CustomConvention.cs

using System.Linq;
using System.Reflection;
using Fixie;
using Fixie.Conventions;

namespace CategoriesDemo.Tests
{
    public class CustomConvention : Convention
    {
        public CustomConvention(RunContext runContext)
        {
            var desiredCategories = runContext.Options["include"].ToArray();
            var shouldRunAll = !desiredCategories.Any();

            Classes
                .NameEndsWith("Tests");

            Cases
                .Where(method => method.Void())
                .ZeroParameters()
                .Where(method => shouldRunAll || MethodHasAnyDesiredCategory(method, desiredCategories));
        }

        static bool MethodHasAnyDesiredCategory(MethodInfo method, string[] desiredCategories)
        {
            return Categories(method).Any(testCategory => desiredCategories.Contains(testCategory.Name));
        }

        static CategoryAttribute[] Categories(MethodInfo method)
        {
            return method.GetCustomAttributes<CategoryAttribute>(true).ToArray();
        }
    }
}