Dotnet Testing Assertions

Overview

Standardise test assertions on open-source libraries, preferring AwesomeAssertions (the community fork of FluentAssertions) for fluent, readable assertions with better error messages. Enforce OSS license revalidation for all test dependencies.

When to Use

• Establishing or reviewing test conventions for a new project
• Introducing or upgrading assertion libraries in existing projects
• Migrating from FluentAssertions to AwesomeAssertions
• Reviewing PRs that add new test dependencies
• Evaluating assertion library license compliance

Core Workflow

1. Verify the assertion library is open-source (check license)
2. Add AwesomeAssertions package reference to test projects
3. Update using statements to reference AwesomeAssertions namespace
4. Write fluent assertions using Should() extension methods
5. Use BeEquivalentTo for DTOs with exclusions for generated fields
6. Use BeCloseTo for time assertions to avoid brittle exact matches
7. Run tests to verify assertions work correctly

Core

When to use

• Establishing or reviewing test conventions.
• Introducing or upgrading assertion libraries.

Defaults

• Prefer AwesomeAssertions (formerly FluentAssertions) for fluent assertions (open source).
• Test framework selection is separate (e.g., xUnit) and may be governed elsewhere.

Note: AwesomeAssertions is the community-maintained fork of FluentAssertions after the original project returned to open source governance. The API is largely compatible, making migration straightforward for existing FluentAssertions users.

Review rules

• Do not introduce assertion libraries that are not open source.
• Apply the OSS license revalidation gate (see dotnet-open-source-first-governance).

Load: examples

• Assert equivalence for DTOs and read models.
• Assert exception types and messages where stable.
• Use precise assertions for time, GUIDs, collections, and nullable scenarios.

Load: advanced

• Avoid brittle assertions that over-specify implementation details.
• Prefer structural assertions for API contract tests.
• For non-deterministic values (timestamps/IDs), assert invariants and ranges.

Load: enforcement

• Reject PRs that introduce non-open-source assertion libraries.
• Require a documented license verification for new/updated test dependencies.

Load: migration

Migration Checklist (FluentAssertions → AwesomeAssertions)

• Update NuGet packages: Replace FluentAssertions with AwesomeAssertions
• Update using statements: using FluentAssertions; → using AwesomeAssertions;
• Run tests: Verify all existing tests pass (API is largely compatible)
• Check breaking changes: Review release notes for any API differences
• Update CI: Ensure package restore works in CI pipeline
• Document decision: Add ADR noting the migration and rationale

Sample Assertion Diff

// Package reference
- <PackageReference Include="FluentAssertions" Version="6.12.0" />
+ <PackageReference Include="AwesomeAssertions" Version="7.0.0" />

// Using statement
- using FluentAssertions;
+ using AwesomeAssertions;

// Assertions remain the same
result.Should().NotBeNull();
result.Items.Should().HaveCount(3);
result.CreatedAt.Should().BeCloseTo(DateTime.UtcNow, TimeSpan.FromSeconds(1));

Common Assertion Patterns

// Object equivalence (ignoring specific properties)
actual.Should().BeEquivalentTo(expected, options => options
    .Excluding(x => x.Id)
    .Excluding(x => x.CreatedAt));

// Collection assertions
items.Should().ContainSingle(x => x.Name == "Test");
items.Should().BeInAscendingOrder(x => x.Priority);
items.Should().AllSatisfy(x => x.IsValid.Should().BeTrue());

// Exception assertions
action.Should().Throw<ValidationException>()
    .WithMessage("*required*");

// Async assertions
await action.Should().ThrowAsync<NotFoundException>();

xUnit vs AwesomeAssertions Comparison

Red Flags - STOP

These statements indicate assertion anti-patterns:

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