Enabled category inheritance, fix #2306

Author: AndreyAkinshin

src/BenchmarkDotNet.Annotations/Attributes/BenchmarkCategoryAttribute.cs

@@ -3,7 +3,7 @@
 
 namespace BenchmarkDotNet.Attributes
 {
-    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Assembly)]
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Assembly, AllowMultiple = true)]
     public class BenchmarkCategoryAttribute : Attribute
     {
         public string[] Categories { get; }

src/BenchmarkDotNet/Running/BenchmarkConverter.cs

@@ -248,11 +248,11 @@ private static IEnumerable<ParameterInstances> GetArgumentsDefinitions(MethodInf
         private static string[] GetCategories(MethodInfo method)
         {
             var attributes = new List<BenchmarkCategoryAttribute>();
-            attributes.AddRange(method.GetCustomAttributes(typeof(BenchmarkCategoryAttribute), false).OfType<BenchmarkCategoryAttribute>());
-            var type = method.DeclaringType;
+            attributes.AddRange(method.GetCustomAttributes(typeof(BenchmarkCategoryAttribute), true).OfType<BenchmarkCategoryAttribute>());
+            var type = method.ReflectedType;
             if (type != null)
             {
-                attributes.AddRange(type.GetTypeInfo().GetCustomAttributes(typeof(BenchmarkCategoryAttribute), false).OfType<BenchmarkCategoryAttribute>());
+                attributes.AddRange(type.GetTypeInfo().GetCustomAttributes(typeof(BenchmarkCategoryAttribute), true).OfType<BenchmarkCategoryAttribute>());
                 attributes.AddRange(type.GetTypeInfo().Assembly.GetCustomAttributes().OfType<BenchmarkCategoryAttribute>());
             }
             if (attributes.Count == 0)

tests/BenchmarkDotNet.Tests/Configs/CategoriesTests.cs

@@ -0,0 +1,55 @@
+using System.Linq;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Running;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace BenchmarkDotNet.Tests.Configs
+{
+    public class CategoriesTests
+    {
+        private readonly ITestOutputHelper output;
+
+        public CategoriesTests(ITestOutputHelper output)
+        {
+            this.output = output;
+        }
+
+        [Fact]
+        public void CategoryInheritanceTest()
+        {
+            string Format(BenchmarkCase benchmarkCase) =>
+                benchmarkCase.Descriptor.WorkloadMethod.Name + ": " +
+                string.Join("+", benchmarkCase.Descriptor.Categories.OrderBy(category => category));
+
+            var benchmarkCases = BenchmarkConverter
+                .TypeToBenchmarks(typeof(DerivedClass))
+                .BenchmarksCases
+                .OrderBy(x => x.Descriptor.WorkloadMethod.Name)
+                .ToList();
+            Assert.Equal(2, benchmarkCases.Count);
+
+            output.WriteLine(Format(benchmarkCases[0]));
+            output.WriteLine(Format(benchmarkCases[1]));
+
+            Assert.Equal("BaseMethod: BaseClassCategory+BaseMethodCategory+DerivedClassCategory", Format(benchmarkCases[0]));
+            Assert.Equal("DerivedMethod: BaseClassCategory+DerivedClassCategory+DerivedMethodCategory", Format(benchmarkCases[1]));
+        }
+
+        [BenchmarkCategory("BaseClassCategory")]
+        public class BaseClass
+        {
+            [Benchmark]
+            [BenchmarkCategory("BaseMethodCategory")]
+            public void BaseMethod() { }
+        }
+
+        [BenchmarkCategory("DerivedClassCategory")]
+        public class DerivedClass : BaseClass
+        {
+            [Benchmark]
+            [BenchmarkCategory("DerivedMethodCategory")]
+            public void DerivedMethod() { }
+        }
+    }
+}