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

Short answer

xUnit creates a new test class instance for every [Fact] method to isolate state and prevent interference; it uses the constructor and IDisposable instead of Setup and Teardown.

Explanation

In NUnit, the same class instance is reused across tests unless configured otherwise, which may cause accidental state sharing. xUnit enforces strict isolation: it creates each test class with its constructor and disposes it through Dispose(). To share expensive data across tests, xUnit provides Fixtures through IClassFixture<T>.

Code example

public class CalculatorTests : IDisposable {
    private readonly Calculator _calc = new(); // Clean and new for each Fact

    [Fact]
    public void AddsCorrectly() => Assert.Equal(4, _calc.Add(2, 2));

    public void Dispose() { /* Clean up after every test */ }
}

Common mistakes

• Relying on public class variables to share state between xUnit tests, not realizing each test has a separate instance.

Follow-up questions

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

Sources

• Microsoft Learn — Unit testing C# with xUnit and .NET