Radberi Tdd

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Don't look at it
• Delete means delete

Implement fresh from tests. Period.

Red-Green-Refactor

┌─────────────────┐     ┌─────────────────┐     ┌─────────────────┐
│      RED        │────▶│     GREEN       │────▶│    REFACTOR     │
│ Write failing   │     │ Minimal code    │     │ Clean up        │
│ test            │     │ to pass         │     │ (stay green)    │
└─────────────────┘     └─────────────────┘     └────────┬────────┘
        ▲                                                │
        └────────────────────────────────────────────────┘
                            Next test

RED - Write Failing Test

Write one minimal test showing what should happen.

Good:

[Fact]
public async Task RetryOperation_RetriesThreeTimes_BeforeSuccess()
{
    // Arrange
    var attempts = 0;
    var operation = () =>
    {
        attempts++;
        if (attempts < 3)
        {
            throw new InvalidOperationException("fail");
        }

        return Task.FromResult("success");
    };

    // Act
    var result = await RetryService.ExecuteAsync(operation);

    // Assert
    result.Should().Be("success");
    attempts.Should().Be(3);
}

Clear name, tests real behaviour, one thing.

Bad:

[Fact]
public async Task RetryWorks()
{
    // Arrange
    var mock = Substitute.For<IOperation>();
    mock.ExecuteAsync()
        .Returns(
            x => throw new Exception(),
            x => throw new Exception(),
            x => Task.FromResult("success"));

    // Act
    await _sut.ExecuteAsync(mock.ExecuteAsync);

    // Assert
    await mock.Received(3).ExecuteAsync();
}

Vague name, tests mock not code.

Requirements:

• One behaviour
• Clear name (MethodName_Scenario_ExpectedBehavior)
• Real code (no mocks unless unavoidable)
• AAA pattern (Arrange-Act-Assert)

Verify RED - Watch It Fail

MANDATORY. Never skip.

dotnet test --filter "FullyQualifiedName~RetryOperation_RetriesThreeTimes"

Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature missing (not typos)

Test passes? You're testing existing behaviour. Fix test.

Test errors? Fix error, re-run until it fails correctly.

GREEN - Minimal Code

Write simplest code to pass the test.

Good:

public static class RetryService
{
    public static async Task<T> ExecuteAsync<T>(Func<Task<T>> operation)
    {
        for (var i = 0; i < 3; i++)
        {
            try
            {
                return await operation();
            }
            catch
            {
                if (i == 2)
                {
                    throw;
                }
            }
        }

        throw new InvalidOperationException("Unreachable");
    }
}

Just enough to pass.

Bad:

public static async Task<T> ExecuteAsync<T>(
    Func<Task<T>> operation,
    RetryOptions? options = null)
{
    options ??= RetryOptions.Default;

    for (var i = 0; i < options.MaxRetries; i++)
    {
        try
        {
            return await operation();
        }
        catch (Exception ex) when (options.ShouldRetry(ex))
        {
            await Task.Delay(options.GetDelay(i));
            options.OnRetry?.Invoke(i, ex);
        }
    }
    // YAGNI - over-engineered!
}

Over-engineered.

Don't add features, refactor other code, or "improve" beyond the test.

Verify GREEN - Watch It Pass

MANDATORY.

dotnet test --filter "FullyQualifiedName~RetryOperation_RetriesThreeTimes"

Confirm:

• Test passes
• Other tests still pass
• Output pristine (no errors, warnings)

Test fails? Fix code, not test.

Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Don't add behaviour.

Repeat

Next failing test for next feature.

Good Tests

xUnit 3 Test Naming Convention

// Pattern: MethodName_Scenario_ExpectedBehavior
[Fact]
public async Task Handle_WithValidCommand_ReturnsSuccess()

[Fact]
public async Task Handle_WithDuplicateNumber_ReturnsInvalidResult()

[Fact]
public void Email_WithEmptyValue_ThrowsArgumentException()

// For Theory with multiple cases
[Theory]
[InlineData("")]
[InlineData(" ")]
[InlineData(null)]
public void Email_WithInvalidValue_ThrowsArgumentException(string? email)

Why Order Matters

"I'll write tests after to verify it works"

Tests written after code pass immediately. Passing immediately proves nothing:

• Might test wrong thing
• Might test implementation, not behaviour
• Might miss edge cases you forgot
• You never saw it catch the bug

Test-first forces you to see the test fail, proving it actually tests something.

"I already manually tested all the edge cases"

Manual testing is ad-hoc. You think you tested everything but:

• No record of what you tested
• Can't re-run when code changes
• Easy to forget cases under pressure
• "It worked when I tried it" ≠ comprehensive

Automated tests are systematic. They run the same way every time.

"Deleting X hours of work is wasteful"

Sunk cost fallacy. The time is already gone. Your choice now:

• Delete and rewrite with TDD (X more hours, high confidence)
• Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

"TDD is dogmatic, being pragmatic means adapting"

TDD IS pragmatic:

• Finds bugs before commit (faster than debugging after)
• Prevents regressions (tests catch breaks immediately)
• Documents behaviour (tests show how to use code)
• Enables refactoring (change freely, tests catch breaks)

"Pragmatic" shortcuts = debugging in production = slower.

"Tests after achieve the same goals - it's spirit not ritual"

No. Tests-after answer "What does this do?" Tests-first answer "What should this do?"

Tests-after are biased by your implementation. You test what you built, not what's required. You verify remembered edge cases, not discovered ones.

Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't).

30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work.

Common Rationalizations

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalising "just this once"
• "I already manually tested it"
• "Tests after achieve the same purpose"
• "It's about spirit not ritual"
• "Keep as reference" or "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic"
• "This is different because..."

All of these mean: Delete code. Start over with TDD.

Example: Bug Fix (Email Validation)

Bug: Empty email accepted in CreateRoomCommand

RED:

[Fact]
public async Task Handle_WithEmptyEmail_ReturnsValidationError()
{
    // Arrange
    var command = new InviteMemberCommand(
        Email: string.Empty,
        RoleId: _orgUserRoleId);
    var handler = new InviteMemberHandler(_dbContext, _emailService);

    // Act
    var result = await handler.Handle(command, CancellationToken.None);

    // Assert
    result.IsSuccess.Should().BeFalse();
    result.ValidationErrors.Should().Contain(e => e.ErrorMessage.Contains("Email"));
}

Verify RED:

$ dotnet test --filter "Handle_WithEmptyEmail"
Failed Handle_WithEmptyEmail_ReturnsValidationError
  Expected result.IsSuccess to be False, but found True.

GREEN:

public class InviteMemberHandler : ICommandHandler<InviteMemberCommand, Result>
{
    public async Task<Result> Handle(InviteMemberCommand command, CancellationToken ct)
    {
        if (string.IsNullOrWhiteSpace(command.Email))
        {
            return Result.Invalid(new ValidationError("Email is required"));
        }

        // ... rest of implementation
    }
}

Verify GREEN:

$ dotnet test --filter "Handle_WithEmptyEmail"
Passed Handle_WithEmptyEmail_ReturnsValidationError

REFACTOR: Extract validation to FluentValidation validator for consistency.

public class InviteMemberCommandValidator : AbstractValidator<InviteMemberCommand>
{
    public InviteMemberCommandValidator()
    {
        RuleFor(x => x.Email)
            .NotEmpty()
            .WithMessage("Email is required");
    }
}

Example: Domain Entity TDD

Feature: Room must have positive area

RED:

[Fact]
public void SetArea_WithNegativeValue_ThrowsArgumentOutOfRangeException()
{
    // Arrange
    var room = new Room("101", "Meeting Room", _projectId);

    // Act
    var act = () => room.SetArea(-5.0m);

    // Assert
    act.Should().Throw<ArgumentOutOfRangeException>()
        .WithMessage("*Area*cannot be negative*");
}

GREEN:

public void SetArea(decimal? area)
{
    if (area < 0)
    {
        throw new ArgumentOutOfRangeException(nameof(area), "Area cannot be negative");
    }

    AreaRequired = area;
}

Example: Blazor Component TDD with bUnit

Feature: Room list shows "No rooms" when empty

RED:

[Fact]
public void RoomList_WithNoRooms_ShowsEmptyMessage()
{
    // Arrange
    using var ctx = new TestContext();
    ctx.Services.AddSyncfusionBlazor();

    var queryMediator = Substitute.For<IQueryMediator>();
    queryMediator.Send(Arg.Any<GetRoomsQuery>(), Arg.Any<CancellationToken>())
        .Returns(Result<IReadOnlyList<RoomDto>>.Success(new List<RoomDto>()));
    ctx.Services.AddSingleton(queryMediator);

    // Act
    var cut = ctx.RenderComponent<RoomList>(parameters =>
        parameters.Add(p => p.ProjectId, Guid.NewGuid()));

    // Assert
    cut.Find(".empty-state").TextContent.Should().Contain("No rooms found");
}

GREEN:

@if (!_rooms.Any())
{
    <span class="empty-state">No rooms found</span>
}
else
{
    <SfGrid TValue="RoomDto" DataSource="_rooms">
        @* ... *@
    </SfGrid>
}

Verification Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass (dotnet test)
• Output pristine (no errors, warnings)
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered
• Follows AAA pattern (Arrange-Act-Assert)

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.

Skill References