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Fluent Assertions

"With Fluent Assertions, the assertions look beautiful, natural and most importantly, extremely readable."

What is Fluent Assertions?

Fluent Assertions is a set of .NET extension methods that allow you to more naturally specify the expected outcome of a TDD or BDD-style test.


To verify that a string begins, ends and contains a particular phrase.

string actual = "ABCDEFGHI";

To verify that a collection contains a specified number of elements and that all elements match a predicate.

IEnumerable collection = new[] { 1, 2, 3 };
collection.Should().HaveCount(4, "because we thought we put three items in the collection"))

The nice thing about the second failing example is that it will throw an exception with the message

“Expected <4> items because we thought we put three items in the collection, but found <3>.”

To verify that a particular business rule is enforced using exceptions.

var recipe = new RecipeBuilder()
                    .With(new IngredientBuilder().For("Milk").WithQuantity(200, Unit.Milliliters))
Action action = () => recipe.AddIngredient("Milk", 100, Unit.Spoon);
                    .WithMessage("change the unit of an existing ingredient", ComparisonMode.Substring)

One neat feature is the ability to chain a specific assertion on top of an assertion that acts on a collection or graph of objects.

someObject.Should().BeOfType<Exception>().Which.Message.Should().Be("Other Message");
xDocument.Should().HaveElement("child").Which.Should().BeOfType<XElement>().And.HaveAttribute("attr", "1");

I’ve run into quite a few of these scenarios in which this chaining would make the unit test a lot easier to read.


Nothing is more annoying than a unit test that fails without clearly explaining why. More than often, you need to set a breakpoint and start up the debugger to be able to figure out what went wrong. Jeremy D. Miller once gave the advice to “keep out of the debugger hell” and I can only agree with that.

For instance, only test a single condition per test case. If you don’t, and the first condition fails, the test engine will not even try to test the other conditions. But if any of the others fail, you’ll be on your own to figure out which one. I often run into this problem when developers try to combine multiple related tests that test a member using different parameters into one test case. If you really need to do that, consider using a parameterized test that is being called by several clearly named test cases.

That’s why we designed Fluent Assertions to help you in this area. Not only by using clearly named assertion methods, but also by making sure the failure message provides as much information as possible. Consider this example:


This will be reported as:

Expected string to be “0987654321”, but “1234567890” differs near “123” (index 0).

The fact that both strings are displayed on a separate line is not a coincidence and happens if any of them is longer than 8 characters. However, if that’s not enough, all assertion methods take an optional explanation (the because) that supports formatting placeholders similar to String.Format which you can use to enrich the failure message. For instance, the assertion

new[] { 1, 2, 3 }.Should().Contain(item => item > 3, "at least {0} item should be larger than 3", 1);

will fail with:

Collection {1, 2, 3} should have an item matching (item > 3) because at least 1 item should be larger than 3.

Supported Frameworks and Libraries

It supports the following .NET versions.

It supports the following unit test frameworks:

About versioning

The version numbers of Fluent Assertions releases comply to the Semantic Versioning scheme. In other words, release 1.4.0 only adds backwards-compatible functionality and bug fixes compared to 1.3.0. Release 1.4.1 should only include bug fixes. And if we ever introduce breaking changes, the number increased to 2.0.0.

What do you need to compile the solution?

Who are we?

We are a bunch of developers working for Aviva Solutions who highly value software quality, in particular

Notable contributors from the last couple of months include

The Xamarin version has been built by

If you have any comments or suggestions, please let us know via twitter, through the issues page, or through StackOverflow.

Community Extensions

There are a number of community maintained extension projects. The ones we are aware of a listed below. To add yours please fork the repository and send a pull request.

Special thanks

This project would not have been possible without the support of JetBrains. We thank them generously for providing us with the ReSharper licenses necessary to make us productive developers.