Attribute Interfaces

Charlie Poole edited this page Jan 31, 2016 · 1 revision
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NUnit 3.0 implements a great deal of its functionality in its attributes. This functionality is accessed through a number of standard interfaces, which are implemented by the attributes. This technical note describes the interfaces used on NUnit attributes and recognized by NUnit when loading or running tests.

For ease of understanding, the interfaces are grouped according to the stage in the life-cycle of a test at which they are used. The two primary stages in the life of a test are Load-Time and Execution-Time.

Load-Time Interfaces

Loading tests means loading the assembly into memory and examining its content to discover the classes and fixtures that represent tests. The internal structures that represent tests are built at this time. If requested by the application, information about the tests may be returned for display, as is done in the NUnit GUI runner.


This interface is used by attributes that know how to build a fixture from a user class. IFixtureBuilder is defined as follows:

public interface IFixtureBuilder
    TestSuite BuildFrom(Type type);

Custom fixture builders should examine the provided Type and return an appropriate TestFixture based on it. If the fixture is intended to be an NUnit TestFixture, then the helper class NUnitTestFixtureBuilder may be used to create it.

The following NUnit attributes currently implement this interface:

  • TestFixtureAttribute
  • SetUpFixtureAttribute

It would make more sense for this interface method to return TestFixture rather than TestSuite. We use TestSuite because it is the common base for both TestFixture and SetupFixture. In a future version, we will try to adjust the hierarchy so that all suites based on a class are derived from TestFixture.


This interface is used by attributes that know how to build one or more parameterized TestMethod instances from a MethodInfo. ITestMethodBuilder is defined as follows:

public interface ITestBuilder
    IEnumerable<TestMethod> BuildFrom(MethodInfo method, Test suite);

Custom attributes implementing this interface should examine the MethodInfo and return as many TestMethod instances as it is able to construct, using the parameters available to it. Some attributes will only return a single test, just as TestCaseAttribute does. Others, working like TheoryAttribute may return multiple tests. If no data is available to create tests, an empty collection should be returned.

If the returned tests are standard NUnit TestMethods, the helper class NUnitTestCaseBuilder may be used to create them.

The following NUnit attributes currently implement ITestBuilder:

  • CombiningStrategyAttribute, with the following derived classes:
    • CombinatorialAttribute
    • PairwiseAttribute
    • SequentialAttribute
  • TestCaseAttribute
  • TestCaseSourceAttribute
  • TheoryAttribute


This interface is used by attributes that know how to build a single, non-parameterized test from a MethodInfo. ISimpleTestBuilder is defined as follows:

public interface ISimpleTestBuilder
    TestMethod BuildFrom(MethodInfo method, Test suite);

Custom attributes implementing ISimpleTestFixture should examine the MethodInfo provided and return a single TestMethod instance, as appropriate to that method. The BuildFrom method should never return null, even if the specified method is not valid for a test. In that case, it should return a TestMethod with a RunState of NonRunnable, in order to provide feedback to the user who placed the attribute on the method.

NUnit treats attributes implementing this interface specially. They are ignored if any other attributes are present that implement ITestBuilder. This allows, for example, use of [Test] on a method that also has [Combinatorial] specified, without any error arising. Such usage has existed in NUnit for some time and this special handling of the interface allows us to preserve it.

In the current build, only TestAttribute implements this interface.


The IImplyFixture interface is an empty interface, used solely as a marker:

public interface IImplyFixture

If a class contains any method with an attribute that implements this interface, that class is treated as an NUnit TestFixture without any TestFixture attribute being specified. The following NUnit attributes currently implement this interface:

  • TestAttribute
  • TestCaseAttribute
  • TestCaseSourceAttribute
  • TheoryAttribute


The IApplyToTest interface is used to make modifications to a test immediately after it is constructed. It is defined as follows:

public interface IApplyToTest
    void ApplyToTest(Test test);

The Test Type is quite general and the argument may represent a suite or an individual test case. If the distinction is important, then you must code the attribute to examine the argument and react accordingly.

The interface may appear on the same attribute that is used to construct the test or on a separate attribute. In either case, it will only be called after the test is built.

The order in which ApplyToTest is called on multiple attributes is indeterminate. If two attributes make completely independent changes to a test, then the order is not relevant. But if they both change the same property, or related properties, then it may necessary to make tests in the attribute code to ensure that the correct value 'wins'.

The most common example of this is for attributes that change the RunState of a test. If one attribute is trying to set it to RunState.Ignore, while the other wants it to be RunState.NotRunnable, we would normally expect the 'worst' value to win and for the test to be non-runnable. We can achieve that by code like the following:

// In the attribute setting NotRunable
test.RunState = RunState.NotRunnable;

// In the attribute setting Ignore
if (test.RunState != RunState.NotRunnable)
    test.RunState = RunState.Ignore;

The following NUnit attributes implement IApplyToTest:

  • CategoryAttribute
  • CombiningStrategyAttribute
  • CultureAttribute
  • ExplicitAttribute
  • IgnoreAttribute
  • PlatformAttribute
  • PropertyAttribute (and, through it, a large number of derived attributes)
  • RequiresThreadAttribute
  • TestAttribute
  • TestFixtureAttribute

Execution-Time Interfaces

At execution-time, some or all of the tests that were previously loaded are actually run. Their results are returned and made available to the application.


NUnit tests run within a context, known as the TestExecutionContext. The context for a test case is nested within the context for its containing suite and so on, up to the assembly level. Attributes that implement IApplyToContext are called immediately after the context is created and before the test is run in order to make changes to the context. Once the test execution has completed, the context is discarded so that - effectively - any changes are reverted to their original values.

The IApplyToContext interface is defined as follows:

public interface IApplyToContext
    void ApplyToContext(TestExecutionContext context);

An example of the use of the context may be helpful. One item in the TestExecutionContext is the default timeout value for test cases. When any test is marked with [Timeout(nnn)] the context value is replaced by the supplied argument. The new timeout applies for any test case it appears on and any test case that is contained in a suite that it appears on. When the test or suite completes, the new value is discarded and the value contained in the original context is once against used.

Custom attributes that implement IApplyToContext should modify the TestExecutionContext in accordance with the arguments supplied to them. They are not called after the test is run and have no cleanup to perform.

The NUnit attributes that implement IApplyToContext are as follows:

  • ParallelizableAttribute
  • SetCultureAttribute
  • SetUICultureAttribute
  • TimeoutAttribute


In NUnit 2.x, tests were self-executing objects. In NUnit 3.0, execution is done using command objects, which are constructed for each test. Execution of a single test will generally require multiple nested commands. Some attributes are able to contribute to the chain of commands. For example, MaxTimeAttribute adds a command, which examines the elapsed time to complete a test and fails it if a specified maximum was exceeded.

Attributes add to the command chain by creating a command decorator and implementing the ICommandDecoratorSource interface. The interface is defined as follows:

public interface ICommandDecoratorSource
    IEnumerable<ICommandDecorator> GetDecorators(MethodInfo method);

Custom attributes that require participation in the command chain may create and return a decorator. Note that the decorators returned may vary according to the state of the attribute and that returning an empty collection is valid.


This interface is implemented by command decorators, which are normally separate objects from the attributes. The ICommandDecorator interface is defined as follows:

public interface ICommandDecorator
    CommandStage Stage { get; }
    int Priority { get; }
    TestCommand Decorate(TestCommand command);

There are various stages of command execution. In the current version of NUnit, the following stages are used:

public enum CommandStage
    BelowSetUpTearDown, // Command runs after setup and cleans up before teardown
    AboveSetUpTearDown  // Command runs before setup and cleans up after teardown

This is a preliminary definition, subject to change.

The int value of priority is used to set the order of execution within a single stage. It usually defaults to zero.

The Decorate method should return an appropriate command in which the original command has been nested. For examples, see the implementation of MaxTimeAttribute and ExpectedExceptionAttribute.