Expect

Expression-based Assertion Library for .NET

English | 日本語

Expect is an assertion library for .NET.

The concept of Expect is inspired by power-assert and swift-testing, offering better assertions using Expression Trees.

Why Expect?

In test frameworks like NUnit, assertions are written using Assert.That. Below is an example of a unit test using NUnit:

// Define a record for testing record Person(string Name, int Age); [Test] public void Example() { var person = new Person("Alice", 16); Assert.That(person.Age, Is.GreaterThanOrEqualTo(18)); }

This test will fail, displaying the following message:

Expected: greater than or equal to 18 But was: 16

The problem with assertions using Assert.That is that users are required to distinguish between a large number of Is.** usages. Many test frameworks provide a vast number of APIs to cover all use cases, forcing users to memorize them.

Some assertion libraries adopt a Fluent Interface, such as . However, I believe such APIs should not be used, as they reduce readability. Overly verbose Fluent Interfaces, modeled after natural language, are not necessarily easy to read. Tests should not aim to mimic natural language but instead be programmer-friendly and written in readable code.