Fsharp Testing

This covers the full F# testing stack: xUnit for structure, FsUnit for idiomatic assertions, Unquote for quotation-based test failures that show you exactly what broke, and FsCheck for property-based testing. The examples are solid, from basic unit tests through async patterns, custom generators, and ASP.NET Core integration tests with WebApplicationFactory. The function stub approach for mocking is refreshingly practical compared to heavyweight mocking frameworks. Activate this when you're writing F# tests or setting up a test suite, especially if you want to move beyond basic assertions into property-based testing. The anti-patterns section alone is worth reading.

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last updated: Aug 21, 2026

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Install

$ npx -y skills add affaan-m/ecc --skill fsharp-testing --agent claude-code

Installs into .claude/skills of the current project.

Files

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

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

SKILL.md on GitHub