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:

   

6. Add tag @myMethodTagDecorator to the feature file:

   

7. Build the solution

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

   

You can find the complete Decorator example on GitHub.

Further read

• NUnit Apartment attribute: https://docs.nunit.org/articles/nunit/writing-tests/attributes/apartment.html

• Apartments: https://docs.microsoft.com/en-us/windows/win32/com/processes–threads–and-apartments