses obviously don't apply, the filter is right"	If they "obviously" don't apply, the InlineData row is a 5-second assertion. Write it.
"Status is only ever set during Create, so I'll seed Status = X directly"	Some other handler probably writes it later. Grep for \.Status = across the codebase before believing yourself.
"The Promoter is a different test's concern"	The coupling between Promoter and Reader IS the bug class. One round-trip test makes the coupling explicit.
"Using TimeProvider is overkill for this simple handler"	Every handler that reads UtcNow is one "future engineer mutates upstream state" away from being un-testable. Pay the cost up front.
"Boundary tests are pedantic"	Off-by-one at thresholds is the #1 silent prod bug after status filtering. Three InlineData rows.
"My theory only needs the values I'm asserting about"	The theory is a living spec. Missing rows = silent missing spec = future regression.

Red Flags — STOP and re-test

• A .Where(x => x.Field == constant) with no parametrized test covering every value of Field
• A test that sets a discriminator field with entity.Field = "X" instead of running the handler that produces that value
• A handler calling DateTime.UtcNow directly with no TimeProvider ctor parameter
• A behavior threshold (offset, cutoff, limit) tested with one value instead of three

If you spot any of these in a PR, request the missing tests before approving.

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Anti-Patterns to Avoid

// ❌ WRONG: Multiple unrelated assertions
[Fact]
public void Test_Everything()
{
    // Tests too many things at once
    result.IsSuccess.Should().BeTrue();
    result.Value.Name.Should().Be("Test");
    repository.Received(1).Add(Arg.Any<Entity>());
    unitOfWork.Received(1).SaveChangesAsync(Arg.Any<CancellationToken>());
}

// ✅ CORRECT: Focused tests
[Fact]
public void Handle_Should_ReturnSuccess_When_ValidRequest() { }

[Fact]
public void Handle_Should_AddEntity_When_ValidRequest() { }

[Fact]
public void Handle_Should_CallSaveChanges_When_ValidRequest() { }

// ❌ WRONG: Testing implementation details
repository.Received(1).GetByIdAsync(entityId, CancellationToken);
repository.Received(1).Add(Arg.Any<Entity>());
// ... testing every single call

// ✅ CORRECT: Testing outcomes
result.IsSuccess.Should().BeTrue();
result.Value.Id.Should().NotBeEmpty();

// ❌ WRONG: Shared mutable state
private Entity _sharedEntity;  // Modified by tests, causes flaky tests

// ✅ CORRECT: Fresh setup per test
[Fact]
public void Test()
{
    var entity = CreateEntity();  // Fresh instance
}

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Related Skills

• dotnet-cqrs-command-generator - Commands to test
• dotnet-cqrs-query-generator - Queries to test
• dotnet-domain-entity-generator - Domain entities to test
• dotnet-integration-testing - End-to-end tests