16. Custom attributes

Learning objectives

In this section we will look at some ways of implementing custom NUnit attributes, which can be used to alter test execution.

Intro and motivation

A powerful part of NUnit is that it is very extendable. One of the ways it can be extended is through custom attributes. An example is attributes that implement the IWrapTestMethod interface. This interface has a method for wrapping a TestCommand, which implements a method for executing. Normally these test commands do something, call execute on their inner command and then maybe do something again after the inner command is completed.

In the following three classes an IWrapTestMethod interface is implemented and used in a test:

public class MyAttribute : NUnitAttribute, IWrapTestMethod
{
 public TestCommand Wrap(TestCommand command)
 {
  return new MyCommand(command);
 }
}

public class MyCommand : TestCommand
{
 private TestCommand innerCommand;
 
 public MyCommand(TestCommand command) : base(command.Test)
 {
  innerCommand = command;
 }

 public override TestResult Execute(ITestExecutionContext context)
 {
  Debug.Log("Before");
  var result = innerCommand.Execute(context);
  Debug.Log("After");

  return result;
 }
}

public class MyTests
{
 [Test]
 [MyAttribute]
 public void Test1()
 {
  Debug.Log("The test");
 }
}

When running MyTests.Test1 the following output is printed:

Test1 (0,017s)
---
Before
The test
After

Other interfaces that custom attributes can implement are IWrapSetUpTearDown, IApplyToContext, and IApplyToTest.

Exercise

At Unity we have a goal that an action should never take longer than 500 ms. In the sample 16_CustomAttributes there is a class called MyClass, which has two methods. Both methods are supposed to return true. However someone has made a regression so that one of the two methods takes a long time to run.

The task is to create a new custom attribute, which detects if the test takes longer than 500 ms to run. If that happens, it should fail the test with a descriptive message. Apply that to the two existing tests.

Hints

You can use the class System.Diagnostics.Stopwatch to time how many miliseconds have passed.

Solution

A full solution for the exercise is availiable at 16_CustomAttributes_Solution.

The core of the solution is the execute method in the test command implementation:

public override TestResult Execute(ITestExecutionContext context)
{
 var stopWatch = new Stopwatch();
 stopWatch.Start();
 var result = innerCommand.Execute(context);
 stopWatch.Stop();

 if (stopWatch.ElapsedMilliseconds > 500)
 {
  result.SetResult(ResultState.Failure, $"Test took {stopWatch.ElapsedMilliseconds} ms. That is longer than 500ms!");
 }

 return result;
}

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