FluentAssertions Skill

FluentAssertions provides a fluent interface for writing test assertions in C#. The Sorcha codebase uses it across 1,100+ tests with xUnit. All assertions follow the subject.Should().BeX() pattern.

Quick Start

Basic Value Assertions

// Strings
wallet.Address.Should().NotBeNullOrEmpty();
blueprint.Title.Should().Be("Purchase Order");
error.Message.Should().Contain("validation");

// Numbers
blueprint.Version.Should().Be(5);
result.Count.Should().BeGreaterThan(0).And.BeLessThan(100);

// Booleans
result.IsSuccess.Should().BeTrue();
validation.IsValid.Should().BeFalse();

Exception Testing

// Sync exception testing
builder.Invoking(b => b.Build())
    .Should().Throw<InvalidOperationException>()
    .WithMessage("*title*");  // Wildcard matching

// Async exception testing
await Assert.ThrowsAsync<InvalidOperationException>(async () =>
    await _walletManager.CreateWalletAsync("Test", null!, "user1", "tenant1"));

Collection Assertions

blueprint.Participants.Should().HaveCount(2);
blueprint.Actions.Should().NotBeEmpty();
events.Should().ContainSingle(e => e is WalletCreatedEvent);
wallets.Should().AllSatisfy(w => w.Owner.Should().Be(owner));
docket.TransactionIds.Should().ContainInOrder("tx1", "tx2", "tx3");

Key Concepts

Concept	Usage	Example
Should()	Entry point for all assertions	value.Should().Be(expected)
And	Chain multiple assertions	.NotBeNull().And.HaveCount(2)
Which/WhoseValue	Access nested values	.ContainKey("x").WhoseValue.Should().Be("y")
Invoking	Test sync exceptions	obj.Invoking(x => x.Method())
Awaiting	Test async exceptions	obj.Awaiting(x => x.MethodAsync())

Common Patterns

Object Property Assertions

wallet.Should().NotBeNull();
wallet.Name.Should().Be("Test Wallet");
wallet.Status.Should().Be(WalletStatus.Active);
wallet.Metadata.Should().ContainKey("environment").WhoseValue.Should().Be("production");

Time-Based Assertions

docket.TimeStamp.Should().BeCloseTo(DateTime.UtcNow, TimeSpan.FromSeconds(1));
blueprint.UpdatedAt.Should().BeOnOrAfter(beforeBuild).And.BeOnOrBefore(afterBuild);

Validation Result Assertions

result.Validation.IsValid.Should().BeTrue();
result.Errors.Should().BeEmpty();

// Or for failures
result.Success.Should().BeFalse();
result.Validation.Errors.Should().NotBeEmpty();
result.Errors.Should().Contain(e => e.Contains("validation"));

See Also

• patterns
• workflows

Related Skills

• See the xunit skill for test framework patterns
• See the moq skill for mocking dependencies in tests

Documentation Resources

Fetch latest FluentAssertions documentation with Context7.

How to use Context7:

1. Use mcp__context7__resolve-library-id to search for "fluent-assertions"
2. Prefer website documentation (IDs starting with /websites/) over source code repositories when available
3. Query with mcp__context7__query-docs using the resolved library ID

Library ID: /fluentassertions/fluentassertions

Recommended Queries:

• "FluentAssertions basic assertions Should Be"
• "FluentAssertions collection assertions HaveCount Contain"
• "FluentAssertions exception testing ThrowAsync"
• "FluentAssertions object equivalence BeEquivalentTo"