Apartment

ApartmentAttribute is used to specify that tests should run in a particular COM apartment state, either STA (Single-Threaded Apartment) or MTA (Multi-Threaded Apartment).

This is primarily needed for tests that interact with COM objects, Windows Forms controls, or WPF components that require a specific apartment state.

Constructor

ApartmentAttribute(ApartmentState apartmentState)

Parameter	Type	Description
`apartmentState`	`ApartmentState`	The apartment state for the test. Must be `STA` or `MTA` (not `Unknown`).

ApartmentState Values

Value	Description
`ApartmentState.STA`	Single-Threaded Apartment. Required for most UI components (WinForms, WPF, COM controls).
`ApartmentState.MTA`	Multi-Threaded Apartment. The default for NUnit tests.

Applies To

Test Methods	Test Fixtures (Classes)	Assembly
✅	✅	✅

When applied at multiple levels, the most specific level takes precedence.

Examples

Assembly Level

// All tests in this assembly will use the STA by default
[assembly: Apartment(ApartmentState.STA)]

Test Fixture Level

[TestFixture]
[Apartment(ApartmentState.STA)]
public class WinFormsTests
{
    [Test]
    public void TestRunsInSTA()
    {
        // All tests in this fixture run in the STA
        Assert.That(Thread.CurrentThread.GetApartmentState(), Is.EqualTo(ApartmentState.STA));
    }
}

Test Method Level

[TestFixture]
public class MixedApartmentTests
{
    [Test]
    [Apartment(ApartmentState.STA)]
    public void TestRequiringSTA()
    {
        Assert.That(Thread.CurrentThread.GetApartmentState(), Is.EqualTo(ApartmentState.STA));
    }

    [Test]
    [Apartment(ApartmentState.MTA)]
    public void TestRequiringMTA()
    {
        Assert.That(Thread.CurrentThread.GetApartmentState(), Is.EqualTo(ApartmentState.MTA));
    }
}

Notes

When this attribute is not specified, tests run in the MTA by default.
When running tests in parallel, tests are scheduled to execute on a thread with the specified apartment state.
When parallel execution is disabled, a new thread may be created if the current thread doesn't have the correct apartment state.
This attribute replaces the obsolete RequiresMTA and RequiresSTA attributes.
The ApartmentState.Unknown value is not allowed and will cause an error.

STA Tests and Parallel Execution

NUnit has exactly one worker thread for parallelizable STA tests, however high the level of parallelism is set. As a consequence:

STA tests never run in parallel with each other, even when they are marked [Parallelizable]. They are queued and executed one at a time on that single STA worker, which is why tests in different STA fixtures report the same thread id.
STA tests do run in parallel with MTA tests. The STA worker is active at the same time as the MTA workers, so a parallelizable STA test can overlap with any parallelizable MTA test.
Non-parallelizable STA tests have their own single worker, which runs after all parallelizable work has finished.

If you need several STA tests to run at the same time, use RequiresThread with ApartmentState.STA instead. Each such test gets its own new STA thread, so parallelizable tests marked that way can overlap with each other.

See Framework Parallel Test Execution for the details of the queues involved.