Creating a custom xUnit [Trait]

After creating my blog posts on writing tests with xUnit in a TDD fashion, I remembered I had only briefly touched upon Traits and not at all how to implement custom ones. Time to remedy that.

There is not a lot I can say more about traits than I did in my original post, all the way at the bottom.

With the [Trait] attribute, I can add metadata to tests. The first parameter is the key of the metadata, the second parameter is the value that goes with the key. This can be a bit confusing when starting to work with the xUnit framework. I like to think about traits as a sort of claim from claims-based authentication.

What I can do, is elaborate on creating custom attributes. My original information came from the only blog post I found that described xUnits new way of extending Traits. I think that counting constructor parameters and tight coupling of Traits and TraitDiscoverers is not the most effective way of handling Traits. Here is hopefully a better way. All code is available on my GitHub account, the traits can be found in CustomXunitTrait/CustomXunitTrait.Tests/Infrastructure/.

Simple Trait

A simple Trait is an attribute without derived classes. It stands on its own. For example: an analyst on the team has described a lot of the functionality of the application in test cases. I as a developer want to add the test case identifier to the appropriate unit test. This allows the documentation of the feature to be quickly found in the analysis. Take a look at the Trait below to see what I mean.

[TraitDiscoverer("CustomXunitTrait.Tests.Infrastructure.TestCaseDiscoverer", "CustomXunitTrait.Tests")]
[AttributeUsage(AttributeTargets.Method)]
public class TestCaseAttribute : Attribute, ITraitAttribute
{
  public string TestCase { get; set; }
  public TestCaseAttribute(string testCase)
  {
    TestCase = testCase;
  }
}

With a little workaround, the discoverer can easily use the properties of the custom Trait.

public class TestCaseDiscoverer : ITraitDiscoverer
{
  private const string Key = "TestCase";
  public IEnumerable<KeyValuePair<string, string>> GetTraits(IAttributeInfo traitAttribute)
  {
    string testCase;
    var attributeInfo = traitAttribute as ReflectionAttributeInfo;
    var testCaseAttribute = attributeInfo?.Attribute as TestCaseAttribute;
    if (testCaseAttribute != null)
    {
      testCase = testCaseAttribute.TestCase;
    }
    else
    {
      var constructorArguments = traitAttribute.GetConstructorArguments().ToArray();
      testCase = constructorArguments[0].ToString();
    }
    yield return new KeyValuePair<string, string>(Key, testCase);
  }
}

To access the attribute, I cast the IAttributeInfo object to a ReflectionAttributeInfo object which allows me to access the actual attribute that is being parsed. The property contains an object, since I don’t see an easy way to couple the ITraitDiscoverer to an instance of ITraitAttribute. If you wonder why the discoverer can’t accept a generic type that implements ITraitAttribute, I suggest you read my Points of improvements paragraph first.

Now that I can access the attribute, I can easily get the values that were passed along to it and saved in its properties. This is a lot clearer to use than the GetConstructorArguments() that is used in the else part of the code. I inserted that as a fallback when the casts should fail. Failing on the cast can also be interesting because it would force you to investigate the problem.

Complex Trait

A complex Trait is an attribute that has derived classes for every possible option I want to make available. For example: I build an abstract CategoryAttribute to allow categories to be specified and then provide derived ComponentCategoryAttribute and EndToEndCategoryAttribute to indicate which tests are component or end-to-end tests.

[TraitDiscoverer("CustomXunitTrait.Tests.Infrastructure.CategoryDiscoverer", "CustomXunitTrait.Tests")]
[AttributeUsage(AttributeTargets.Method)]
public abstract class CategoryAttribute : Attribute, ITraitAttribute
{
  public abstract string Type { get; }
}
public class ComponentCategoryAttribute : CategoryAttribute
{
  public override string Type => "Component";
}
public class EndToEndCategoryAttribute : CategoryAttribute
{
  public override string Type => "EndToEnd";
}

This now allows me to write a very easy to understand ITraitDiscoverer that can handle both categories.

public class CategoryDiscoverer : ITraitDiscoverer
{
  private const string Key = "Category";
  public IEnumerable<KeyValuePair<string, string>> GetTraits(IAttributeInfo traitAttribute)
  {
    var attributeInfo = traitAttribute as ReflectionAttributeInfo;
    var category = attributeInfo?.Attribute as CategoryAttribute;
    var value = category?.Type ?? string.Empty;
    yield return new KeyValuePair<string, string>(Key, value);
  }
}

Look at how clean this is solved now. I cast the IAttributeInfo object to a ReflectionAttributeInfo instance, I cast the Attribute property to the CategoryAttribute class and I can access the value stored inside to indicate the correct category of the test.

How to use these Traits

[Fact]
[TestCase("Biz001")]
[ComponentCategory]
public void FirstTest()
{...}
[Fact]
[TestCase("Biz002")]
[EndToEndCategory]
public void SecondTest()
{...}

Unfortunately, at the time of writing, the custom categories are pretty useless since there is no immediate support for them in the ReSharper test discoverer. They can be convenient to indicate the purpose of the test.

Possible improvements

There are two issues with the implementation that I would like to see changed.

First, the ITraitDiscoverer passes an IAttributeInfo object to the GetTraits method. The IAttributeInfo should contain the Attribute property without having to cast it to a specific type. The Attribute property would of course need a cast, but not the object passed to the method. I think the Attribute property is a pretty important object to pass to the discoverer, who knows what people would want to do with it to construct the IEnumerable&lt;KeyValuePair&lt;string, string&gt;&gt; return type.

The other improvement is on the attribute itself. The problem I have is that the TraitDiscoverer attribute that is applied to a custom Trait class needs a type and assembly name as a string to locate the ITraitDiscoverer. If I refactor the name, namespace or assembly, then I have to change the text in all the attributes I created. I wonder why tests can’t see if an attribute implements the ITraitAttribute. The ITraitAttribute could then accept a generic type that expects an ITraitDiscoverer to be passed along.  Example:

[AttributeUsage(AttributeTargets.Method)]
public class TestCaseAttribute : Attribute, ITraitAttribute<TestCaseDiscoverer>
{...}

This would mean that the ITraitDiscoverer could not be made generic, as this would create a circular reference if the attributes and discoverers would be located in two separate assemblies.

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