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".

fsharp
module OrderValidationTests

open Xunit
open Swensen.Unquote

[<Fact>]
let ``PlaceOrder returns success when request is valid`` () =
    let request = { CustomerId = "cust-123"; Items = [ validItem ] }
    let result = OrderService.placeOrder request
    test <@ Result.isOk result @>

[<Fact>]
let ``order total sums item prices`` () =
    let items = [ { Sku = "A"; Quantity = 2; Price = 10m }
                  { Sku = "B"; Quantity = 1; Price = 5m } ]
    let total = Order.calculateTotal items
    test <@ total = 25m @>

[<Fact>]
let ``validated email rejects empty input`` () =
    let result = ValidatedEmail.create ""
    test <@ Result.isError result @>

Async Tests

fsharp
[<Fact>]
let ``PlaceOrder returns success when request is valid`` () = task {
    let deps = createTestDeps ()
    let request = { CustomerId = "cust-123"; Items = [ validItem ] }

    let! result = OrderService.placeOrder deps request

    test <@ Result.isOk result @>
}

[<Fact>]
let ``PlaceOrder returns error when items are empty`` () = task {
    let deps = createTestDeps ()
    let request = { CustomerId = "cust-123"; Items = [] }

    let! result = OrderService.placeOrder deps request

    test <@ Result.isError result @>
}

Parameterized Tests with Theory

fsharp
[<Theory>]
[<InlineData("")>]
[<InlineData("   ")>]
let ``PlaceOrder rejects empty customer ID`` (customerId: string) =
    let request = { CustomerId = customerId; Items = [ validItem ] }
    let result = OrderService.placeOrder request
    result |> should be (ofCase <@ Error @>)

[<Theory>]
[<InlineData("", false)>]
[<InlineData("a", false)>]
[<InlineData("user@example.com", true)>]
[<InlineData("user+tag@example.co.uk", true)>]
let ``IsValidEmail returns expected result`` (email: string, expected: bool) =
    test <@ EmailValidator.isValid email = expected @>

Property-Based Testing with FsCheck

Using FsCheck.xUnit

fsharp
open FsCheck
open FsCheck.Xunit

[<Property>]
let ``order total is always non-negative`` (items: NonEmptyList<PositiveInt * decimal>) =
    let orderItems =
        items.Get
        |> List.map (fun (qty, price) ->
            { Sku = "SKU"; Quantity = qty.Get; Price = abs price })
    let total = Order.calculateTotal orderItems
    total >= 0m

[<Property>]
let ``serialization roundtrips`` (order: Order) =
    let json = JsonSerializer.Serialize order
    let deserialized = JsonSerializer.Deserialize<Order> json
    deserialized = order

Custom Generators

fsharp
type OrderGenerators =
    static member ValidEmail () =
        gen {
            let! user = Gen.elements [ "alice"; "bob"; "carol" ]
            let! domain = Gen.elements [ "example.com"; "test.org" ]
            return $"{user}@{domain}"
        }
        |> Arb.fromGen

[<Property(Arbitrary = [| typeof<OrderGenerators> |])>]
let ``valid emails pass validation`` (email: string) =
    EmailValidator.isValid email

Mocking Dependencies

Function Stubs (Preferred)

fsharp
let createTestDeps () =
    let mutable savedOrders = []
    { FindOrder = fun id -> task { return Map.tryFind id testData }
      SaveOrder = fun order -> task { savedOrders <- order :: savedOrders }
      SendNotification = fun _ -> Task.CompletedTask }

[<Fact>]
let ``PlaceOrder saves the confirmed order`` () = task {
    let mutable saved = []
    let deps =
        { createTestDeps () with
            SaveOrder = fun order -> task { saved <- order :: saved } }

    let! _ = OrderService.placeOrder deps validRequest

    test <@ saved.Length = 1 @>
}

NSubstitute for .NET Interfaces

fsharp
open NSubstitute

[<Fact>]
let ``calls repository with correct ID`` () = task {
    let repo = Substitute.For<IOrderRepository>()
    repo.FindByIdAsync(Arg.Any<Guid>(), Arg.Any<CancellationToken>())
        .Returns(Task.FromResult(Some testOrder))

    let service = OrderService(repo)
    let! _ = service.GetOrder(testOrder.Id, CancellationToken.None)

    do! repo.Received(1).FindByIdAsync(testOrder.Id, Arg.Any<CancellationToken>())
}

ASP.NET Core Integration Tests

fsharp
type OrderApiTests (factory: WebApplicationFactory<Program>) =
    interface IClassFixture<WebApplicationFactory<Program>>

    let client =
        factory.WithWebHostBuilder(fun builder ->
            builder.ConfigureServices(fun services ->
                services.RemoveAll<DbContextOptions<AppDbContext>>() |> ignore
                services.AddDbContext<AppDbContext>(fun options ->
                    options.UseInMemoryDatabase("TestDb") |> ignore) |> ignore))
            .CreateClient()

    [<Fact>]
    member _.``GET order returns 404 when not found`` () = task {
        let! response = client.GetAsync($"/api/orders/{Guid.NewGuid()}")
        test <@ response.StatusCode = HttpStatusCode.NotFound @>
    }

Test Organization

tests/
  MyApp.Tests/
    Unit/
      OrderServiceTests.fs
      PaymentServiceTests.fs
    Integration/
      OrderApiTests.fs
      OrderRepositoryTests.fs
    Properties/
      OrderPropertyTests.fs
    Helpers/
      TestData.fs
      TestDeps.fs

Common Anti-Patterns

| Anti-Pattern | Fix | |---|---| | Testing implementation details | Test behavior and outcomes | | Mutable shared test state | Fresh state per test | | Thread.Sleep in async tests | Use Task.Delay with timeout, or polling helpers | | Asserting on sprintf output | Assert on typed values and pattern matches | | Ignoring CancellationToken | Always pass and verify cancellation | | Skipping property-based tests | Use FsCheck for any function with clear invariants |

Related Skills

• dotnet-patterns - Idiomatic .NET patterns, dependency injection, and architecture
• csharp-testing - C# testing patterns (shared infrastructure like WebApplicationFactory and Testcontainers applies to F# too)

Running Tests

bash
# Run all tests
dotnet test

# Run with coverage
dotnet test --collect:"XPlat Code Coverage"

# Run specific project
dotnet test tests/MyApp.Tests/

# Filter by test name
dotnet test --filter "FullyQualifiedName~OrderService"

# Watch mode during development
dotnet watch test --project tests/MyApp.Tests/