Income Tax Calculator

An ASP.NET Core solution that calculates net salary for a UK-style progressive income tax system: annual tax is the sum of the tax paid within each band — 0% on the first £5,000, 20% on £5,000–£20,000, 40% above £20,000 — and monthly figures are the annual ones divided by 12.

The tax model is deliberately simple. The point of the project is the architecture around it: a layered multi-project solution with a clean dependency flow, dependency injection, EF Core with migrations, ASP.NET Core Identity, Swagger, and a test suite that runs without any external infrastructure.

⚠️ The calculation is intentionally simplified (no personal allowance, National Insurance or other deductions). It is a demo, not tax advice.

Solution structure

Project	Role
TaxCalculator (Web)	MVC app — the UI (Razor + Bootstrap + jQuery + toastr) that calls the API
TaxCalculator.Api (WebApi)	REST API — POST /api/salary, Swagger UI in Development
TaxCalculator.Services	Business logic — tax calculation and salary breakdown
TaxCalculator.Repositories	Data access — tax band repository behind an interface
TaxCalculator.Db	EF Core model, DbContext, migrations, ASP.NET Core Identity user store
TaxCalculator.UnitTests	Unit tests for the services
TaxCalculator.IntegrationTests	API integration tests (in-memory TestServer + EF InMemory)

Dependency flow: Web → WebApi → Services → Repositories → Db. Services depend only on repository interfaces, so the calculation logic is unit-testable in isolation.

Getting started

Requires the .NET 8 SDK.

dotnet build
dotnet test                                        # all tests, no SQL Server needed

dotnet run --project TaxCalculator.Api             # API + Swagger UI
dotnet run --project TaxCalculator                 # MVC UI

The MVC UI expects the API at the address configured in TaxCalculator/appsettings.json (ApiSettings:ApiUrl).

Database

Tax bands live in SQL Server (ConnectionStrings:DefaultConnection in TaxCalculator.Api/appsettings.json). Apply the EF Core migrations to create and seed the database:

dotnet ef database update --project TaxCalculator.Db --startup-project TaxCalculator.Api

Example

POST /api/salary with { "grossAnnualSalary": 27000 } returns:

	Annual	Monthly
Gross salary	27,000.00	2,250.00
Tax paid	5,800.00	483.33
Net salary	21,200.00	1,766.67

Tax breakdown: 0% × 5,000 + 20% × 15,000 + 40% × 7,000 = 5,800.

Testing

• Unit tests cover TaxCalculatorService and SalaryService, including edge cases (salary inside a single band, salary spanning all bands, negative input).
• Integration tests boot the full API in a WebApplicationFactory, swap SQL Server for the EF Core InMemory provider with seeded tax bands, and verify the endpoint end-to-end.