By default, xUnit runs all test cases in a test class synchronously. This package extends the default test framework to execute tests in parallel.

dotnet add package Meziantou.Xunit.ParallelTestFramework

By default, all tests will run in parallel. Selectively disable parallelization by adding the DisableParallelization attribute to a collection or theory.

// All tests are run in parallel
public class ParallelTests
{
    [Fact]
    public void Test1() => Thread.Sleep(2000);

    [Fact]
    public void Test2() => Thread.Sleep(2000);

    [Theory]
    [InlineData(0), InlineData(1), InlineData(2)]
    public void Test3(int value) => Thread.Sleep(2000);

    // This test runs in parallel with other tests
    // However, its test cases are run sequentially
    [Theory]
    [DisableParallelization]
    [InlineData(0), InlineData(1), InlineData(2)]
    public void Test4(int value) => Thread.Sleep(2000);
}

// This collection runs in parallel with other collections
// However, its tests cases are run sequentially
[DisableParallelization]
public class SequentialTests
{
    [Fact]
    public void Test1() => Thread.Sleep(2000);

    [Fact]
    public void Test2() => Thread.Sleep(2000);
}

Previous versions of this package relied on built in xUnit attributes instead of exposing a dedicated DisableParallelization attribute. For backwards compatibility, parallelization can also be disabled by adding an explicit Collection attribute or Theory attribute with DisableDiscoveryEnumeration enabled.

// All tests are run in parallel
public class ParallelTests
{
    [Fact]
    public void Test1() => Thread.Sleep(2000);

    [Fact]
    public void Test2() => Thread.Sleep(2000);

    [Theory]
    [InlineData(0), InlineData(1), InlineData(2)]
    public void Test3(int value) => Thread.Sleep(2000);

    // This test runs in parallel with other tests
    // However, its test cases are run sequentially because of DisableDiscoveryEnumeration
    [Theory]
    [MemberData(nameof(GetData), DisableDiscoveryEnumeration = true)]
    public void Test4(int value) => Thread.Sleep(2000);

    public static TheoryData<int> GetData() =>  new() { { 0 }, { 1 } };
}

// This collection runs in parallel with other collections
// However, its tests cases are run sequentially because the Collection is explicit
[Collection("Sequential")]
public class SequentialTests
{
    [Fact]
    public void Test1() => Thread.Sleep(2000);

    [Fact]
    public void Test2() => Thread.Sleep(2000);
}

The code is greatly inspired by the sample from Travis Mortimer: xunit/xunit#1986 (comment)