Walley Backend Developer Code Challenge

Introduction

Welcome! We're excited to see how you approach building backend payment services at Walley.

This challenge uses a simplified Checkout API — a service that handles orders and refunds. The project is partially built: one endpoint works, the structure is in place, and your job is to extend, fix, and test it.

Time expectation: 2-3 hours

What we're looking for:

• Solid C# and .NET fundamentals
• Clean, readable code that follows existing patterns
• Thoughtful error handling and validation
• Testing instincts
• Pragmatic decision-making

Getting Started

cd src/Walley.Checkout.Api
dotnet restore
dotnet run

The API runs at https://localhost:5001 (or http://localhost:5000). Swagger UI is available at /swagger.

To run the existing tests:

cd src/Walley.Checkout.Api.Tests
dotnet test

The Scenario

You're working on Walley's Checkout API. The service handles customer orders — creating them, retrieving them, processing refunds, and calculating totals. A colleague started building it but moved to another team. You're picking up where they left off.

Take a few minutes to read through the existing code before starting. The project has a clear structure: controllers, services, models — and one fully working endpoint (GET /api/orders) as a reference.

Your Tasks

Task 1: Add the "Create Order" Endpoint

Implement POST /api/orders that accepts a new order and returns the created order.

Requirements:

• Accept a request body with customer info and order lines
• Validate the input (no empty orders, valid amounts, required fields)
• Calculate the total from the order lines
• Assign a new ID and set the initial status to Pending
• Return 201 Created with the order and a Location header

Look at the existing models and the GET endpoint for conventions to follow.

Task 2: Fix the Bug in RefundService

The ProcessRefundAsync method in RefundService has a bug. It doesn't work as intended.

Find it, fix it, and briefly explain (in a code comment) what was wrong.

Task 3: Write Unit Tests for OrderService

The OrderService has no tests. Write unit tests that cover the important behaviors.

xUnit and NSubstitute are already referenced in the test project. Focus on meaningful tests — quality over quantity.

Task 4: Improve Error Handling

The GET /api/orders/{id} endpoint currently returns a generic 500 if anything goes wrong. Improve the error handling so the API returns appropriate status codes and useful error responses.

Consider: what should happen when an order isn't found? When the ID format is invalid? How should unexpected errors be handled?

Task 5 (Open-ended): What Would You Improve?

If you had more time, what would you change or add? Write your thoughts in a short section at the bottom of this README or as a separate IMPROVEMENTS.md file.

This isn't about writing a lot — a few bullet points or a paragraph is fine. We're curious about what catches your eye.

Project Structure

src/
├── Walley.Checkout.Api/
│   ├── Controllers/
│   │   └── OrdersController.cs      # API endpoints
│   ├── Services/
│   │   ├── IOrderService.cs          # Order service interface
│   │   ├── OrderService.cs           # Order business logic
│   │   ├── IRefundService.cs         # Refund service interface
│   │   └── RefundService.cs          # Refund logic (has a bug!)
│   ├── Models/
│   │   ├── Order.cs                  # Order domain model
│   │   ├── OrderLine.cs              # Order line item
│   │   ├── Refund.cs                 # Refund model
│   │   └── ApiErrorResponse.cs       # Error response DTO
│   ├── Middleware/
│   │   └── ExceptionMiddleware.cs    # Global exception handler
│   ├── Program.cs
│   └── Walley.Checkout.Api.csproj
└── Walley.Checkout.Api.Tests/
    ├── RefundServiceTests.cs         # Example tests (for reference)
    └── Walley.Checkout.Api.Tests.csproj

Tips

• Follow existing patterns — consistency matters more than cleverness
• Start with Task 1, it builds your understanding of the codebase
• Don't over-engineer — this is a small service, treat it like one
• Commit as you go — we like seeing your thought process
• Read the existing code first — the answers are in the patterns

Questions?

If anything is unclear, reach out to me at bob.jelica@walley.se — we're happy to help.

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Note: We don't expect perfection in 2-3 hours. We want to see how you think, prioritize, and write code in a real-world codebase. Quality over quantity.

Good luck! 🚀