How do [Fact] and [Theory] differ in xUnit, and how do you pass datasets with [InlineData]?

Short answer

[Fact] is one test that runs once with fixed conditions, while [Theory] accepts parameters and runs multiple times for each [InlineData] or [MemberData] row.

Explanation

To test an email validator with ten different inputs, writing ten [Fact] methods is unnecessary duplication. Use [Theory] and [InlineData('[email protected]', true)] to pass valid and invalid values to the same test and check edge cases in one concise method.

Code example

[Theory]
[InlineData(2, true)]
[InlineData(3, false)]
[InlineData(4, true)]
public void IsEven_ReturnsCorrectResult(int number, bool expected) {
    Assert.Equal(expected, MathUtils.IsEven(number));
}

Common mistakes

• Passing complex, non-constant objects to [InlineData]; it accepts only constants, so use [MemberData] or [ClassData] for complex objects.

Follow-up questions

• What is xUnit's design philosophy, and how does it isolate tests by creating a new class instance for each test?

Sources

• Microsoft Learn — Create unit tests with xUnit