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16. Custom attributes

Implement custom NUnit attributes, which can be used to alter test execution.
Read time 2 minutesLast updated 14 days ago

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;}