FluentAssertions Skill

FluentAssertions provides a fluent API for expressing test assertions in C#. In VanDaemon, it's used alongside xUnit and Moq for testing services, controllers, plugins, and the JsonFileStore persistence layer. The library produces clear failure messages that explain exactly what went wrong.

Quick Start

Basic Value Assertions

// From VanDaemon test patterns
result.Should().NotBeNull();
result.Should().Be(expectedValue);
tank.CurrentLevel.Should().BeInRange(0, 100);
tank.IsActive.Should().BeTrue();

Collection Assertions

// Testing tank service returns
var tanks = await tankService.GetAllTanksAsync();
tanks.Should().NotBeNull();
tanks.Should().HaveCount(3);
tanks.Should().AllSatisfy(t => t.IsActive.Should().BeTrue());
tanks.Should().Contain(t => t.Type == TankType.FreshWater);

Async Operation Assertions

// Testing async service methods
await service.Invoking(s => s.GetTankByIdAsync(Guid.Empty))
    .Should().ThrowAsync<ArgumentException>();

var result = await service.GetAllControlsAsync();
result.Should().BeEquivalentTo(expectedControls);

Key Concepts

Concept	Usage	Example
`Should()`	Entry point for all assertions	`value.Should().Be(5)`
`BeEquivalentTo()`	Deep object comparison	`actual.Should().BeEquivalentTo(expected)`
`AllSatisfy()`	Assert condition on all items	`list.Should().AllSatisfy(x => x.IsActive.Should().BeTrue())`
`Invoking()`	Test exception throwing	`obj.Invoking(x => x.Method()).Should().Throw<T>()`
`Because()`	Custom failure message	`value.Should().Be(5, "configuration requires this")`

Common Patterns

Service Response Validation

When: Testing application services like TankService, ControlService

var mockFileStore = new Mock<IJsonFileStore>();
mockFileStore.Setup(x => x.LoadAsync<List<Tank>>("tanks.json", It.IsAny<CancellationToken>()))
    .ReturnsAsync(testTanks);

var service = new TankService(logger, mockFileStore.Object);
var result = await service.GetAllTanksAsync();

result.Should().NotBeNull();
result.Should().HaveCount(2);
result.First().Name.Should().Be("Fresh Water");

Entity State Assertions

When: Validating domain entity properties

var tank = new Tank { Type = TankType.FreshWater, AlertLevel = 10 };
tank.Type.Should().Be(TankType.FreshWater);
tank.AlertLevel.Should().BeGreaterThanOrEqualTo(0);
tank.AlertWhenOver.Should().BeFalse("fresh water alerts when level drops below threshold");

Related Skills

xunit skill for test framework fundamentals
moq skill for mocking dependencies before asserting results
csharp skill for language features used in assertions

fluent-assertions