DYN-6732: Fix 'ReferenceEqualityComparer' hash collisions

src/Engine/ProtoCore/FFI/CLRObjectMarshaler.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using System.Runtime.CompilerServices;
 using System.Xml;
 using Autodesk.DesignScript.Interfaces;
 using DesignScript.Builtin;
@@ -1394,8 +1395,17 @@ void core_Dispose(ProtoCore.RuntimeCore sender)
     }
 
     /// <summary>
-    /// This class compares two CLR objects. It is used in CLRObjectMap to 
-    /// avoid hash collision. 
+    /// This class compares two CLR objects using reference equality. It is used in CLRObjectMap
+    /// to map CLR object instances to their marshaled StackValue representations. Uses
+    /// <see cref="RuntimeHelpers.GetHashCode"/> to get the object's identity hash code, which
+    /// ensures well-distributed hash codes even when objects have value-based hash codes that
+    /// collide (e.g., Point objects with identical coordinates).
+    /// <para>
+    /// <b>Note:</b> The hash code computation is intentionally aligned with the reference
+    /// equality behavior used by <see cref="object.ReferenceEquals"/>. This ensures consistent
+    /// semantics between equality comparison and hash code generation, which is a requirement
+    /// for proper <see cref="IEqualityComparer{T}"/> implementation.
+    /// </para>
     /// </summary>
     public class ReferenceEqualityComparer: IEqualityComparer<object>
     {
@@ -1406,7 +1416,7 @@ bool IEqualityComparer<object>.Equals(object x, object y)
 
         public int GetHashCode(object obj)
         {
-            return obj.GetHashCode();
+            return RuntimeHelpers.GetHashCode(obj);
         }
     }
 

test/Engine/ProtoTest/FFITests/ReferenceEqualityComparerTests.cs

@@ -0,0 +1,135 @@
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
+using NUnit.Framework;
+using ReferenceEqualityComparer = ProtoFFI.ReferenceEqualityComparer;
+
+namespace ProtoFFITests
+{
+    [TestFixture]
+    public class ReferenceEqualityComparerTests
+    {
+        /// <summary>
+        /// Test class that simulates geometry objects with value-based hash codes
+        /// (similar to Point objects with identical coordinates)
+        /// </summary>
+        private class TestPoint
+        {
+            public double X { get; set; }
+            public double Y { get; set; }
+            public double Z { get; set; }
+
+            public TestPoint(double x, double y, double z)
+            {
+                X = x;
+                Y = y;
+                Z = z;
+            }
+
+            // Value-based hash code (like Point.ComputeHashCode in LibG)
+            public override int GetHashCode()
+            {
+                int hash = 17;
+                hash = hash * 23 + X.GetHashCode();
+                hash = hash * 23 + Y.GetHashCode();
+                hash = hash * 23 + Z.GetHashCode();
+                return hash;
+            }
+
+            public override bool Equals(object obj)
+            {
+                if (obj == null || GetType() != obj.GetType())
+                    return false;
+
+                var other = (TestPoint)obj;
+                return X == other.X && Y == other.Y && Z == other.Z;
+            }
+        }
+
+        [Test]
+        [Category("UnitTests")]
+        public void ProducesDifferentHashCodesForDifferentInstances()
+        {
+            // Arrange: Create multiple objects with identical values but different instances
+            var point1 = new TestPoint(0, 0, 0);
+            var point2 = new TestPoint(0, 0, 0);
+            var point3 = new TestPoint(0, 0, 0);
+
+            var comparer = new ReferenceEqualityComparer();
+
+            // Act: Get hash codes using ReferenceEqualityComparer
+            int hash1 = comparer.GetHashCode(point1);
+            int hash2 = comparer.GetHashCode(point2);
+            int hash3 = comparer.GetHashCode(point3);
+
+            // Assert: Different instances should produce different hash codes
+            // (even though they have identical values)
+            Assert.AreNotEqual(hash1, hash2, "Different instances should produce different hash codes");
+            Assert.AreNotEqual(hash1, hash3, "Different instances should produce different hash codes");
+            Assert.AreNotEqual(hash2, hash3, "Different instances should produce different hash codes");
+        }
+
+        [Test]
+        [Category("UnitTests")]
+        public void UsesIdentityHashCode()
+        {
+            // Arrange
+            var point = new TestPoint(1, 2, 3);
+            var comparer = new ReferenceEqualityComparer();
+
+            // Act
+            int comparerHash = comparer.GetHashCode(point);
+            int identityHash = RuntimeHelpers.GetHashCode(point);
+
+            // Assert: ReferenceEqualityComparer should use RuntimeHelpers.GetHashCode
+            Assert.AreEqual(identityHash, comparerHash,
+                "ReferenceEqualityComparer should use RuntimeHelpers.GetHashCode for identity-based hashing");
+        }
+
+        [Test]
+        [Category("UnitTests")]
+        public void DictionaryLookupPerformance_NoCollisions()
+        {
+            // Arrange: Create dictionary using ReferenceEqualityComparer
+            var dictionary = new Dictionary<object, string>(new ReferenceEqualityComparer());
+
+            // Create objects with identical values but different instances
+            const int count = 20;
+            var objects = new TestPoint[count];
+            for (int i = 0; i < count; i++)
+            {
+                objects[i] = new TestPoint(0, 0, 0); // All have identical coordinates
+                dictionary[objects[i]] = $"Value_{i}";
+            }
+
+            // Act & Assert: All lookups should succeed and be fast (O(1))
+            for (int i = 0; i < count; i++)
+            {
+                Assert.IsTrue(dictionary.TryGetValue(objects[i], out string value),
+                    $"Lookup should succeed for object at index {i}");
+                Assert.AreEqual($"Value_{i}", value,
+                    $"Retrieved value should match for object at index {i}");
+            }
+
+            // Verify that objects with same values but different instances are treated as different
+            var newPoint = new TestPoint(0, 0, 0);
+            Assert.IsFalse(dictionary.ContainsKey(newPoint),
+                "New instance with same values should not be found (reference equality)");
+        }
+
+        [Test]
+        [Category("UnitTests")]
+        public void ReferenceEqualitySemantics()
+        {
+            // Arrange
+            var point1 = new TestPoint(1, 2, 3);
+            var point2 = new TestPoint(1, 2, 3); // Same values, different instance
+            IEqualityComparer<object> comparer = new ReferenceEqualityComparer();
+
+            // Act & Assert: Reference equality should be used, not value equality
+            Assert.IsFalse(comparer.Equals(point1, point2),
+                "Different instances should not be equal (reference equality)");
+            Assert.IsTrue(comparer.Equals(point1, point1),
+                "Same instance should be equal to itself");
+        }
+    }
+}