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:

• Use mcp__context7__resolve-library-id to search for "fluent-assertions"

• Prefer website documentation (IDs starting with /websites/ ) over source code repositories when available

• 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"