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F Testing Patterns

F# testing patterns with xUnit, FsUnit, Unquote, FsCheck property-based testing, integration tests, and test organization best practices.

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Buy it · $59 Read it before you buy $59 Written by affaan-m · unverified publisher
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MITpaid listing
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What it does

F# testing patterns with xUnit, FsUnit, Unquote, FsCheck property-based testing, integration tests, and test organization best practices. Use when writing F# tests with xUnit, FsUnit, Unquote, or FsCheck.

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securitytesting
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Security & Testing

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SKILL.md7.9 kB · 282 lines
--- name: fsharp-testing description: F# testing patterns with xUnit, FsUnit, Unquote, FsCheck property-based testing, integration tests, and test organization best practices. Use when writing F# tests with xUnit, FsUnit, Unquote, or FsCheck. metadata: origin: ECC ---
8# F# Testing Patterns
9
10Comprehensive testing patterns for F# applications using xUnit, FsUnit, Unquote, FsCheck, and modern .NET testing practices.
11
12## When to Activate
13
14- Writing new tests for F# code
15- Reviewing test quality and coverage
16- Setting up test infrastructure for F# projects
17- Debugging flaky or slow tests
18
19## Test Framework Stack
20
21| Tool | Purpose |
22|---|---|
23| **xUnit** | Test framework (standard .NET ecosystem choice) |
24| **FsUnit.xUnit** | F#-friendly assertion syntax for xUnit |
25| **Unquote** | Assertion library using F# quotations for clear failure messages |
26| **FsCheck.xUnit** | Property-based testing integrated with xUnit |
27| **NSubstitute** | Mocking .NET dependencies |
28| **Testcontainers** | Real infrastructure in integration tests |
29| **WebApplicationFactory** | ASP.NET Core integration tests |
30
31## Unit Tests with xUnit + FsUnit
32
33### Basic Test Structure
34
35```fsharp
36module OrderServiceTests
37
38open Xunit
39open FsUnit.Xunit
40
41[<Fact>]
42let `create sets status to Pending` () =
43 let order = Order.create "cust-1" [ validItem ]
44 order.Status |> should equal Pending
45
46[<Fact>]
47let `confirm changes status to Confirmed` () =
48 let order = Order.create "cust-1" [ validItem ]
49 let confirmed = Order.confirm order
50 confirmed.Status |> should be (ofCase <@ Confirmed @>)
51```
52
53### Assertions with Unquote
54
55Unquote uses F# quotations so failure messages show the full expression that failed, not just "expected X got Y".
56
57```fsharp
58module OrderValidationTests
59
60open Xunit
61open Swensen.Unquote
62
63[<Fact>]
64let `PlaceOrder returns success when request is valid` () =
65 let request = { CustomerId = "cust-123"; Items = [ validItem ] }
66 let result = OrderService.placeOrder request
67 test <@ Result.isOk result @>
68
69[<Fact>]
70let `order total sums item prices` () =
71 let items = [ { Sku = "A"; Quantity = 2; Price = 10m }
72 { Sku = "B"; Quantity = 1; Price = 5m } ]
73 let total = Order.calculateTotal items
74 test <@ total = 25m @>
75
76[<Fact>]
77let `validated email rejects empty input` () =
78 let result = ValidatedEmail.create ""
79 test <@ Result.isError result @>
80```
81
82### Async Tests
83
84```fsharp
85[<Fact>]
86let `PlaceOrder returns success when request is valid` () = task {
87 let deps = createTestDeps ()
88 let request = { CustomerId = "cust-123"; Items = [ validItem ] }
89
90 let! result = OrderService.placeOrder deps request
91
92 test <@ Result.isOk result @>
93}
94
95[<Fact>]
96let `PlaceOrder returns error when items are empty` () = task {
97 let deps = createTestDeps ()
98 let request = { CustomerId = "cust-123"; Items = [] }
99
100 let! result = OrderService.placeOrder deps request
101
102 test <@ Result.isError result @>
103}
104```
105
106### Parameterized Tests with Theory
107
108```fsharp
109[<Theory>]
110[<InlineData("")>]
111[<InlineData(" ")>]
112let `PlaceOrder rejects empty customer ID` (customerId: string) =
113 let request = { CustomerId = customerId; Items = [ validItem ] }
114 let result = OrderService.placeOrder request
115 result |> should be (ofCase <@ Error @>)
116
117[<Theory>]
118[<InlineData("", false)>]
119[<InlineData("a", false)>]
120[<InlineData("user@example.com", true)>]
121[<InlineData("user+tag@example.co.uk", true)>]
122let `IsValidEmail returns expected result` (email: string, expected: bool) =
123 test <@ EmailValidator.isValid email = expected @>
124```
125
126## Property-Based Testing with FsCheck
127
128### Using FsCheck.xUnit
129
130```fsharp
131open FsCheck
132open FsCheck.Xunit
133
134[<Property>]
135let `order total is always non-negative` (items: NonEmptyList<PositiveInt * decimal>) =
136 let orderItems =
137 items.Get
138 |> List.map (fun (qty, price) ->
139 { Sku = "SKU"; Quantity = qty.Get; Price = abs price })
140 let total = Order.calculateTotal orderItems
141 total >= 0m
142
143[<Property>]
144let `serialization roundtrips` (order: Order) =
145 let json = JsonSerializer.Serialize order
146 let deserialized = JsonSerializer.Deserialize<Order> json
147 deserialized = order
148```
149
150### Custom Generators
151
152```fsharp
153type OrderGenerators =
154 static member ValidEmail () =
155 gen {
156 let! user = Gen.elements [ "alice"; "bob"; "carol" ]
157 let! domain = Gen.elements [ "example.com"; "test.org" ]
158 return $"{user}@{domain}"
159 }
160 |> Arb.fromGen
161
162[<Property(Arbitrary = [| typeof<OrderGenerators> |])>]
163let `valid emails pass validation` (email: string) =
164 EmailValidator.isValid email
165```
166
167## Mocking Dependencies
168
169### Function Stubs (Preferred)
170
171```fsharp
172let createTestDeps () =
173 let mutable savedOrders = []
174 { FindOrder = fun id -> task { return Map.tryFind id testData }
175 SaveOrder = fun order -> task { savedOrders <- order :: savedOrders }
176 SendNotification = fun _ -> Task.CompletedTask }
177
178[<Fact>]
179let `PlaceOrder saves the confirmed order` () = task {
180 let mutable saved = []
181 let deps =
182 { createTestDeps () with
183 SaveOrder = fun order -> task { saved <- order :: saved } }
184
185 let! _ = OrderService.placeOrder deps validRequest
186
187 test <@ saved.Length = 1 @>
188}
189```
190
191### NSubstitute for .NET Interfaces
192
193```fsharp
194open NSubstitute
195
196[<Fact>]
197let `calls repository with correct ID` () = task {
198 let repo = Substitute.For<IOrderRepository>()
199 repo.FindByIdAsync(Arg.Any<Guid>(), Arg.Any<CancellationToken>())
200 .Returns(Task.FromResult(Some testOrder))
201
202 let service = OrderService(repo)
203 let! _ = service.GetOrder(testOrder.Id, CancellationToken.None)
204
205 do! repo.Received(1).FindByIdAsync(testOrder.Id, Arg.Any<CancellationToken>())
206}
207```
208
209## ASP.NET Core Integration Tests
210
211```fsharp
212type OrderApiTests (factory: WebApplicationFactory<Program>) =
213 interface IClassFixture<WebApplicationFactory<Program>>
214
215 let client =
216 factory.WithWebHostBuilder(fun builder ->
217 builder.ConfigureServices(fun services ->
218 services.RemoveAll<DbContextOptions<AppDbContext>>() |> ignore
219 services.AddDbContext<AppDbContext>(fun options ->
220 options.UseInMemoryDatabase("TestDb") |> ignore) |> ignore))
221 .CreateClient()
222
223 [<Fact>]
224 member _.`GET order returns 404 when not found` () = task {
225 let! response = client.GetAsync($"/api/orders/{Guid.NewGuid()}")
226 test <@ response.StatusCode = HttpStatusCode.NotFound @>
227 }
228```
229
230## Test Organization
231
232```
233tests/
234 MyApp.Tests/
235 Unit/
236 OrderServiceTests.fs
237 PaymentServiceTests.fs
238 Integration/
239 OrderApiTests.fs
240 OrderRepositoryTests.fs
241 Properties/
242 OrderPropertyTests.fs
243 Helpers/
244 TestData.fs
245 TestDeps.fs
246```
247
248## Common Anti-Patterns
249
250| Anti-Pattern | Fix |
251|---|---|
252| Testing implementation details | Test behavior and outcomes |
253| Mutable shared test state | Fresh state per test |
254| Thread.Sleep in async tests | Use Task.Delay with timeout, or polling helpers |
255| Asserting on sprintf output | Assert on typed values and pattern matches |
256| Ignoring CancellationToken | Always pass and verify cancellation |
257| Skipping property-based tests | Use FsCheck for any function with clear invariants |
258
259## Related Skills
260
261- dotnet-patterns - Idiomatic .NET patterns, dependency injection, and architecture
262- csharp-testing - C# testing patterns (shared infrastructure like WebApplicationFactory and Testcontainers applies to F# too)
263
264## Running Tests
265
266```bash
267# Run all tests
268dotnet test
269
270# Run with coverage
271dotnet test --collect:"XPlat Code Coverage"
272
273# Run specific project
274dotnet test tests/MyApp.Tests/
275
276# Filter by test name
277dotnet test --filter "FullyQualifiedName~OrderService"
278
279# Watch mode during development
280dotnet watch test --project tests/MyApp.Tests/
281```
282
In the file
SKILL.md1,029 words
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LicenceMIT
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$59 once
F Testing Patterns · MIT · affaan-m
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LicenceMIT — the author’s, unchanged by this purchase
Paid throughStripe, once, on the card you add at the checkout
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