Decorators

SpecFlow supports decorators which can be used in feature files. Decorators can be used to convert a tag in a feature file to an attribute in the generated code behind file.

Example decorator

Say we want to add an NUnit Apartment attribute to a test method in the generated code behind file (a file with extension .feature.cs) to specify that the test should be running in a particular apartment, either the STA or the MTA. For this, we can use a decorator which we need to register in a generator plugin so that the decorator can have its effect during the code behind file generation.

Steps to follow:

  1. Create a SpecFlow project with test framework NUnit using the project template provided by the SpecFlow Visual Studio extension. Learn more

  2. Create a GeneratorPlugin. You can follow the steps here or you can use the sample generator plugin project as a basis

  3. Create a Decorator (which is a class which implements interfaces like ITestMethodTagDecorator, ITestMethodDecorator, etc.):

    • ITestMethodDecorator is called always

    • ITestMethodTagDecorator is called only if the scenario has at least one tag

    public class MyMethodTagDecorator : ITestMethodTagDecorator
    {
        public static readonly string TAG_NAME = "myMethodTagDecorator";
        private readonly ITagFilterMatcher _tagFilterMatcher;
    
        public MyMethodTagDecorator(ITagFilterMatcher tagFilterMatcher)
        {
            _tagFilterMatcher = tagFilterMatcher;
        }
    
        public bool CanDecorateFrom(string tagName, TestClassGenerationContext generationContext, CodeMemberMethod testMethod)
        {
            return _tagFilterMatcher.Match(TAG_NAME, tagName);
        }
    
        public void DecorateFrom(string tagName, TestClassGenerationContext generationContext, CodeMemberMethod testMethod)
        {
            var attribute = new CodeAttributeDeclaration(
                "NUnit.Framework.ApartmentAttribute",
                new CodeAttributeArgument(
                    new CodeFieldReferenceExpression(
                        new CodeTypeReferenceExpression(typeof(System.Threading.ApartmentState)),
                        "STA")));
    
            testMethod.CustomAttributes.Add(attribute);
        }
    
        public int Priority { get; }
        public bool RemoveProcessedTags { get; }
        public bool ApplyOtherDecoratorsForProcessedTags { get; }
    }
    
  4. Register the Decorator in the Initialize method of the GeneratorPlugin:

    public void Initialize(GeneratorPluginEvents generatorPluginEvents, GeneratorPluginParameters generatorPluginParameters,
    UnitTestProviderConfiguration unitTestProviderConfiguration)
    {
        // Register the decorator
        generatorPluginEvents.RegisterDependencies += RegisterDependencies;
    }
    
    private void RegisterDependencies(object sender, RegisterDependenciesEventArgs eventArgs)
    {
        eventArgs.ObjectContainer.RegisterTypeAs<MyMethodTagDecorator, ITestMethodTagDecorator>(MyMethodTagDecorator.TAG_NAME);
    }
    
  5. Install the GeneratorPlugin NuGet package to the SpecFlow project. Once the installation finishes, it should look like this:

    Generator plugin installed

  6. Add tag @myMethodTagDecorator to the feature file:

    Add decorator to feature file

  7. Build the solution

  8. Check the generated code behind file (.feature.cs) if it contains the NUnit Apartment attribute:

    Attribute added

You can find the complete Decorator example on GitHub.