diff --git a/SysML2.NET.Tests/Extend/AttributeUsageExtensionsTestFixture.cs b/SysML2.NET.Tests/Extend/AttributeUsageExtensionsTestFixture.cs
index c226fe18..83c58ebe 100644
--- a/SysML2.NET.Tests/Extend/AttributeUsageExtensionsTestFixture.cs
+++ b/SysML2.NET.Tests/Extend/AttributeUsageExtensionsTestFixture.cs
@@ -1,44 +1,72 @@
 // -------------------------------------------------------------------------------------------------
 // <copyright file="AttributeUsageExtensionsTestFixture.cs" company="Starion Group S.A.">
-// 
+//
 //   Copyright 2022-2026 Starion Group S.A.
-// 
+//
 //   Licensed under the Apache License, Version 2.0 (the "License");
 //   you may not use this file except in compliance with the License.
 //   You may obtain a copy of the License at
-// 
+//
 //        http://www.apache.org/licenses/LICENSE-2.0
-// 
+//
 //    Unless required by applicable law or agreed to in writing, software
 //    distributed under the License is distributed on an "AS IS" BASIS,
 //    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 //    See the License for the specific language governing permissions and
 //    limitations under the License.
-// 
+//
 // </copyright>
 // ------------------------------------------------------------------------------------------------
 
 namespace SysML2.NET.Tests.Extend
 {
     using System;
-    
+
     using NUnit.Framework;
 
-    using SysML2.NET.Core.POCO.Systems.Attributes ;
+    using SysML2.NET.Core.POCO.Core.Classifiers;
+    using SysML2.NET.Core.POCO.Core.Features;
+    using SysML2.NET.Core.POCO.Systems.Attributes;
+    using SysML2.NET.Extensions;
 
     [TestFixture]
     public class AttributeUsageExtensionsTestFixture
     {
         [Test]
-        public void ComputeAttributeDefinition_ThrowsNotSupportedException()
+        public void VerifyComputeAttributeDefinition()
         {
-            Assert.That(() => ((IAttributeUsage)null).ComputeAttributeDefinition(), Throws.TypeOf<NotSupportedException>());
+            Assert.That(() => ((IAttributeUsage)null).ComputeAttributeDefinition(), Throws.TypeOf<ArgumentNullException>());
+
+            var emptySubject = new AttributeUsage();
+
+            Assert.That(emptySubject.ComputeAttributeDefinition(), Has.Count.EqualTo(0));
+
+            var subject = new AttributeUsage();
+            var attributeDefinition = new AttributeDefinition();
+            var bareClassifier = new Classifier();
+
+            subject.AssignOwnership(new FeatureTyping { Type = attributeDefinition });
+            subject.AssignOwnership(new FeatureTyping { Type = bareClassifier });
+
+            // Only the IDataType (AttributeDefinition) is returned; bare Classifier is excluded.
+            Assert.That(subject.ComputeAttributeDefinition(), Is.EquivalentTo(new[] { attributeDefinition }));
         }
-        
+
         [Test]
-        public void ComputeIsReference_ThrowsNotSupportedException()
+        public void VerifyComputeIsReference()
         {
-            Assert.That(() => ((IAttributeUsage)null).ComputeIsReference(), Throws.TypeOf<NotSupportedException>());
+            Assert.That(() => ((IAttributeUsage)null).ComputeIsReference(), Throws.TypeOf<ArgumentNullException>());
+
+            var emptySubject = new AttributeUsage();
+
+            Assert.That(emptySubject.ComputeIsReference(), Is.True);
+
+            var populatedSubject = new AttributeUsage();
+            var attributeDefinition = new AttributeDefinition();
+            populatedSubject.AssignOwnership(new FeatureTyping { Type = attributeDefinition });
+
+            // Always true regardless of subject state — AttributeUsages are always referential.
+            Assert.That(populatedSubject.ComputeIsReference(), Is.True);
         }
     }
 }
diff --git a/SysML2.NET/Extend/AttributeUsageExtensions.cs b/SysML2.NET/Extend/AttributeUsageExtensions.cs
index 78dc8528..9d52443e 100644
--- a/SysML2.NET/Extend/AttributeUsageExtensions.cs
+++ b/SysML2.NET/Extend/AttributeUsageExtensions.cs
@@ -22,6 +22,7 @@ namespace SysML2.NET.Core.POCO.Systems.Attributes
 {
     using System;
     using System.Collections.Generic;
+    using System.Linq;
 
     using SysML2.NET.Core.Core.Types;
     using SysML2.NET.Core.Root.Namespaces;
@@ -63,16 +64,25 @@ internal static class AttributeUsageExtensions
         /// <summary>
         /// Computes the derived property.
         /// </summary>
+        /// <remarks>
+        /// Walks <c>OwnedRelationship</c> → <c>IFeatureTyping</c> → <c>Type</c> directly,
+        /// filtering to <c>IDataType</c>. The AttributeUsage POCO's explicit-interface
+        /// <c>IUsage.definition</c> impl delegates to <c>this.attributeDefinition</c>,
+        /// which would route back into this method → stack overflow. Bypassing the
+        /// instance property mirrors the technique in
+        /// <see cref="UsageExtensions.ComputeDefinition" />.
+        /// </remarks>
         /// <param name="attributeUsageSubject">
         /// The subject <see cref="IAttributeUsage"/>
         /// </param>
         /// <returns>
         /// the computed result
         /// </returns>
-        [System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
         internal static List<IDataType> ComputeAttributeDefinition(this IAttributeUsage attributeUsageSubject)
         {
-            throw new NotSupportedException("Create a GitHub issue when this method is required");
+            return attributeUsageSubject == null
+                ? throw new ArgumentNullException(nameof(attributeUsageSubject))
+                : [..attributeUsageSubject.OwnedRelationship.OfType<IFeatureTyping>().Select(featureTyping => featureTyping.Type).OfType<IDataType>()];
         }
 
         /// <summary>
@@ -84,10 +94,11 @@ internal static List<IDataType> ComputeAttributeDefinition(this IAttributeUsage
         /// <returns>
         /// the computed result
         /// </returns>
-        [System.Diagnostics.CodeAnalysis.ExcludeFromCodeCoverage]
         internal static bool ComputeIsReference(this IAttributeUsage attributeUsageSubject)
         {
-            throw new NotSupportedException("Create a GitHub issue when this method is required");
+            return attributeUsageSubject == null
+                ? throw new ArgumentNullException(nameof(attributeUsageSubject))
+                : true;
         }
 
     }