Fsharp Testing

# F# Testing Patterns

Comprehensive testing patterns for F# applications using xUnit, FsUnit, Unquote, FsCheck, and modern .NET testing practices.

## When to Activate

- Writing new tests for F# code
- Reviewing test quality and coverage
- Setting up test infrastructure for F# projects
- Debugging flaky or slow tests

## Test Framework Stack

| Tool | Purpose |
|---|---|
| **xUnit** | Test framework (standard .NET ecosystem choice) |
| **FsUnit.xUnit** | F#-friendly assertion syntax for xUnit |
| **Unquote** | Assertion library using F# quotations for clear failure messages |
| **FsCheck.xUnit** | Property-based testing integrated with xUnit |
| **NSubstitute** | Mocking .NET dependencies |
| **Testcontainers** | Real infrastructure in integration tests |
| **WebApplicationFactory** | ASP.NET Core integration tests |

## Unit Tests with xUnit + FsUnit

### Basic Test Structure

```fsharp
module OrderServiceTests

open Xunit
open FsUnit.Xunit

[<Fact>]
let ``create sets status to Pending`` () =
    let order = Order.create "cust-1" [ validItem ]
    order.Status |> should equal Pending

[<Fact>]
let ``confirm changes status to Confirmed`` () =
    let order = Order.create "cust-1" [ validItem ]
    let confirmed = Order.confirm order
    confirmed.Status |> should be (ofCase <@ Confirmed @>)
```

### Assertions with Unquote

Unquote uses F# quotations so failure messages show the full expression that failed, not just "expected X got Y".