LevelOfParallelism

LevelOfParallelismAttribute is an assembly-level attribute used to specify the maximum number of worker threads that may execute tests simultaneously. This controls how many tests can run in parallel.

Constructor

LevelOfParallelismAttribute(int level)
Parameter Type Description
level int The maximum number of worker threads to be created by the framework for running tests.

Applies To

Test Methods Test Fixtures (Classes) Assembly
❌ ❌ ✅

Typically placed in AssemblyInfo.cs or a global usings file.

Default Value

If this attribute is not specified, NUnit uses Environment.ProcessorCount or 2, whichever is greater. For example, on an 8-core machine, the default is 8 worker threads.

Example

The following code, placed in AssemblyInfo.cs, limits parallel execution to 3 worker threads:

using NUnit.Framework;

[assembly: LevelOfParallelism(3)]

Notes

  1. This attribute controls the maximum number of parallel workers. The actual number may be lower depending on the tests and their parallelization settings.
  2. Tests must be marked with [Parallelizable] to actually run in parallel. This attribute only sets the upper limit on worker threads.
  3. Setting the level to 1 effectively disables parallelism (only one test runs at a time).
  4. This value can be overridden using the --workers command-line option in the NUnit console runner.
  5. Consider your test dependencies and shared resources when setting this value. Higher parallelism can cause issues with tests that share state.

See Also