FluentAssertions vs Shouldly: Assertion Libraries for Modern .NET

 

Introduction

Assertions are the voice of your tests.

They’re the part everyone reads during:

• code reviews

• CI failures

• late-night debugging sessions

Two libraries dominate the modern .NET space:

• FluentAssertions

• Shouldly

Both dramatically improve upon classic Assert.Equal(...). But they differ in philosophy, syntax, and licensing, and that last point matters more than many teams expect.

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What Assertion Libraries Are Really For

Assertions should:

1. Clearly express intent

2. Produce readable failure messages

3. Stay out of the way

If your assertions are noisy, your tests become harder to understand than the code they test.

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FluentAssertions: Overview

FluentAssertions uses a chainable, fluent syntax:

result.Should().Be(42);

It’s expressive, powerful, and very popular.

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Shouldly: Overview

Shouldly uses a BDD-style “should” syntax:

result.ShouldBe(42);

It focuses on:

• minimal ceremony

• extremely readable failure messages

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Simple Comparison

FluentAssertions

user.Name.Should().Be("Alice");

Shouldly

user.Name.ShouldBe("Alice");

Both are clear, but Shouldly removes the fluent chain entirely.

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Failure Messages (Huge Difference)

FluentAssertions Failure

Expected result to be 5, but found 4.

Shouldly Failure

result
    should be 5
    but was 4

Shouldly is famous for pointing directly at the expression you wrote, not just the values.

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Collections

FluentAssertions

users.Should().ContainSingle(u => u.Id == id);

Shouldly

users.Count.ShouldBe(1);
users.Single().Id.ShouldBe(id);

FluentAssertions shines for:

• complex collection assertions

• deep object graphs

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Object Equivalence

FluentAssertions

actual.Should().BeEquivalentTo(expected);

Supports:

• deep comparisons

• excluding properties

• custom rules

Shouldly

actual.ShouldBe(expected);

Simple, but less configurable.

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Exception Assertions

FluentAssertions

action.Should()
      .Throw<InvalidOperationException>()
      .WithMessage("*invalid*");

Shouldly

Should.Throw<InvalidOperationException>(() => action());

Both are readable. FluentAssertions offers more chaining options.

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Async Assertions

FluentAssertions

await action.Should()
            .ThrowAsync<InvalidOperationException>();

Shouldly

await Should.ThrowAsync<InvalidOperationException>(action);

Comparable clarity—Shouldly remains minimal.

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The Licensing Elephant in the Room

FluentAssertions Licensing (Important)

• Versions before v8: permissive open-source

• v8+: requires a commercial license for commercial use

This caught many teams off-guard during upgrades.

Shouldly Licensing

• MIT License

• Fully open-source

• No commercial restrictions

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Why This Matters

For commercial projects:

• Legal approval

• Procurement

• CI dependency policies

A “small” assertion library suddenly becomes a legal decision.

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Team Readability & PR Reviews

Shouldly tests read like sentences:

result.ShouldBeGreaterThan(0);

This is especially valuable when:

• onboarding juniors

• reviewing large test suites

• debugging CI failures

FluentAssertions is powerful, but sometimes verbose.

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Performance & Overhead

Both are fast enough for real-world testing.

However:

• FluentAssertions has a larger API surface

• Shouldly has fewer abstractions

In massive test suites, simplicity matters.

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When FluentAssertions Is the Right Choice

Choose FluentAssertions if:

• You need deep equivalency rules

• You do heavy collection assertions

• You already have a license or are non-commercial

• You value expressive chaining

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When Shouldly Is the Better Default

Choose Shouldly if:

• You want zero licensing risk

• You value minimal syntax

• You want the clearest failure messages

• You prefer BDD-style tests

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Recommended Modern Stack (2025)

For new commercial .NET projects:

xUnit / NUnit + NSubstitute + Shouldly

This stack:

• avoids licensing surprises

• produces readable tests

• encourages behaviour-focused testing

• stays refactor-friendly

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Final Verdict

Assertion libraries don’t just assert values, they shape how you think about tests.

If you want:

• power and configurability → FluentAssertions

• clarity and simplicity → Shouldly

For most teams starting fresh today, Shouldly is the safer, cleaner default.