Test-Driven Development (TDD) in ASP.NET Core

Introduction

TDD — Red, Green, Refactor — builds ShopNest's promo code discount feature test-first: write failing test, minimal code to pass, then clean up. Honest take: great for domain logic; less ideal for UI scaffolding.

After this article you will

• Apply Red-Green-Refactor cycle
• TDD a service method from failing test outward
• Choose mocking strategy during TDD
• Know when TDD helps vs slows delivery
• Avoid common TDD pitfalls

Prerequisites

• Article 57 — Performance and Load Testing
• ShopNest Order Service / API from prior modules

Concept deep-dive

RED   → Write failing test for ApplyPromoCodeAsync
GREEN → Minimal implementation: if code=="SAVE10" discount=10%
REFACTOR → Extract IPromoRuleEngine, remove duplication

// Step 1 — RED (test fails — no implementation yet)
[Fact]
public async Task ApplyPromoCode_ValidCode_AppliesTenPercent()
{
    var order = new Order { Subtotal = 1000m };
    await _sut.ApplyPromoCodeAsync(order, "SAVE10");
    Assert.Equal(100m, order.Discount);
    Assert.Equal(900m, order.Total);
}

// Step 2 — GREEN (minimal)
public Task ApplyPromoCodeAsync(Order order, string code)
{
    if (code == "SAVE10") order.ApplyDiscount(order.Subtotal * 0.10m);
    return Task.CompletedTask;
}

// Step 3 — REFACTOR — PromoRuleEngine with pluggable rules

When TDD helps: pricing, tax, inventory rules. When it slows: CRUD scaffolding, spike/prototype, heavy UI — use tests after or integration tests instead.

Hands-on — ShopNest Building a Feature TDD-Style

1. Promo code feature entirely TDD from first test.
2. Tests: invalid code, expired code, max discount cap, stackable rules.
3. Refactor to PromoRuleEngine + IPromoRepository.
4. API endpoint added last with integration test.

Common errors & best practices

• Writing tests after code and calling it TDD — order matters.
• Tests too large — one behavior per test drives design.
• Skipping refactor step — debt accumulates in green phase.
• TDD on everything including layout pages — wrong tool.

Interview questions

Q: Red-Green-Refactor?
A: Fail test → pass minimally → improve design without changing behavior.

Q: TDD drawbacks?
A: Learning curve; slower initial UI work; requires discipline.

Q: Emergent design?
A: API of classes shaped by testability needs over upfront UML.

Summary

• TDD excels on ShopNest pricing and promo rules
• Red-Green-Refactor is the core rhythm
• Mock at boundaries while driving service design
• Skip TDD for pure CRUD — add integration tests instead

Previous: Performance and Load Testing
Next: Deploying to IIS

FAQ